FORMATION OF ZIRCONIUM DIBORIDE AND OTHER METAL BORIDES BY VOLUME COMBUSTION SYNTHESIS AND MECHANOCHEMICAL PROCESS
A THESIS SUBMITTED TO THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCES OF MIDDLE EAST TECHNICAL UNIVERSITY
BY
BARIŞ AKGÜN
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN METALLURGICAL AND MATERIALS ENGINEERING
FEBRUARY 2008
Approval of the thesis:
FORMATION OF ZIRCONIUM DIBORIDE AND OTHER METAL BORIDES BY VOLUME COMBUSTION SYNTHESIS AND MECHANOCHEMICAL PROCESS
submitted by BARIŞ AKGÜN in partial fulfillment of the requirements for the degree of Master of Science in Metallurgical and Materials Engineering Department, Middle East Technical University by,
Prof. Dr. Canan Özgen ______Dean, Graduate School of Natural and Applied Sciences
Prof. Dr. Tayfur Öztürk ______Head of Department, Metallurgical and Materials Engineering
Prof. Dr. Naci Sevinç ______Supervisor, Metallurgical and Materials Engineering Dept., METU
Examining Committee Members:
Prof. Dr. Ahmet Geveci ______Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Naci Sevinç ______Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Yavuz A. Topkaya ______Metallurgical and Materials Engineering Dept., METU
Prof. Dr. İshak Karakaya ______Metallurgical and Materials Engineering Dept., METU
Prof. Dr. Gülhan Özbayoğlu ______Mining Engineering Dept., METU
Date: 01.02.2008
I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work.
Name, Last name : Barış Akgün
Signature :
iii
ABSTRACT
FORMATION OF ZIRCONIUM DIBORIDE AND OTHER METAL BORIDES BY VOLUME COMBUSTION SYNTHESIS AND MECHANOCHEMICAL PROCESS
Akgün, Barış
M.Sc., Department of Metallurgical and Materials Engineering
Supervisor: Prof. Dr. Naci Sevinç
February 2008, 109 pages
The aim of this study was to produce zirconium diboride (ZrB2) and other metal borides such as lanthanum hexaboride (LaB6) and cerium hexaboride
(CeB6) by magnesiothermic reduction (reaction of metal oxide, boron oxide and magnesium) using volume combustion synthesis (VCS) and mechanochemical process (MCP).
Production of ZrB2 by VCS in air occurred with the formation of side products, Zr2ON2 and Mg3B2O6 in addition to MgO. Formation of Zr2ON2 was prevented by conducting VCS experiments under argon atmosphere.
Wet ball milling was applied before leaching for easier removal of Mg3B2O6.
Leaching in 5 M HCl for 2.5 hours was found to be sufficient for removal of
MgO and Mg3B2O6. By MCP, 30 hours of ball milling was enough to produce ZrB2 where 10% of excess Mg and B2O3 were used. MgO was easily removed when MCP products were leached in 1 M HCl for 30 minutes.
Complete reduction of ZrO2 could not be achieved in either production
iv method because of the stability of ZrO2. Hence, after leaching VCS or MCP products, final product was composed of ZrB2 and ZrO2.
Formation of LaB6 and CeB6 were very similar to each other via both methods. Mg3B2O6 appeared as a side product in the formation of both borides by VCS. After wet ball milling, products were leached in 1 M HCl for 15 hours and pure LaB6 or CeB6 was obtained. As in ZrB2 production, 30 hours of ball milling was sufficient to form these hexaborides by MCP. MgO was removed after leaching in 1 M HCl for 30 minutes and the desired hexaboride was obtained in pure form.
Keywords: Zirconium Diboride, Lanthanum Hexaboride, Cerium
Hexaboride, Volume Combustion Synthesis, Mechanochemical Process,
Acid Leaching.
v
ÖZ
HACİMSEL TUTUŞMA SENTEZLEMESİ VE MEKANOKİMYASAL YÖNTEM İLE ZİRKONYUM DİBORÜR VE DİĞER METAL BORÜRLERİN OLUŞUMU
Akgün, Barış
Y. Lisans, Metalurji ve Malzeme Mühendisliği Bölümü
Tez Yöneticisi: Prof. Dr. Naci Sevinç
Şubat 2008, 109 sayfa
Bu çalışmanın amacı magnezyotermik indirgeme (metal oksit, bor oksit ve magnezyum’un reaksiyonu) ile hacimsel tutuşma sentezi (HTS) ve mekanokimyasal yöntem (MKY) kullanılarak zirkonyum diborür (ZrB2) ile lantan hekzaborür (LaB6) ve seryum hekzaborür (CeB6) gibi diğer metal borürleri üretmektir.
Hava ortamında HTS ile ZrB2 üretimi, MgO’e ek olarak yan ürünlerin,
Zr2ON2 ve Mg3B2O6, oluşumu ile birlikte görülmüştür. Zr2ON2’ün oluşumu
HTS deneylerinin argon atmosferinde yapılmasıyla engellenmiştir.
Mg3B2O6’ın kolay giderilmesi için liç öncesinde ıslak öğütme uygulanmıştır.
5 M HCl içinde 2,5 saat yapılan liç MgO ve Mg3B2O6’ın giderilebilmesi için yeterli bulunmuştur. MKY ile 30 saatlik öğütme %10 fazla Mg ve B2O3 kullanıldığında ZrB2 üretmek için yeterlidir. MKY ürünleri 1 M HCl içinde
30 dakika liç edildiğinde, MgO kolayca giderilmiştir. ZrO2’in kararlılığından dolayı, her iki yöntemle de ZrO2’in tam indirgenmesi sağlanamamıştır. Bu
vi yüzden, HTS veya MKY ürünlerinin liçinden sonra, son ürün ZrB2 ve
ZrO2’ten oluşmaktadır.
Her iki yöntemle de LaB6 ve CeB6 oluşumu birbirlerine çok benzemektedir.
Mg3B2O6 her iki borürün HTS ile oluşumunda yan ürün olarak görülmüştür.
Islak öğütmeden sonra, ürünler 1 M HCl içinde 15 saat liçe tabi tutulmuştur ve saf LaB6 veya CeB6 elde edilmiştir. ZrB2 üretiminde olduğu gibi, 30 saatlik öğütme bu börürlerin MKY ile oluşması için yeterlidir. 1 M HCl içinde 30 dakika liçden sonra, MgO giderilmiş ve istenen hekzaborür saf halde elde edilebilmiştir.
Anahtar Kelimeler: Zirkonyum Diborür, Lantan Hekzaborür, Seryum
Hekzaborür, Hacimsel Tutuşma Sentezi, Mekanokimyasal Yöntem, Asit
Liçi.
vii
To my family
& in memory of my maternal uncle Ergun Erenkuş
viii
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to my supervisor Prof. Dr. Naci
Sevinç for his guidance, supervision, support, patience and encouragement throughout this study. I would also like to thank to Prof. Dr. Yavuz
Topkaya for his valuable advices and guidance.
I am very grateful to Dr. H. Erdem Çamurlu for his helps and support during this work. I should thank to Necmi Avcı for XRD analysis and
Cengiz Tan for SEM analysis.
I must thank to Çağla Özgit for her endless patience, motivation and support. I am also thankful to my best friends; Fatih Güngör, Alp Kol,
Özgür Gülse, Serkan Yalçınkaya, Hakan Cenkçiler, H. Volkan Yaycı, Emre
Erdem, Ali Mutlu, Ender Erdem for sharing my hard and bad times with me.
I must also express my special thanks to Ergin Büyükakıncı, Volkan Kayasü,
Arda Çetin, Deniz Keçik, Metehan Erdoğan and Evren Tan for their help and support.
Finally, I would like to express my deepest gratitude to my family Kemal,
Berrin, Başak, Nimet Akgün and Gönül Erenkuş for their endless love and support during my life.
ix This study was supported by National Boron Research Institute (BOREN)
Project No: BOREN-2006-Ç-04.
x
TABLE OF CONTENTS
ABSTRACT...... iv
ÖZ...... vi
DEDICATION...... viii
ACKNOWLEDGEMENTS...... ix
TABLE OF CONTENTS ...... xi
LIST OF TABLES ...... xiv
LIST OF FIGURES ...... xv
CHAPTER
1 INTRODUCTION...... 1
2 LITERATURE SURVEY...... 4
2.1 Binary Metal Borides ...... 4
2.1.1 Classification of Metal Borides...... 6
2.1.2 Boride Structures...... 7
2.1.3 Production Methods ...... 8
2.2 Zirconium Diboride ...... 9
2.2.1 Crystal Structure of Zirconium Diboride...... 10
2.2.2 Properties of Zirconium Diboride...... 11
2.2.3 Applications of Zirconium Diboride ...... 14
2.3 Lanthanum Hexaboride and Cerium Hexaboride ...... 15
2.3.1 Crystal Structure of Lanthanum Hexaboride and Cerium
Hexaboride...... 15
2.3.2 Properties of Lanthanum Hexaboride and Cerium Hexaboride... 16
xi 2.3.3 Applications of Lanthanum Hexaboride and Cerium Hexaboride
...... 18
2.4 Production Methods ...... 19
2.4.1 Volume Combustion Synthesis ...... 20
2.4.2 Mechanochemical Process...... 21
2.4.3 Thermite Reactions...... 22
2.5 Previous Works on the Production of Zirconium Diboride, Lanthanum
Hexaboride and Cerium Hexaboride...... 24
2.5.1 Previous Works on the Production of Zirconium Diboride...... 24
2.5.2 Previous Works on the Production of Lanthanum Hexaboride and
Cerium Hexaboride...... 32
3 EXPERIMENTAL PROCEDURE ...... 35
3.1 The Starting Materials...... 35
3.1.1 Calcination of Boric Acid ...... 35
3.1.2 Preparation of Reactant Mixtures ...... 37
3.2 Processing Methods ...... 38
3.2.1 Volume Combustion Synthesis Experiments and Determination of
the Ignition Temperature ...... 38
3.2.2 Ball Milling Operation ...... 42
3.2.2.1 Ball Milling Operation before Volume Combustion Synthesis
Experiments ...... 46
3.2.2.2 Ball Milling Operation after Volume Combustion Synthesis
Experiments ...... 47
3.2.2.3 Ball Milling Operation for Mechanochemical Process...... 48
3.3 Leaching Process ...... 48
3.4 Analysis Methods...... 52
4 RESULTS AND DISCUSSION ...... 55
xii 4.1 Formation of Zirconium Diboride ...... 55
4.1.1 Volume Combustion Synthesis (VCS)...... 55
4.1.2 Mechanochemical Process (MCP)...... 71
4.2 Formation of Lanthanum Hexaboride ...... 79
4.2.1 Volume Combustion Synthesis (VCS)...... 79
4.2.2 Mechanochemical Process (MCP)...... 85
4.3 Formation of Cerium Hexaboride...... 89
4.3.1 Volume Combustion Synthesis (VCS)...... 89
4.3.2 Mechanochemical Process (MCP)...... 93
5 CONCLUSION ...... 97
REFERENCES ...... 100
xiii
LIST OF TABLES
TABLES
Table 2.1 Melting points of some metals and their borides [1, 5 – 7, 11 –
15] ...... 5
Table 2.2 Microhardness values of some metal borides, some carbides
and diamond [5 – 7, 16, 17] ...... 5
Table 2.3 Periodic classification of some binary borides [6] ...... 6
Table 2.4 Properties of ZrB2 [5 – 7, 19 – 21]...... 12
Table 2.5 Properties of LaB6 and CeB6 [1, 5 – 7, 9, 16, 45, 52, 53] ...... 17
Table 2.6 Differences between hexaboride and tungsten cathodes [5, 57] .. 19
Table 3.1 Properties of starting powders ...... 38
Table 3.2 Experimental parameters used in ball milling before volume
combustion synthesis experiments...... 46
Table 3.3 Experimental parameters used in ball milling after volume
combustion synthesis experiments...... 47
Table 3.4 Experimental parameters used in ball milling for
mechanochemical process...... 48
xiv
LIST OF FIGURES
FIGURES
Figure 2.1 Crystal structure of ZrB2 [19] ...... 10
Figure 2.2 Hexagonal close packing of ZrB2 [6]...... 11
Figure 2.3 ZrB2 rocket nozzle [7]...... 14
Figure 2.4 Crystal structure of MB6 [6]...... 15
Figure 2.5 Thermionic electron emitter made of CeB6 [57]...... 19
Figure 2.6 CS modes (a) SHS (b) VCS [60]...... 21
Figure 3.1 Nickel crucible used in the calcination of boric acid...... 36
Figure 3.2 Schematic diagram of the pot furnace...... 39
Figure 3.3 Experimental set – up used in the volume combustion
synthesis experiments...... 41
Figure 3.4 Retsch PM 100 high – energy planetary ball mill [84]...... 42
Figure 3.5 (a) 250 ml stainless steel jar, (b) The aeration cover, (c) The
safety closure device [85]...... 44
Figure 3.6 The clamped and balanced jar in the ball mill [84]...... 45
Figure 3.7 Graphic display of the ball mill [84] ...... 45
Figure 3.8 Experimental set – up used in leaching process ...... 50
Figure 3.9 Experimental set – up for filtration...... 52
Figure 3.10 Rigaku Multiflex Powder X-Ray diffractometer...... 53
Figure 3.11 JSM – 6400 scanning electron microscope...... 54
xv Figure 4.1 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture
which was ignited in the furnace preheated to 900 0C in air
atmosphere ...... 56
Figure 4.2 XRD pattern of the sample ignited in the furnace preheated to
900 0C for ZrB2 production ...... 57
Figure 4.3 XRD patterns for ZrB2 production (a) Sample ignited in the
furnace preheated to 900 0C, (b) Sample in (a) leached in 1 M
HCl/water solution for 15 hours...... 58
Figure 4.4 SEM micrographs for ZrB2 production (a) Sample ignited in
the furnace preheated to 900 0C, (b) Sample in (a) leached in 1
M HCl/water solution for 15 hours...... 59
Figure 4.5 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture
which was ball milled for 15 hours and then ignited in the
furnace preheated to 900 0C under argon atmosphere...... 61
Figure 4.6 XRD patterns for ZrB2 production (a) Sample ball milled for
15 hours, (b) Sample in (a) ignited in the furnace preheated to
900 0C under argon atmosphere ...... 62
Figure 4.7 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture
which was ball milled for 7.5 hours and then ignited in the
furnace preheated to 900 0C under argon atmosphere...... 63
Figure 4.8 XRD patterns for ZrB2 production (a) Sample ball milled for
7.5 hours, (b) Sample in (a) ignited in the furnace preheated to
900 0C under argon atmosphere ...... 64
Figure 4.9 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture
containing 40% excess Mg and B2O3, which was ball milled
for 7.5 hours and then ignited in the furnace preheated to 900
0C under argon atmosphere ...... 65
xvi Figure 4.10 XRD pattern of the sample containing 40% excess Mg and
B2O3, ball milled for 7.5 hours and then ignited in the furnace
under argon atmosphere for ZrB2 production ...... 66
Figure 4.11 XRD patterns for ZrB2 production (a) Sample containing 40%
excess Mg and B2O3, ball milled for 7.5 hours and ignited in
the furnace under argon atmosphere, (b) Sample in (a)
leached in 1 M HCl/water solution for 15 hours...... 67
Figure 4.12 XRD patterns for ZrB2 production (a) Sample containing 40%
excess Mg and B2O3, ball milled for 7.5 hours and ignited in
the furnace under argon atmosphere, (b) Sample in (a) ball
milled for 7 hours in ethanol medium and then leached in 5
M HCl/water solution for 2.5 hours...... 68
Figure 4.13 SEM micrographs for ZrB2 production (a) Sample ball milled
for 7.5 hours, (b) Sample in (a) ignited in the furnace
preheated to 900 0C under argon atmosphere, (c) Sample in
(b) ball milled for 7 hours in ethanol medium and then
leached in 5 M HCl/water solution for 2.5 hours...... 70
Figure 4.14 XRD patterns for ZrB2 production (a) Sample ball milled for
15 hours, (b) Sample ball milled for 20 hours, (c) Sample ball
milled for 25 hours, (d) Sample ball milled for 30 hours...... 72
Figure 4.15 XRD pattern of the sample ball milled continuously for 30
hours for ZrB2 production ...... 73
Figure 4.16 XRD patterns for ZrB2 production (a) Sample containing 10%
excess Mg and B2O3, ball milled for 30 hours, (b) Sample in (a)
leached in 1 M HCl/water solution for 30 minutes...... 75
xvii Figure 4.17 XRD patterns for ZrB2 production (a) Sample containing 30%
excess Mg and B2O3 ball milled for 40 hours, (b) Sample in (a)
leached in 1 M HCl/water solution for 30 minutes...... 76
Figure 4.18 XRD patterns for ZrB2 production (a) Leached product of the
sample containing 30% excess Mg and B2O3 ball milled for 40
hours, (b) Leached product of the sample containing 10%
excess Mg and B2O3 ball milled for 30 hours ...... 77
Figure 4.19 SEM micrographs for ZrB2 production (a) Sample containing
30% excess Mg and B2O3 ball milled for 40 hours, (b) Sample
in (a) leached in 1 M HCl/water solution for 30 minutes ...... 78
Figure 4.20 Ignition temperature of La2O3 – B2O3 – Mg reactant mixture
which was ignited in the furnace preheated to 1000 0C under
argon atmosphere...... 80
Figure 4.21 XRD pattern of the sample ignited in the furnace preheated to
1000 0C under argon atmosphere for LaB6 production ...... 81
Figure 4.22 XRD patterns for LaB6 production (a) Sample ignited in the
furnace preheated to 1000 0C, (b) Sample in (a) leached in 1 M
HCl/water solution for 15 hours...... 82
Figure 4.23 XRD patterns for LaB6 production (a) Sample ignited in the
furnace preheated to 1000 0C, (b) Sample in (a) ball milled for
7 hours in ethanol medium and then leached in 1 M
HCl/water solution for 15 hours...... 83
Figure 4.24 SEM micrographs for LaB6 production (a) Sample ignited in
the furnace preheated to 1000 0C, (b) Sample in (a) ball milled
for 7 hours in ethanol medium and then leached in 1 M
HCl/water solution for 15 hours...... 84
xviii Figure 4.25 XRD pattern of the sample ball milled for 30 hours for LaB6
production ...... 86
Figure 4.26 XRD patterns for LaB6 production (a) Sample ball milled for
30 hours, (b) Sample in (a) leached in 1 M HCl/water solution
for 30 minutes...... 87
Figure 4.27 SEM micrographs for LaB6 production (a) Sample ball milled
for 30 hours, (b) Sample in (a) leached in 1 M HCl/water
solution for 30 minutes ...... 88
Figure 4.28 Ignition temperature of CeO2 – B2O3 – Mg reactant mixture
which was ignited in the furnace preheated to 1000 0C under
argon atmosphere...... 90
Figure 4.29 XRD patterns for CeB6 production (a) Sample ignited in the
furnace preheated to 1000 0C, (b) Sample in (a) leached in 1 M
HCl/water solution for 15 hours...... 91
Figure 4.30 XRD patterns for CeB6 production (a) Sample ignited in the
furnace preheated to 1000 0C, (b) Sample in (a) ball milled for
7 hours in ethanol medium and then leached in 1 M
HCl/water solution for 15 hours...... 92
Figure 4.31 SEM micrographs for CeB6 production (a) Sample ignited in
the furnace preheated to 1000 0C, (b) Sample in (a) ball milled
for 7 hours in ethanol medium and then leached in 1 M
HCl/water solution for 15 hours...... 92
Figure 4.32 XRD patterns for CeB6 production (a) Sample ball milled for
30 hours, (b) Sample in (a) leached in 1 M HCl/water solution
for 30 minutes...... 94
xix Figure 4.33 SEM micrographs for CeB6 production (a) Sample ball milled
for 30 hours, (b) Sample in (a) leached in 1 M HCl/water
solution for 30 minutes ...... 95
xx
CHAPTER 1
INTRODUCTION
Boron minerals and compounds are used in various applications such as, glass industry, detergents, enamel frits, glazes, pulp bleaching, flame retardants, metallurgy, magnets, agriculture, nuclear industry, photography, textile finishing, armor protection and advanced composites
[1, 2]. These applications increase the importance of boron (B). Boron is a rare element and its concentration in earth’s crust is 3 ppm [3]. Elemental boron is hard, brittle and semi – metallic. Its pure form is an amorphous black powder [1]. In nature, boron does not appear in elemental form. It is always combined with oxygen to form borates. Among 230 borate minerals, tincal (Na2B4O7.10H2O) and colemanite (Ca2B6O11.5H2O) are the commercial ones [1, 4]. World boron reserves are determined according to the amount of boron oxide (B2O3) in the borate minerals. The total world reserves of boron is 885 million tons of contained B2O3. Turkey owns 64% (563 million tons of B2O3) of the world boron reserves. It is followed by Russia (11%) and
USA (9%) [4]. These statistics reveal the importance of Turkey in boron industry.
Following the World War II, researchers became aware of the significance of the materials for high temperature applications; this progress was the main
1 stimulus for the use of boron in binary metal borides (ZrB2, TiB2, LaB6, CeB6, etc.) [5, 6]. Their properties such as high melting point, extreme hardness, strength, chemical stability, high electrical conductivity, high wear resistance and high oxidation resistance at very high temperatures made them commercially attractive materials [1, 5, 6]. They are used in a very large number of applications like cutting tools, high temperature electrical contacts, rocket motor components, jet engine components and nozzles for molten metal handling. Commercially, ZrB2 and TiB2 are the most demanded metal borides. ZrB2 is used in spray nozzles, pumps for molten metal transportation and composites [1]. Hexaborides of lanthanum and cerium are used as thermionic cathode material due to their high electrical conductivity and low work function [7 – 9]. The methods used to produce binary metal borides are solid state reaction between the metal and elemental boron at temperatures above 1000 0C, reduction of boric oxide and metal oxide mixture with aluminum, magnesium or carbon, reduction of metal oxide with elemental boron, fused salt electrolysis and heating metal and boron or volatile boron compound together (vapor phase reaction) [1, 7].
In this work, formation of zirconium diboride (ZrB2) and other metal borides such as lanthanum hexaboride (LaB6), cerium hexaboride (CeB6) by magnesiothermic reduction with volume combustion synthesis (VCS) and mechanochemical processing (MCP) were studied. Metal oxide (ZrO2 or
La2O3 or CeO2), boron oxide (B2O3) and magnesium (Mg) powders were used as starting materials. B2O3 was obtained from the calcination of boric acid (H3BO3).
2 In addition to the metal boride and magnesium oxide (MgO), formation of undesired minor phases was observed during volume combustion synthesis experiments. The volume combustion synthesis and mechanochemical processing products were subjected to HCl acid leaching in order to remove
MgO and side products.
The produced powders before and after leaching were analyzed by X – Ray
Diffraction (XRD) and Scanning Electron Microscopy (SEM). XRD was used to identify the formed phases. SEM gave information about the topography of the produced powders.
Following this brief introduction, Chapter 2 will present detailed information about properties, production methods and applications of metal borides. Especially, it will focus on ZrB2, LaB6 and CeB6. Previous studies on the formation of these borides will also be given in this chapter.
In Chapter 3, experimental procedure for both volume combustion synthesis and mechanochemical processing will be explained. The starting materials, calcination of boric acid, preparation of reactant mixtures and analysis methods will be explained under this chapter. The experimental results will be given and discussed in Chapter 4. The conclusions derived from this experimental work will be presented in Chapter 5.
3
CHAPTER 2
LITERATURE SURVEY
This chapter is intended to give detailed information about binary metal borides, their applications and production routes. It will focus especially on
ZrB2, LaB6 and CeB6. Previous studies about these borides will also be presented in this part. Theoretical background of the present study will also be described in this chapter.
2.1 Binary Metal Borides
High melting point and very high hardness values are the most remarkable properties of the binary metal borides [10]. Melting points of some metals and their borides are given in Table 2.1. It is seen from this table that the melting points of metal borides are significantly higher than the melting points of their metals [5]. Microhardness values of some metal borides, some carbides and diamond are presented in Table 2.2. Hardness of diamond and carbides are given in order to point out the severity of the hardness values obtained by these metal borides. Other important properties of binary metal borides are high thermal shock resistance, resistance to corrosion, high electrical conductivity, high wear resistance and chemical stability at high temperatures [1, 5 – 7, 10]. Metal borides are
4 brittle. As temperature increases, they become more ductile but at the same time their hardness decreases [6].
Table 2.1 Melting points of some metals and their borides [1, 5 – 7, 11 – 15].
Metal Metal Tm (0C) Metal Boride Metal Boride Tm (0C)
Ti 1700 TiB2 2920
Zr 1850 ZrB2 3050
Hf 2225 HfB2 3240
Ca 842 CaB6 2230
La 920 LaB6 2150
Ce 795 CeB6 2190
Table 2.2 Microhardness values of some metal borides, some carbides and diamond [5 – 7, 16, 17].
Metal Boride Microhardness (kg/mm2)
TiB2 3250 – 3400
ZrB2 2200 – 2250
HfB2 2900
CaB6 2750
LaB6 2520 – 2770
WC 1800 – 2400
SiC 3350
Diamond 6000 – 10000
5 Metal borides have a metallic appearance. They are generally grey in color with a few exceptions; e.g. LaB6 has purple and CeB6 has blue color [6].
2.1.1 Classification of Metal Borides
Metal borides are classified according to the groups of metal in the periodic table [6]. Periodic classification of some binary borides is shown in Table
2.3. The most stable and the most refractory borides are the borides of metals in the area bordered by Ti, Nb, Mo, W, actinide metals and lanthanides [18].
Table 2.3 Periodic classification of some binary borides [6].
I II III IV V VI VII VIII
2nd Be2B LiB4 Period BeB2
3rd NaB6 MgB2 Period
4th CaB4 ScB2 Ti3B4 CrB Mn3B4 Fe2B KB6 VB2 Period CaB6 ScB4 TiB2 CrB2 MnB2 FeB
5th YB2 ZrB2 Nb3B4 MoB SrB6 TcB2 RuB2 Period YB4 ZrB12 NbB2 Mo2B5
6th WB ReB BaB6 HfB2 TaB2 Os2B5 Period WB2 ReB2
LaB4 Lanthanides CeB6 PrB6 NdB6 SmB6 LaB6
ThB4 UB2 Actinides PuB6 ThB6 UB4
6 The problem of conducting chemical analysis on the binary metal borides has not been solved till now. Metal borides still do not have any standard samples [7].
2.1.2 Boride Structures
Physical, chemical and thermal properties of materials depend strongly on their crystal structures. The ratio of boron to metal (B:M) is an important parameter in determining the properties and electronic structure of metal borides. A change in B:M ratio influences the electronic structure of the boron and this causes formation of one, two or three dimensional networks of boron [19]. Boride structures can be divided into four groups [5];
1) Borides consisting of isolated boron atoms: M4B (B:M = 1:4), M2B (B:M =
1:2), M3B2 (B:M = 2:3), M5B3 (B:M = 3:5)
2) Borides consisting of boron chains: MB (B:M = 1:1), M3B4 (B:M = 4:3)
3) Borides consisting of two dimensional networks of boron atoms: MB2
(B:M = 2:1), M2B5 (B:M = 5:2)
4) Borides consisting of three dimensional networks of boron atoms: MB4
(B:M = 4:1), MB6 (B:M = 6:1), MB12 (B:M = 12:1), MB70 (B:M = 70:1)
7 2.1.3 Production Methods
The methods used to prepare metal borides are;
a) Solid state reaction of metal and boron (Reaction 2.1) or hydride of the
metal and boron at temperatures above 1000 0C [1, 6, 7].
M + xB = MBx (2.1)
b) Reduction of metal oxide by boron [1, 6, 7].
3MO2 + 10B = 3MB2 + 2B2O3 (2.2)
c) Reduction of boron halide and metal halide with hydrogen at
temperatures between 1000 and 1300 0C (direct vapor deposition) [1, 6,
7].
MCl4 + 2BBr3 + 5H2 = MB2 + 4HCl + 6HBr (2.3)
d) Reaction of boron halide, metal or metal oxide and hydrogen [6, 7].
e) Fused salt electrolysis of mixtures of metal oxide and boron oxide in an
electrolyte. Electrolyte composition determines the operating
temperature of the cell which is generally between 950 and 1100 0C [1, 6,
7].
8 f) Reduction of metal oxide and boron oxide mixtures by carbon
(carbothermic reduction, Reaction 2.4), by aluminum (aluminothermic
reduction, Reaction 2.5) or by magnesium (magnesiothermic reduction,
Reaction 2.6) at high temperatures [1, 6, 7].
MO2 + B2O3 + 5C = MB2 + 5CO (2.4)
3MO2 + 3B2O3 + 10Al = 3MB2 + 5Al2O3 (2.5)
MO2 + B2O3 + 5Mg = MB2 + 5MgO (2.6)
g) Reduction of metal oxide by boron carbide (B4C). Free carbon is
sometimes used in the reactions [6, 7].
2MO2 + B4C + 3C = 2MB2 + 4CO (2.7)
Metal borides can be obtained commercially both in the form of raw materials and finished shapes. Raw materials are in the form of powder or granules. Hot pressing or powder metallurgy techniques using binders are the methods used to fabricate the finished shapes from boride powders [7].
2.2 Zirconium Diboride
Zirconium diboride (ZrB2) belongs to the Group IV of transition metal borides [20]. Besides ZrB2, zirconium also forms a dodecaboride (ZrB12) which is stable above 1600 0C [5, 6]. Commercially, TiB2 and ZrB2 are the
9 most widely used metal borides [1]. Crystal structure, properties and some applications of ZrB2 are presented below.
2.2.1 Crystal Structure of Zirconium Diboride
ZrB2 (B:M = 2:1) is made up of two dimensional networks of boron atoms.
Its crystal structure is hexagonal and one unit cell contains only one ZrB2 formula. The structure is AlB2 type and consists of B – B, Zr – Zr and Zr – B bonds. The lattice parameters are a = b = 3. 170 Å and c = 3.533 Å. The ratio of c to a (c/a) is 1.114 [5, 19, 21]. The crystal structure of ZrB2 is shown in
Figure 2.1 where zirconium is designated as metal.
Figure 2.1 Crystal structure of ZrB2 [19].
10 The structure is composed of two dimensional hexagonal networks of boron atoms packed between layers of metal atoms which are also hexagonally close – packed. Each zirconium atom has 6 equidistant zirconium neighbors in its layer and 12 equidistant boron neighbors (6 of them are above zirconium layer and the other 6 are below zirconium layer). In the same manner, each boron atom has 3 equidistant boron neighbors in its plane and
6 equidistant zirconium neighbors (3 above the boron layer and 3 below the boron layer) [5, 6, 19]. Hexagonal close packing of ZrB2 is shown in Figure
2.2.
Figure 2.2 Hexagonal close packing of ZrB2 [6].
2.2.2 Properties of Zirconium Diboride
ZrB2 is one of the most stable binary metal borides [6]. It has very high melting point, hence; it is also called as ultra high temperature ceramic
(UHTC) [19, 22, 23]. ZrB2 is an important boride not only for its high melting points but also for its excellent and attractive properties like high strength, extreme hardness, high thermal conductivity, high electrical
11 conductivity, chemical stability at high temperatures, high corrosion resistance, wear resistance and superb thermal shock resistance [19, 20, 22 –
30]. The only disadvantageous property of ZrB2 is its brittleness [6]. The numerical values of some of these properties are listed in Table 2.4.
Table 2.4 Properties of ZrB2 [5 – 7, 19 – 21].
Crystal Structure Hexagonal – AlB2 structure
Melting Temperature (Tm) 3050 0C
Density 6.1 g/cm3
Hardness 2200 – 2250 kg/mm2
Thermal Expansion Coefficient 5.9 x 10-6 K-1
Heat Capacity at 25 0C 48.2 J.(mol.K)-1
Thermal Conductivity 60 W.(m.K)-1
Electrical Resistivity 7 – 10 µohm.cm
Enthalpy of Formation at 25 0C - 328 kJ/mole
The strong bonding between Zr – Zr, Zr – B, and B – B control the material properties. Especially, Zr – B and B – B bonds are responsible for very high melting point and extreme hardness of ZrB2 [19].
The ratio of the melting point of metal boride to the melting point of its pure metal (Tm (metal boride)/ Tm (metal)) is a parameter in determining the stability of borides. As the ratio increases, the boride becomes more stable.
12 ZrB2 has the second highest ratio after TiB2 and it has the highest melting point in Group IV metal borides [5, 6].
Electrical resistivity value shows its high conductivity. ZrB2 (7 – 10 µohm.cm) is a better electrical conductor than its parent metal (Zr (40 – 42 µohm.cm)) [7,
31]. This value is recorded on powder samples. The single crystals of ZrB2 have probably smaller resistivities. In addition to electrical resistivity, its thermal expansion coefficient is much smaller than its pure metal form [5,
6].
Strong covalent bonding, low self diffusion coefficient, high melting point and high vapor pressure make densification of ZrB2 difficult [19, 23, 25, 27,
32]. Several methods are employed in order to densify ZrB2. The most common ones are hot pressing (HP), pressureless sintering (PS) and spark plasma sintering (SPS) [19]. Complete densification of pure ZrB2 is attained by pressure assisted sintering at temperatures above 1900 0C. Sintering temperatures can be lowered and the final density can be improved by the use of several sintering additives like Fe, Ni Co, W and C, which are liquid phase formers. When metals like Ni and Fe are used as sintering aids, the formation of secondary phases is observed. These residual phases affect the properties of ZrB2 in a negative manner [24, 25, 33]. Ceramic additives are generally used to produce dense ZrB2 at lower temperatures. Monteverde et al. [24, 33] used sintering additives such as Si3N4 in pure ZrB2 or Si3N4 –
Al2O3 – Y2O3 in ZrB2 based composites and produced very dense ZrB2 at reduced temperatures (1700 0C). Not only sinterability but also strength of
ZrB2 was improved. In another study, Monteverde et al. [34] reduced the
13 sintering temperature to 1600 0C by TiB2 and Ni additions. The sintering temperature was reduced to 1850 0C when only Ni was added.
2.2.3 Applications of Zirconium Diboride
The properties of ZrB2 make it an important candidate for several applications. Some applications of ZrB2 and ZrB2 based ceramics are as follows; refractory linings [19, 20, 24, 26], nozzles [19, 20, 24], armor [20, 24], electronic devices [20, 24], cutting tools [19, 26], engine components for hypersonic flight [19, 24, 26, 32, 35, 36], thermal protection for atmospheric re – entry vehicles [19, 24, 32, 37], molten metal crucibles [32, 35, 37], high temperature electrodes [19, 26, 32, 35, 37, 38] and protective coatings for steel [39]. A rocket nozzle made of ZrB2 is shown in Figure 2.3.
Figure 2.3 ZrB2 rocket nozzle [7].
14 2.3 Lanthanum Hexaboride and Cerium Hexaboride
Lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) are the borides of rare earth metals [5]. They are classified in the group of lanthanides in metal borides. In addition to the hexaboride, both La and Ce can form tetraborides (LaB4 or CeB4) [6]. Crystal structure, properties and some applications of LaB6 and CeB6 are described below.
2.3.1 Crystal Structure of Lanthanum Hexaboride and Cerium Hexaboride
LaB6 and CeB6 (B:M = 6:1) are made up of three dimensional networks of boron. Their crystal structure is cubic and one unit cell contains one formula weight of LaB6 or CeB6 [5, 6]. Their prototype structure is CaB6 [40]. The MB6 type of structure is shown in Figure 2.4.
Figure 2.4 Crystal structure of MB6 [6].
15 As shown in Figure 2.4, boron atoms in the MB6 structure are linked together with B – B bonds forming a three dimensional B6 octahedron, the relationship between metal – octahedron can also be considered as in CsCl type structures where Cs site is occupied by rare earth ion and Cl site is occupied by B6 octahedron. Each octahedron is surrounded by eight metal atoms, and vice versa. A rigid but open framework is formed by the linking of all octahedron with neighbors in all six directions [5, 6, 41 – 43]. The lattice parameter of LaB6 is a = 4.153 Å [7, 42] and that of CeB6 is a = 4.137 Å
[7, 42].
2.3.2 Properties of Lanthanum Hexaboride and Cerium Hexaboride
Among the rare earth borides, hexaborides are considered as the most stable borides, especially LaB6 and CeB6 [42]. Hexaborides of La and Ce combine several advantageous properties such as low electrical resistivity, high melting point, high hardness, low work function, high mechanical strength, low volatility at high temperatures and high chemical stability. These properties are the result of their specific atomic arrangement [9, 44 – 49].
The main difference between transition metal borides and rare earth metal borides is rare earth metals tend to form boron rich compounds, however; transition metals have a tendency to form metal rich compounds. Hence, rare earth metal borides in the situation of hexaborides consist of three dimensional B6 octahedrons in which boron atoms are bonded to each other covalently. Besides, octahedrons are linked to each other by covalent bonds forming a strong framework. This is the reason of stability, hardness and
16 high melting point of rare earth hexaborides [8, 41, 45, 50, 51]. Some properties of LaB6 and CeB6 are given in Table 2.5.
Table 2.5 Properties of LaB6 and CeB6 [1, 5 – 7, 9, 16, 45, 52, 53].
LaB6 CeB6
Cubic - CaB6 Cubic - CaB6 Crystal Structure structure structure
Melting Temperature (Tm) 2150 0C 2190 0C
Density 4.72 g/cm3 4.79 g/cm3
Hardness 2520 – 2770 kg/mm2 No data found
Thermal Expansion Coefficient 6.4 x 10-6 K-1 7.3 x 10-6 K-1
Electrical Resistivity 15 µohm.cm 29 µohm.cm
Work Function 2. 74 eV 2.5 eV
Enthalpy of Formation at 25 0C - 287 kJ/mole - 339 kJ/mole
As can be seen from Table 2.5, hexaborides of La and Ce have very low work functions, which can be defined as the minimum energy required to remove an electron from a material. In the case of thermionic work function, the required energy is supplied by heat energy. Because of their low work functions, these materials are classified as electron offering materials. The combination of low work function, low volatility and high melting point makes them suitable for many applications which will be described in the following section [8 – 10, 44].
17 When subjected to thermal shock, CeB6 and LaB6 become very brittle [44,
46]. Their thermal shock resistance is increased by using them with ZrB2 in composites [54, 55].
2.3.3 Applications of Lanthanum Hexaboride and Cerium Hexaboride
As mentioned before, LaB6 and CeB6 can be named as electron offering materials. Mainly due to their low work function, low evaporation rate and high electrical conductivity, they are used as cathode materials [9]. Some applications of LaB6 and CeB6 are thermionic electron emitter [8 – 10, 44 –
46, 48, 49, 51, 56, 57], sensors for high resolution detectors, electrical coatings for resistors, high energy optical systems [48] and structural materials when used in composites [58].
In electron microscope applications such as scanning electron microscope and transmission electron microscope, thermionic electron emitters made of
LaB6 and CeB6 provide better brightness, longer service life and lower operation temperature when compared to conventional tungsten filaments because of their low work function [9, 51, 56, 57].
When hexaborides are compared with each other, CeB6 has longer service life due to its lower work function and lower evaporation rate at higher temperatures [9, 57]. The differences between hexaboride and tungsten cathodes at typical operating temperatures (near 1500 0C) are shown in
Table 2.6. In Figure 2.5, CeB6 thermionic electron emitter is presented.
18 Table 2.6 Differences between hexaboride and tungsten cathodes [5, 57].
Tungsten LaB6 CeB6 Filament
Work Function 4.5 eV 2.74 eV 2.5 eV
Evaporation Rate NA 2.2 x 10-9 g/cm2.sec 1.6 x 10-9 g/cm2.sec
Operating Vacuum 10-5 torr 10-7 torr 10-7 torr
Service Life 30 – 100 hr 1,000 + hr 1,500 + hr
Brightness 106 A/cm2.sr 107 A/cm2.sr 107 A/cm2.sr
Figure 2.5 Thermionic electron emitter made of CeB6 [57].
2.4 Production Methods
In this study, ZrB2, LaB6 and CeB6 were produced by magnesiothermic reduction using volume combustion synthesis (VCS) and mechanochemical process (MCP). VCS and MCP are discussed below.
19 2.4.1 Volume Combustion Synthesis
Before VCS, it is necessary to give general information about combustion synthesis (CS). CS is a method used to produce advanced functional and structural ceramics, composites, alloys and nanomaterials [59]. Fast heating rates, high temperatures and short reaction times are the unique and important properties of CS process. It has many advantages over conventional methods used to produce advanced materials such as use of exothermicity of the chemical reaction, use of very simple equipment, production of high purity materials due to evaporation of impurities, formation of unique microstructures due to rapid cooling after reaction and stabilization of metastable phases [59, 60].
CS can occur in two modes shown in Figure 2.6, first one is self – propagating high – temperature synthesis (SHS) and the second one is volume combustion synthesis (VCS). In SHS, the sample is heated locally by an external heat source which can be a tungsten coil or laser. After the local initiation, self – sustaining propagation of the reaction occurs through the mixture of reactants and the desired products are obtained. In VCS, different than SHS, the entire sample is heated uniformly in a controlled manner until the reaction occurs simultaneously throughout the volume.
VCS products are uniform in microstructure and phase composition due to simultaneous occurrence of the reaction [60]. During VCS, a sudden increase in temperature indicates that the reaction is ignited, therefore; it is also called thermal explosion (TE) [61]. Ignition temperature of reactions can be lowered by mechanical activation [62].
20
(a) (b)
Figure 2.6 CS modes (a) SHS (b) VCS [60].
2.4.2 Mechanochemical Process
Mechanochemical process (MCP) is a powder process in which chemical reactions and phase transformations take place due to application of mechanical energy. It is simply the conversion of mechanical energy into chemical energy. In the literature, MCP is also called as mechanosynthesis or mechanochemical synthesis. High energy ball mills such as SPEX mill and planetary ball mill are used to perform MCP [63].
Reactions with a large negative free energy change are feasible at room temperature, however; at ambient temperatures the occurrence of them is limited by kinetic reasons. Due to repeated cold welding and fracturing of powder particles during MCP, the size of the particles is reduced. As a result of reduction in particle size, the area of contact between reactant powder particles increases by repeatedly bringing fresh surfaces into contact which eliminates the necessity of diffusion. Hence, the reactions,
21 which require high temperatures, occur at lower temperatures such as room temperature [63].
Reaction kinetics in MCP can be of two types [63];
1) Gradual transformation of reaction during each collision, or
2) A combustion reaction can happen if the enthalpy of reaction is high.
In the second type, after a critical milling time, called as ignition time, a combustion reaction can be initiated. At ignition time a sudden increase in temperature occurs and then it slowly decreases to ambient temperature
[63].
Important experimental parameters that affect the MCP are milling temperature, ball to powder weight ratio, use of a process control agent, relative proportion of the reactants and the grinding ball diameter. They have several effects on the process; ignition time decreases as ball to powder weight ratio or grinding ball diameter increases. Process control agent generally slows down the combustion, however; it is used to prevent particle agglomeration. In MCP, generally excess amount of reactants are used in order to compensate the surface oxidation losses [63].
2.4.3 Thermite Reactions
Non – oxide ceramics (borides, carbides, silicides and nitrides) can be synthesized by a self sustaining regime if a strong reducing metal is used.
This type of combustion reaction is called thermite reaction. Thermite
22 reactions are preferred when direct elemental solid state reaction can not generate sufficient amount of heat to be self sustaining or when the elemental powders cost too much [64].
Generally, magnesium (magnesiothermic reduction) or aluminum
(aluminothermic reduction) is chosen as reducing agent in thermite reactions. The reaction yields a metal oxide as a by – product. It is then removed from the product by leaching in order to obtain pure non – oxide ceramics. When the removal of the formed metal oxide is taken into account, magnesium seems to be a better choice as a reducing agent since magnesium oxide (MgO) is easily leachable with hydrochloric (HCl) acid.
On the other hand, the by – product of aluminothermic reduction (Al2O3) can not be removed from the reduction product easily due to its stability.
Hence, reduction product is directly used as alumina based composites [64].
As thermite reactions are highly exothermic, the reaction becomes self sustaining after the ignition of the reactant mixture [59]. In the present study magnesium was used as the reducing agent. The general representation of magnesiothermic reduction reaction (Reaction 2.8) [64] and the specific magnesiothermic reduction reactions (Reaction 2.9, 2.10,
2.11) related to this study and their enthalpies per mole are given below.
MaOb + BcOd + (b + d) Mg = MaBc + (b + d) MgO (2.8)
ZrO2 + B2O3 + 5Mg = ZrB2 + 5MgO ∆H = - 989 kJ (2.9)
La2O3 + 6B2O3 + 21Mg = 2LaB6 + 21MgO ∆H = - 3895 kJ (2.10)
23 CeO2 + 3B2O3 + 11Mg = CeB6 + 11MgO ∆H = - 2104 kJ (2.11)
The enthalpies of the reactions are calculated from the data complied by
Barin [52], Mueller et al. [53] and Turkdogan [65].
2.5 Previous Works on the Production of Zirconium Diboride, Lanthanum
Hexaboride and Cerium Hexaboride
Previous studies on the production of ZrB2, LaB6 and CeB6 will be covered in this section. Magnesiothermic production methods will be described in detail, other conventional methods will be explained briefly.
2.5.1 Previous Works on the Production of Zirconium Diboride
Production of ZrB2 was studied by many scientists. Several methods were developed for the formation of high purity ZrB2 ceramics. One of them is the formation of ZrB2 from elemental powders. Synthesis of borides directly from their elements allows controlling the composition and purity of the resulting boride [7]. Direct synthesis from elements requires very high activation temperatures. Makino et al. [66] determined the activation temperature of Reaction 2.12 as 1467 0C.
Zr + 2B = ZrB2 (2.12)
Cooper [67] developed a method for the preparation of metal borides like titanium and zirconium. In this method, briquetted mixtures of metal
24 hydride and high purity boron were heated slowly in hydrogen atmosphere. The reaction occurred at 1400 0C.
Fused salt electrolysis is another process for the formation ZrB2. In the fused salt electrolysis, the process starts with the liberation of free alkali or alkaline earth metal. Liberated metal reduces the boric oxide in the bath to free boron and part of the metal reacts with the free boron to form metal boride. Baths composed of CaO – CaF2 – B2O3 – ZrO2 are used in the electrolytic production of ZrB2 [7].
ZrB2 coatings can be produced by vapor phase deposition. Campbell et al.
[68] studied the deposition of refractory metal borides through the reaction of metal halide and boron halide in the presence of hydrogen. They found that the coatings of ZrB2 as well as TiB2 were deposited on substrates upon heating the reactant mixture to temperatures between 1000 and 1300 0C.
Gebhardt et al. [69] deposited hard and brittle ZrB2 coatings through
Reaction 2.13 at 1400 0C, however; considerable amount of excess boron was detected in the coatings. Low vapor pressure of zirconium tetrachloride was shown as the reason of deposition of excess boron. Wang et al [70] claimed that ZrB2 was the only phase that could form in Zr – B – Cl – H system using
Reaction 2.13. It was found that deposition rate and surface morphology depends on the reduction of boron trichloride by hydrogen.
ZrCl4 + 2BCl3 + 5H2 = ZrB2 + 10HCl (2.13)
Nanocrystalline ZrB2 powders were produced by Chen et al. [71] through the reaction of ZrCl4 and NaBH4 (Reaction 2.14) in a temperature range of
25 500 – 700 0C. According to their study, crystallinity and crystallite size increased as temperature was increased. Crystallinity was found to be very poor when the reaction time was less than 6 hours. Besides, it was found that below 450 0C, reaction did not occur.
ZrCl4 + 2NaBH4 = ZrB2 + 2HCl + 2NaCl + 3H2 (2.14)
Reduction processes are the most important methods for synthesizing ZrB2 ceramics. Among several reducing agents, carbon, aluminum, boron carbide and magnesium are the most common ones [19].
In the case of carbothermic reduction, there can be two chemical routes for the manufacturing of ZrB2. First method uses mixtures of ZrO2, B2O3 and carbon (Reaction 2.15) and the second uses mixtures of ZrO2, B4C and carbon (Reaction 2.16) [19, 64]. The advantage of carbothermic reduction over other reduction processes is that no oxide residues are left in the product; sometimes it can be very difficult to remove these oxides [5].
ZrO2 + B2O3 + 5C = ZrB2 + 5CO (2.15)
2ZrO2 + B4C + 3C = 2ZrB2 + 4CO (2.16)
Both methods are very similar to each other, however; when B4C is used as a boron source, less by – product CO is produced and the reaction occurs at lower temperatures. Reactions 2.15 and 2.16 are very endothermic, hence; they are favorable at very high temperatures. Reaction 2.15 is favorable above 1500 0C and 2.16 is favorable above 1400 0C. Generally temperatures
26 near 2000 0C are employed to synthesize ZrB2 by carbothermic reduction
[19, 64].
Zhao et al. [72] synthesized ZrB2 powder at temperatures 1400 0C and 1600
0C using Reaction 2.16. They observed that as the synthesis temperature was increased, carbon and oxygen contents of the product decreased but at the same time particle size increased. They also noted an intermediate reaction at lower synthesis temperatures. Due to reaction of ZrO2 with B4C, a volatile intermediate boron compound (B2O3) formed. Boron losses were detected because of volatile B2O3. They advised using excess boron in order to compensate boron losses during synthesizing high purity ZrB2 powder.
Mishra et al. [73] investigated the defect concentration of the powders of zirconium diboride produced by carbothermic reduction (by both reaction routes) and compared with those produced by self propagated high temperature synthesis (magnesiothermic production). In non – SHS technique, ZrB2 powders were synthesized by heating reactant mixtures in a graphite furnace and soaking them for 1 hour at 1800 0C. Then, the samples were cooled down to room temperature by a slow cooling rate (25 0C/min).
Defect concentration was found to be larger in the powders which were produced by SHS techniques due to higher heating and cooling rates.
When aluminum is used as a reducing agent, the reaction is classified as thermite type reaction. As described before in Section 2.4.3, aluminothermic reduction is highly exothermic, therefore; once the reaction is ignited, it becomes self sustaining [59, 64]. Aluminothermic production of ZrB2 is shown in Reaction 2.17 [74, 75]. Since the oxide product, Al2O3, is very
27 stable, it can not be removed from ZrB2 very easily. Therefore, reaction product is directly used as ZrB2 – Al2O3 composite [64].
3ZrO2 + 3B2O3 + 10Al = 3ZrB2 + 5Al2O3 (2.17)
Mishra et al. [75] synthesized ZrB2 – Al2O3 composites by SHS method using
Reaction 2.17. Ignition temperature of the reaction was calculated as 1230 0C and combustion temperature was found to be around 1900 to 2000 0C. They claimed that melting of aluminum might have been responsible for the SHS reaction. Mishra et al. [76] in another study detected the formation of solid solutions of Al in ZrB2, (Al, Zr) B2, before full conversion into ZrB2 – Al2O3 composite.
Reduction by magnesium is another type of thermite reaction. The oxide product of magnesiothermic reaction is MgO. This makes it favorable over aluminothermic reduction processes when pure boride powders are needed because removal of MgO by acid leaching is very easy [64]. In this study,
ZrB2 was produced by magnesiothermic reduction using Reaction 2.9.
Reaction 2.9 is not a single step reduction reaction. Firstly, B2O3 and ZrO2 are reduced to elemental boron and zirconium by magnesium. The resulting boron and zirconium react with each other and form ZrB2.
Magnesiothermic production of metal borides using Reaction 2.8 was studied by various scientists. Sundaram et al. [77] investigated the reaction chemistry of TiB2 formation through Reaction 2.18 by SHS.
28 TiO2 + B2O3 + 5Mg = TiB2 + 5MgO (2.18)
In their study, in addition to the expected phases, TiB2 and MgO, formation of Mg2TiO4 and Mg3B2O6 as minor phases was observed when the experiment was conducted both in air and argon atmosphere. They investigated the formation of these minor phases. 3Mg – B2O3 mixtures were ignited both in air and argon atmosphere. Mg3B2O6 formed in air, however; under argon atmosphere it did not although the reaction was favorable.
Based on these results they explained the formation of Mg3B2O6 in air through Reaction 2.19; on the other hand under argon atmosphere Mg3B2O6 formed due to Reaction 2.20 [77].
3Mg + 3/2O2 + B2O3 = Mg3B2O6 (2.19)
3MgO + B2O3 = Mg3B2O6 (2.20)
Since the production of ZrB2 by magnesiothermic production is very similar to production of TiB2, Mg3B2O6 formation can be observed as a minor phase during Reaction 2.9. Weimin et al. [78] also studied the reaction chemistry of
TiO2 – B2O3 – Mg system and observed Mg3B2O6 formation when the experiment was conducted under argon atmosphere. They found that using excessive amount of Mg in reactant mixture decreased the amount Mg3B2O6 phase in final product.
Boric acid (H3BO3) can also be used as a boron source in magnesiothermic reduction reactions (Reaction 2.21).
29 ZrO2 + 2H3BO3 + 5Mg = ZrB2 + 5MgO + 3H2O (2.21)
Khanra et al. [79, 80] in several studies investigated the production of ZrB2 from ZrO2 – H3BO3 – Mg system under argon atmosphere. The results deducted from these works are as follows; (i) ignition temperature of the exothermic reaction was found to be 794 0C, (ii) in the products, unreacted
ZrO2 is observed which indicates an incomplete conversion. Using excess
Mg did not affect the results and complete conversion could not be achieved. This was explained by the very high oxide stability of ZrO2, (iii) synthesized powder by SHS process if subjected to a second SHS, conversion from ZrO2 to ZrB2 was increased but not complete, (iv) formation of Mg3B2O6 was expected, however; it was not detected during
XRD. Absence of Mg3B2O6 could not be explained, (v) following the cooling of the product, MgO was easily leached out by using dilute HCl solution.
Mechanism of Reaction 2.21 was explained by Khanra et al. [79]. During
SHS, H3BO3 was reduced to Mg3B2O6 and MgB4 (Reactions 2.22 and 2.23) but
ZrO2 was partially reduced to elemental Zr. Incomplete reduction was the reason of unreacted ZrO2 in SHS products. At the same time, B2O3, arisen from Reaction 2.22, was reduced to elemental boron (Reaction 2.24).
2H3BO3 = 3H2O + B2O3 (2.22)
4B2O3 + 7Mg = MgB4 + 2Mg3B2O6 (2.23)
B2O3 + 3Mg = 2B + 3MgO (2.24)
30 Following these reactions, formation of ZrB2 was resulted from the reaction of MgB4 and elemental Zr or elemental boron and elemental Zr (Reactions
2.25, 2.26 and 2.27) [79].
MgB4 + Zr = MgB2 + ZrB2 (2.25)
MgB2 + Zr = ZrB2 + Mg (2.26)
Zr + 2B = ZrB2 (2.27)
As explained in Section 2.4.2, reactions which require high temperatures can occur at lower temperatures even at room temperature by the application of mechanical energy if the reaction is highly exothermic [63]. Hence, ZrB2 can be produced at room temperature by MCP using magnesiothermic reduction.
Setoudeh and Welham [81] used MCP for synthesizing ZrB2 powders at room temperature through Reaction 2.9. MCP was conducted in a tumbling mill and ball to powder ratio was taken as 43:1. Grinding jar was vacuumed and filled with argon gas in order to create an argon atmosphere. The tumbling was operated at 165 rpm. At the end of 15 hours of milling, formation of ZrB2 was observed, however; as in the previous studies complete conversion of ZrO2 could not be achieved. Some ZrO2 remained unreacted. They suggested using excess Mg and B2O3 for complete conversion. Different than other methods, formation of an intermediate phase was not observed during MCP. The reaction products were leached in
31 0.1 M HCl solution for 1 hour and MgO was removed. Their final product was composed of ZrB2 and unreacted ZrO2.
2.5.2 Previous Works on the Production of Lanthanum Hexaboride and
Cerium Hexaboride
As in ZrB2, hexaborides of La and Ce can be prepared by several methods.
Direct synthesis from elemental powders can be a possible way to synthesize LaB6 and CeB6, however; studies on this topic dates back to very early times, hence; information about their activation temperatures could not be found.
Hexaborides can also be synthesized by fused salt electrolysis. In electrolytic production, bath compositions in MgO – MgF2 – B2O3 – La2O3 and MgO – MgF2 – B2O3 – CeO2 systems are used to synthesize LaB6 and
CeB6, respectively [7].
As explained before, vapor phase deposition is an alternative method for the production of metal borides. Zhang et al. [9, 49] produced single crystalline LaB6 and CeB6 nanowires by vapor deposition through Reactions
2.28 and 2.29.
2LaCl3 + 12BCl3 + 21H2 = 2LaB6 + 42HCl (2.28)
2CeCl3 + 12BCl3 + 21H2 = 2CeB6 + 42HCl (2.29)
32 A tube furnace was used in order to conduct the experiments. It was operated at 1150 0C for LaB6 deposition and at 1125 0C for CeB6 deposition.
Trichlorides of La and Ce were vaporized in the tube furnace and BCl3 gas was introduced to the system. LaB6 or CeB6 nanowires were deposited on silicon substrates coated with platinum or gold. The experiments were conducted in an atmosphere composed of 5% H2 and 95% N2 [9, 49].
A different method was put forth for the deposition of LaB6 and CeB6 nanowires by Xu et al. [48] and Zou et al. [56]. Instead of trichlorides of La and Ce, they used elemental powders (Reactions 2.30 and 2.31) and they created an atmosphere of 50% H2 and 50% N2. Advantage of this procedure is the use of self catalyst method, however; in previous studies [9, 49] platinum or gold was used as catalysts.
La + 6BCl3 + 9H2 = LaB6 + 18HCl (2.30)
Ce + 6BCl3 + 9H2 = CeB6 + 18HCl (2.31)
Although formation of hexaborides by reduction processes is an attractive topic, there are only a few studies on the production of LaB6 and CeB6 by these methods.
Carbothermic production of LaB6 was studied by Wenyuan et al. [82]. In their work, they investigated the reaction mechanism of La2O3 – H3BO3 – C system (Reaction 2.32).
La2O3 + 12H3BO3 + 21C= 2LaB6 + 21CO + 18H2O (2.32)
33 According to their study, first of all calcination of H3BO3 occurred at temperatures between 82 and 390 0C. Following the calcination, La2O3 and formed B2O3 reacted to form LaBO3 and at the same time, La2O3, formed
B2O3 and C reacted to form LaBO3 and B4C in the temperature range of 836 to 1400 0C. Further reaction of LaBO3 and B4C (Reaction 2.33) at 1450 0C, resulted in the formation of LaB4 and elemental boron. Finally, these two phases reacted to form LaB6 at 1500 0C (Reaction 2.34) [82].
LaBO3 + 3B4C = LaB4 + 9B + 3CO (2.33)
LaB4 + 2B = LaB6 (2.34)
Markovskii et al. [83] studied the formation of several metal borides and represented the magnesiothermic production of hexaborides as follows;
MnOm + 3nB2O3 + (m + 9n) Mg = nMB6 + (m + 9n) MgO (2.35)
Formation mechanism of metal borides was explained as; metallic Mg reduced both metal oxide and boron oxide to free metal and boron then free metal and boron reacted to yield metal boride, however; process parameters were not presented in their study [83].
As explained before, number of studies on the formation of hexaborides by reduction process using carbon, aluminum or magnesium is very limited.
Besides, no studies were found about the production of LaB6 and CeB6 by magnesiothermic reduction using combustion synthesis and mechanochemical process.
34
CHAPTER 3
EXPERIMENTAL PROCEDURE
In this study, metal borides such as zirconium diboride (ZrB2), lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) were produced by following two different methods. These methods are volume combustion synthesis (VCS) and mechanochemical process (MCP). In this chapter, experimental procedure will be discussed in three parts. First, the starting materials will be discussed, followed by production methods and leaching process. Finally, the techniques used to analyze the produced powder will be explained.
3.1 The Starting Materials
In this section calcination of boric acid and preparation of reactant mixtures are explained. In addition to these properties of used powders are given.
3.1.1 Calcination of Boric Acid
Calcination of boric acid (H3BO3) was conducted in order to produce boron oxide. The calcination reaction is given in Reaction 3.1.
35 2H3BO3 = B2O3 + 3H2O (3.1)
It was performed in a nickel crucible which was 50 mm in height and 45 mm in diameter. The capacity of the crucible was 40 grams of H3BO3. The nickel crucible used is shown in Figure 3.1.
Figure 3.1 Nickel crucible used in the calcination of boric acid.
The calcination of H3BO3 was conducted by the following procedure: (i)
Placing the nickel crucible together with 40 grams of H3BO3 into the pot furnace which was heated to 900 0C, (ii) Keeping the crucible at this temperature for 45 minutes in order to remove all H2O, (iii) Taking the crucible out of the furnace and pouring the molten B2O3 onto a stainless steel plate. After the solidification of B2O3, it was separated from the stainless steel plate and ground into small pieces in a ceramic mortar.
36 3.1.2 Preparation of Reactant Mixtures
The reactant mixtures in both volume combustion synthesis and mechanochemical process experiments were prepared according to the following reactions.
ZrO2 + B2O3 + 5Mg = ZrB2 + 5MgO (3.2)
La2O3 + 6B2O3 + 21Mg = 2LaB6 + 21MgO (3.3)
CeO2 + 3B2O3 + 11Mg = CeB6 + 11MgO (3.4)
In volume combustion synthesis experiments, 4 grams of reactant mixture was prepared. In the case of mechanochemical process, a different parameter was taken into account, which was the ball to powder ratio.
Hence, 16 grams of reactant mixture was prepared in mechanochemical process experiments. This amount was 1/15 of the weight of the 8 stainless steel balls. Reactants, amounts of which were determined from the stoichiometry of the reactions, were mixed in a ceramic mortar and then processed by volume combustion synthesis or mechanochemical synthesis.
The properties of reactant powders used in the experiments are given in
Table 3.1. H3BO3 was not used as starting powder; B2O3 obtained from the calcination of H3BO3 was used as the starting powder.
37 Table 3.1 Properties of starting powders.
Powder Purity Particle Size (μm) Company
ZrO2 > 98.5% > 15 µm Merck
La2O3 99.9% Not specified Aldrich
CeO2 99.9% < 5 µm Aldrich
H3BO3 > 99.8% Not specified Merck
Mg > 99% < 300 µm Aldrich
3.2 Processing Methods
In this section, the procedure applied for volume combustion synthesis experiments and determinations of ignition temperature are discussed.
Besides, detailed information about ball milling operation is given.
3.2.1 Volume Combustion Synthesis Experiments and Determination of the Ignition Temperature
Volume combustion synthesis experiments were conducted in a pot furnace under argon atmosphere; some experiments were conducted in air atmosphere. The pot furnace was heated by Kanthal resistance wires. Gemo
PC107 temperature controller system was connected to the furnace in order to keep temperature constant with an accuracy of ± 5 0C. Furnace temperature was measured by using a K – type thermocouple in an alumina protection tube. Insulators were used in order to avoid heat losses. The pot
38 furnace had an inside diameter of 128 mm and a depth of 136 mm. The schematic diagram of the pot furnace used is shown in Figure 3.2.
Thermocouple Hole
Refractory Lid
Insulator
Insulator Insulator
Kanthal Resistance Wire
Soil Bed
Insulator
Figure 3.2 Schematic diagram of the pot furnace.
Reactant mixture prepared according to the stoichiometry of the Reactions
3.2, 3.3 or 3.4 was charged into a graphite crucible. The graphite crucible was 90 mm in height and 50 mm in diameter. After charging, the crucible was closed with a lid made of graphite again. Two holes were opened through the lid of the crucible; the aim of the first hole was to place a K – type thermocouple. This thermocouple, which had an inconel protection tube, was used to indicate temperature of the reactants and to determine the ignition temperature of the reactions. It was connected to a Tetcis PC990 temperature indicator unit. The temperature indicator unit was directly linked with a computer with a RS 232 Interface; temperature and time data
39 were recorded by a computer program called Opik 04. The temperature versus time graphs were obtained by using this recorded data. The second hole was opened in order to blow argon gas through an alumina pipe when the experiments were performed under argon atmosphere. Before loading the crucible into the furnace, the crucible was flushed with argon gas for 15 minutes with a flow rate of 400 ml/min to remove air from it completely.
During experiments, argon flow continued with the same flow rate. Flow rate was adjusted by using a flow meter. Argon gas was supplied from
Habaş A.Ş. with 99.998% purity. The second hole was kept closed when there was not any argon blow.
The closed crucible was placed into the preheated pot furnace and temperature of the reactant mixture started to increase. Due to the highly exothermic nature of Reactions 3.2, 3.3 and 3.4, at the ignition temperature a heat release was observed which resulted in a sudden increase in temperature. This rise in temperature indicated the beginning of the reaction. Then, the crucible was taken out of the furnace and left for cooling in air. In the case of argon atmosphere, argon flow continued during cooling outside the furnace also. The experimental set – up used in the volume combustion synthesis experiments is shown in Figure 3.3 schematically.
40
Overflow Exit Computer Argon Gas Flow Meter Cylinder
Alumina Separator for Pipe Overflow RS 232 Interface 41 Thermocouple Temperature Controller Thermocouple Alumina Protection Tube
Sample Crucible Temperature Indicator
Figure 3.3 Experimental set – up used in the volume combustion synthesis experiments. 3.2.2 Ball Milling Operation
For ball milling operation a Retsch PM 100 planetary ball mill was used. It is a high energy ball mill which can operate between 100 and 650 rpm. It is shown in Figure 3.4.
Figure 3.4 Retsch PM 100 high – energy planetary ball mill [84].
Planetary ball mill owes its name to the planet – like movement of its vials
[63]. The grinding jars are arranged eccentrically on the sun wheel of the planetary ball mill. The direction of movement of the sun wheel is opposite to that of the grinding jars. The grinding balls in the grinding jars are subjected to superimposed rotational movements, which are called Coriolis forces. Due to the different speeds of grinding balls and grinding jar, an interaction is produced between frictional and impact forces. This
42 interaction releases high dynamic energies. The interplay between frictional and impact forces produces the high and very effective degree of size reduction of the planetary ball mill [84].
Before ball milling process, the reactant mixture was put into the 250 ml stainless steel jar of the planetary ball mill together with stainless steel balls.
The size and the number of grinding balls were determined depending on the purpose of milling operation. This will be explained in the following sections. The jar was closed with an aeration cover and a safety closure device was placed onto the cover and its screws were tightened. The 250 ml jar, its aeration cover and safety closure device are presented in Figure 3.5
(a), (b) and (c), respectively.
43
(a) (b)
(c)
Figure 3.5 (a) 250 ml stainless steel jar, (b) The aeration cover, (c) The safety closure device [85].
After the tightening of the screws of the safety closure device, the remaining air in the jar was taken out by using a vacuum pump and the jar was flushed with argon gas for 10 minutes so as to evacuate the air from the jar completely. Following the flushing, the jar was filled with argon gas in order to create an argon atmosphere in the grinding jar. Then, the jar was weighed and placed into the planetary ball mill. The clamps were closed and balance adjustment was made using a counterweight according to the jar’s weight as measured before placing it into the ball mill. The clamped and balanced jar is shown in Figure 3.6.
44 Clamp
Counterweight
Grinding Balance Jar Adjustment Switch Supporting Disk
Figure 3.6 The clamped and balanced jar in the ball mill [84].
As a last step, experimental parameters such as grinding time, speed and service interval were entered via graphic display and the ball mill was started to operate. Graphic display is shown in Figure 3.7.
Figure 3.7 Graphic display of the ball mill [84].
45 3.2.2.1 Ball Milling Operation before Volume Combustion Synthesis
Experiments
The aim of ball milling before volume combustion synthesis experiments was to reduce the particle size and achieve perfect mixing of the reactants.
This method was applied in the production ZrB2. The reason of applying ball milling before ignition in furnace will be discussed in Chapter 4. The reactant mixture was prepared according to the stoichiometry of Reaction
3.2 and ball milled by following the procedure explained in the previous section. The experimental parameters used in ball milling before ignition in furnace are presented in Table 3.2. After the application of ball milling for different times, the reactant mixture was ignited in the furnace and the ignition temperature of the ball milled sample was determined.
Table 3.2 Experimental parameters used in ball milling before volume combustion synthesis experiments.
Numbers Milling Milling Milling Atmosphere and sizes of Speed Time Medium balls (rpm) (hour) Ball 8 balls with Milling Argon 20 mm 300 15 before diameter Ignition Dry Ball Milling 8 balls with Argon 20 mm 300 7.5 diameter
46 3.2.2.2 Ball Milling Operation after Volume Combustion Synthesis
Experiments
In the production of metal borides by volume combustion synthesis, in addition to the formed major phases some side products also formed. After leaching some of these side products could not be removed. Therefore, an alternative method was tried after ignition in furnace before leaching process. This was the milling of the volume combustion synthesis products.
The aim of milling after ignition was to reduce the particle sizes of products, free the entrapped minor phases and increase the specific surface area of all particles. As a result, particle – acid solution contact was raised and the side products were removed more easily. Ball milling was applied according to the procedure explained before, however; a process control agent (PCA) was added in order to avoid agglomeration and increase the efficiency of milling. Therefore, ethanol (C2H5OH) was selected as PCA and the ball milling was conducted in ethanol medium. The amount of ethanol used during ball milling was 50 ml. The experimental parameters used in ball milling after ignition in furnace are presented in Table 3.3.
Table 3.3 Experimental parameters used in ball milling after volume combustion synthesis experiments.
Numbers Milling Milling Ball Milling Atmosphere and sizes of Speed Time Milling Medium balls (rpm) (hour) after Ball 50 balls Ignition Milling in Argon with 10 mm 150 7 Ethanol diameter
47 3.2.2.3 Ball Milling Operation for Mechanochemical Process
Mechanochemical process experiments were performed following the procedure as described in section 3.2.2. In the production of all metal borides 8 stainless steel grinding balls with 20 mm diameter were used. In the production of all metal borides, milling time was taken as 30 hours. 40 hours of milling was performed only in the formation of ZrB2 in addition to
30 hours. Experimental parameters used in the mechanochemical processing experiments are presented in Table 3.4.
Table 3.4 Experimental parameters used in ball milling for mechanochemical process.
Numbers Milling Milling Milling Atmosphere and sizes of Speed Time Medium balls (rpm) (hour) Ball 8 balls with Milling Argon 20 mm 300 30 for MCP diameter Dry Ball Milling 8 balls with Argon 20 mm 300 40 diameter
3.3 Leaching Process
The last step in the production of metal borides was the leaching process. It was performed after the formation of metal borides by volume combustion synthesis or mechanochemical process. The objective of leaching process was removing the major phase, magnesium oxide (MgO), from the reaction
48 products and obtaining pure metal boride, however; besides MgO some side products also occurred in the formation of metal borides by volume combustion synthesis and some iron (Fe) was detected after mechanochemical process experiments due to the wear of stainless steel jar and balls. The formed side products were magnesium borate (Mg3B2O6) and lanthanum borate (LaBO3). LaBO3 was observed only in LaB6 production.
Hence, leaching was applied not only for removing MgO but also for removing side products and iron.
MgO is known to be soluble in hydrochloric (HCl) acid solution [64].
Therefore, HCl was chosen as the leaching agent. HCl acid solution was supplied from Merck and it contained 37% HCl acid by weight in water.
Molecular weight of HCl is 36.46 g/mol. The density of the solution was 1.19 g/ml. Its molarity was calculated as follows;
g37 g37 1mol g ml mol =HCl of Molarity of =HCl × 1.19× 1000× 12.0762= g 100 g 36.46 g ml l l
Leaching process was conducted in 1 M HCl/water solution which was mol1 prepared by mixing of = 0.0828 l = 82.8 ml of commercial mol/l 12.0762 mol/l
37wt% HCl solution and 917.2 ml deionized water. In the production of
ZrB2 by volume combustion synthesis, leaching was also performed in 5 M
HCl/water solution.
Another important parameter in the leaching process was the slurry density. It is the ratio of the weight of the powder to be leached to the
49 volume of the leach solution. The slurry density was taken as 1/100 gram/milliliter in all leaching processes.
Experimental set – up used in leaching is shown in Figure 3.8. An 800 ml glass beaker was placed onto an Ikamag RCT magnetic stirrer together with a magnetic stirring bar. The volume of leach solution was determined according to the slurry density and put into the glass beaker. The appropriate amount of powder that would be leached was poured into the leach solution and the top of the beaker was closed with parafilm. Finally, the stirrer was started to operate and continued for a predetermined time.
All leaching processes were conducted at room temperature.
Parafilm
Glass Beaker Magnetic Stirring Bar
Ikamag RCT Magnetic Stirrer
Figure 3.8 Experimental set – up used in leaching process.
50 At the end of leaching, the beaker was taken from the magnetic stirrer and the leach solution was filtered. The set – up used for filtration is presented in Figure 3.9. The filter papers, which were supplied from Whatman, were
110 mm in diameter. They were dried, weighed and placed into the Büchner funnel after the preparation of filtration set – up. The vacuum pump which was connected to the flask was started to operate and the filter paper was wetted with deionized water. The leach solution was poured onto the paper very slowly and carefully in order to minimize the splashing of the solution onto the walls of the Büchner funnel. The wet solid remaining on the filter paper was the product of leaching. When all the solution was filtered, the pump was stopped and the filter paper was taken from the Büchner funnel.
Then, it was dried in a drying oven for 20 minutes which was preheated to
90 0C. The dried filter paper was weighed and the weight of the product was calculated. Finally, the leach product was removed from the filter paper by scraping. The solution remaining in the flask after filtering was filtered one more time so as to take the solids that passed through the filter paper during first filtering. The second filtering was conducted following the same procedure described before.
51 Sample on Filter Paper Büchner Funnel
Vacuum Pump Connection Pipe Flask
Residual Solution
Figure 3.9 Experimental set – up for filtration.
3.4 Analysis Methods
Two techniques were used in order to analyze the produced powders; these were X – Ray Diffraction and Scanning Electron Microscopy. X – Ray
Diffraction was used for phase identification and scanning electron microscopy was used for particle size determination and the examination of the morphology of the produced powders.
For X – Ray Diffraction analysis, Rigaku Multiflex Powder X-Ray diffractometer with Cu-Kα radiation (Figure 3.10) was available at the
Department of Metallurgical and Materials Engineering in Middle East
Technical University. The powder was analyzed at a rate of 20/min with
52 0.020 steps. Generally, the analysis was carried out between 200 and 800. The obtained XRD patterns were opened with Qualitative Analysis Program and after smoothing, background subtraction, Kα2 elimination, respectively phase identification was performed.
The equipment used for particle size determination and morphological investigation was JSM – 6400 scanning electron microscope (JEOL), which was available at the Department of Metallurgical and Materials Engineering in Middle East Technical University. Besides topographical information, it is capable of supplying compositional information with the aid of NORAN
System 6 X – Ray Microanalysis System. It is shown in Figure 3.11.
Figure 3.10 Rigaku Multiflex Powder X-Ray diffractometer.
53
Figure 3.11 JSM – 6400 scanning electron microscope.
54
CHAPTER 4
RESULTS AND DISCUSSION
The aim of this chapter is to present the results of conducted experiments explained in the previous chapter. First of all, formation of zirconium diboride by volume combustion synthesis and mechanochemical process using magnesiothermic reduction will be explained. This will be followed by the formation of lanthanum hexaboride and cerium hexaboride by using the same methods.
4.1 Formation of Zirconium Diboride
4.1.1 Volume Combustion Synthesis (VCS)
Volume combustion synthesis experiments were done according to the procedure discussed in Chapter 3. The first step in volume combustion synthesis experiments was to determine the ignition temperature.
Therefore, preliminary experiments were conducted to determine the ignition temperature of Reaction 4.1.
ZrO2 + B2O3 + 5Mg = ZrB2 + 5MgO (4.1)
55 The reactants were mixed stoichiometrically in a ceramic mortar and were placed in the pot furnace. The pot furnace was preheated to 900 0C. Due to the highly exothermic nature of the reaction a sudden increase in temperature indicated that the reaction took place. The temperature vs. time graph of this experiment and the ignition temperature of Reaction 4.1 in air are shown in Figure 4.1.
1000
o 900 758 C
800
) 700
C o (
600
500
400
300 Temperature
200
100
0 0 100 200 300 400 500 600 700 800 900 Time (sec.)
Figure 4.1 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture which was ignited in the furnace preheated to 900 0C in air atmosphere.
According to Figure 4.1 the ignition temperature of Reaction 4.1 was determined as 758 0C. X – Ray Diffraction method was used to analyze the reaction products. XRD pattern of the product of this reaction is given in
Figure 4.2.
56 1. ZrO2 3 2. MgO 2 3. ZrB2
4.Mg3B2O6
5.Zr2ON2
3 2 (a.u.)
5 Intensity 3 5 3 3 3 3 5 3 1 5 1 2 3 3 2 4 4 1 4 4 1 4 1 4
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.2 XRD pattern of the sample ignited in the furnace preheated to 900 0C for ZrB2 production.
As can be seen from the XRD pattern, the formation of ZrB2 was achieved with the formation MgO, however; additional minor phases such as zirconium oxynitride (Zr2ON2) and magnesium borate (Mg3B2O6) were also formed. Besides, ZrO2 could not be converted to ZrB2 completely.
Zr2ON2 may have formed due to the reaction of unreacted ZrO2 with N2 gas in air during the reduction of ZrO2 by Mg. Formation of Mg3B2O6 may have been the result of the reaction between unreacted B2O3 and the formed MgO according to Reaction 4.2.
B2O3 + 3MgO = Mg3B2O6
(4.2)
57 After ignition in furnace, the products were leached in 1 M HCl/water solution at room temperature for 15 hours in order to obtain pure ZrB2 and to remove MgO and the additional minor phases. The slurry density, which was described in Chapter 3, was 1/100 (g/ml). Bilgi [86] studied the production of TiB2 by both volume combustion synthesis and mechanochemical process using magnesiothermic reduction. Mg3B2O6 and some minor phases were also observed in the formation of TiB2. Hence, leaching parameters were determined based on her study. The XRD patterns of the sample ignited in furnace and the leached product of this sample are given in Figure 4.3.
1. ZrO2
2. MgO 3 3. ZrB2
4. Mg3B2O6 5 5.Zr2ON2 3
5 (a.u.) 3 3 3 3 1 5 3 1 5 3 3 (b) 1 4 1 4 4 11 4 1 4
Intensity 3 2 3 2 5 3 5 3 (a) 1 5 5 3 3 3 2 1 41 2 4 1 3 4 1 4 4 3 2
10 20 30 40 50 60 70 80
2θ (deg.)
Figure 4.3 XRD patterns for ZrB2 production (a) Sample ignited in the furnace preheated to 900 0C, (b) Sample in (a) leached in 1 M HCl/water solution for 15 hours.
58 As expected, MgO was completely eliminated after leaching process, however; other minor phases could not be taken away. After leaching in 1
M HCl/water solution for 15 hours, the final product contained ZrB2, ZrO2,
Zr2ON2 and Mg3B2O6. The morphologies of the produced powders were investigated by Scanning Electron Microscope. SEM micrographs of the reaction product and the leached reaction product are given in Figure 4.4.
(a) (b)
Figure 4.4 SEM micrographs for ZrB2 production (a) Sample ignited in the furnace preheated to 900 0C, (b) Sample in (a) leached in 1 M HCl/water solution for 15 hours.
The sample which was produced by volume combustion synthesis consisted of particles of 10 – 20 micrometers and had a sponge like structure, which is shown in Figure 4.4 (a). The leached product of this sample is presented in
Figure 4.4 (b). After leaching operation, agglomerates were broken into pieces and ZrB2 particles were liberated.
It was thought that the formation of minor phases could be prevented if the same experiment was repeated under an argon atmosphere with the same
59 parameters. Therefore, the reactants were again stoichiometrically mixed in a ceramic mortar and placed in the preheated pot furnace. The pot furnace was preheated to 900 0C. After placing the powder mixture in the pot furnace, a rapid heat release and a sudden increase in temperature was not observed, which means that the reaction had not taken place. According to these results, another reactant mixture was prepared and the pot furnace was preheated to its maximum limit which was 1100 0C, however; once more no reaction took place.
The reason why ZrB2 did not form by volume combustion synthesis under argon atmosphere may be the absence of O2 gas. As mentioned before, the formation of ZrB2 in air atmosphere was possible, which is probably due to the heat released during the oxidation of Mg. Mg is chemically active and the oxidation of Mg is an exothermic reaction. As a result, some amount of
Mg was oxidized to MgO when the experiments were performed in air atmosphere. This released heat might have supplied the energy needed for
Reaction 4.1.
This result implied that higher temperatures may be needed for the reduction of ZrO2. Another possible solution was to decrease ignition temperature by achieving perfect mixing and decreasing particle size.
Reduction in particle size leads to increase in specific surface area of powder particles and consequently powder particles become more active.
For this reason, powder mixture was ball milled for several hours for decreasing particle size and perfect mixing. The powder mixture was then ignited in furnace.
60 Based on the ideas explained above, 15 hours of ball milling was applied before ignition in furnace. After the application of mechanical energy for the mentioned time interval, the powder mixture was taken from the stainless steel jar and ignited in the preheated pot furnace under argon atmosphere.
The pot furnace was preheated to 900 0C as in the previous experiments.
This time the reduction reaction occurred under argon atmosphere and as a consequence of reduction in particle size and perfect mixing the ignition temperature was lowered to 224 0C. The temperature vs. time graph of this experiment is given in Figure 4.5.
350
300 ) 224 oC C
o 250 (
200
150
Temperature 100
50
0 0 20 40 60 80 100 120 140 Time (sec.)
Figure 4.5 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture which was ball milled for 15 hours and then ignited in the furnace preheated to 900 0C under argon atmosphere.
61 The XRD patterns of this sample before and after ignition in furnace are given in Figure 4.6.
1. ZrO2 2 2. MgO 3
3. ZrB2
4. Mg3B2O6 2 6. Mg 3 (a.u.)
3 1 3 3 3 1 3 2 1 (b) 1 21 1 3 1 14 4 4 1 4 1 1 1 1
Intensity 1 6 1 6 1 1 1 6 1 1 1 6 6 (a) 11 1 1 1 11 1 6 1 1 1 1 6 6 111 1
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.6 XRD patterns for ZrB2 production (a) Sample ball milled for 15 hours, (b) Sample in (a) ignited in the furnace preheated to 900 0C under argon atmosphere.
Figure 4.6 (a) indicates that starting powder mixture did not go into reaction after the application of mechanical energy for 15 hours; powder mixture still contained ZrO2, Mg and B2O3. Due to its amorphous structure, B2O3 can not be seen in Figure 4.6 (a). As can be seen from Figure 4.6 (b), the reduction reaction took place under argon atmosphere. Formation of Zr2ON2 was prevented due to the absence of N2 gas, however; formation of Mg3B2O6 could not be prevented. Furthermore, complete reduction of ZrO2 could not be achieved. The final product contained ZrB2, MgO, Mg3B2O6 and ZrO2.
62 The volume combustion synthesis experiment occurred with a small explosion and due to this explosion some of the final product was lost.
Therefore, it was decided to conduct the experiment with less active powders and to achieve this, starting powder mixture was ball milled for
7.5 hours instead of 15 hours and then ignited in the preheated pot furnace.
Again the pot furnace was preheated to 900 0C. The temperature vs. time graph of this experiment is given in Figure 4.7.
500 450 400 282 oC C)
o 350 300 250 200 150 ( Temperature 100 50 0 0 20406080100120140
Time (sec.)
Figure 4.7 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture which was ball milled for 7.5 hours and then ignited in the furnace preheated to 900 0C under argon atmosphere.
The ignition temperature was determined as 282 0C. As expected, the ignition temperature of the 7.5 hours ball milled sample was greater than the 15 hours ball milled sample. In addition, the explosion during the
63 ignition was not as intense as before. The XRD patterns of this sample before and after ignition in the furnace are given in Figure 4.8.
1. ZrO2 3 2. MgO 2
3. ZrB2
4. Mg3B2O6 3 2 6. Mg (a.u.)
y 3 3 3 3 23 11 1 11 3 (b) 1 2 4 1 1 41 4 1 1 114 Intensit 1 6 1 6 1 1 6 (a) 6 1 11 16 611 1 66 1 1 1 11 1 1 1 1 1111
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.8 XRD patterns for ZrB2 production (a) Sample ball milled for 7.5 hours, (b) Sample in (a) ignited in the furnace preheated to 900 0C under argon atmosphere.
Starting powder mixture did not go into any reaction after 7.5 hours of applying mechanical energy as it is shown in Figure 4.8 (a). As can be easily realized from Figure 4.8 (b), intensities of ZrB2 peaks are much higher than the sample which was ball milled for 15 hours and then ignited in furnace.
Once more, the formation of Zr2ON2 was prevented due to the absence of N2 gas, however; formation of Mg3B2O6 could not be prevented. Furthermore, complete reduction of ZrO2 could not be achieved. When intensities of ZrB2 peaks, which are higher when compared to the previous experiments, are
64 analyzed, it was decided to conduct experiments on samples that were ball milled for 7.5 hours. To achieve complete reduction of ZrO2, excess Mg was added to the starting powder mixture. Besides, B2O3 was added to the starting powder mixture as an excess amount in order to avoid smearing of
Mg on the walls of stainless steel jar. Hence, 40% excess Mg and 40% excess
B2O3 were added. After preparation of the powder mixture, it was ball milled for 7.5 hours and then ignited in furnace. The effect of excess amounts of Mg and B2O3 on the ignition temperature is shown in Figure 4.9.
500
450 314 oC 400 ) 350 C o ( 300
250
200
150 Temperature
100 50
0 0 20 40 60 80 100 120 140 160 Time (sec.)
Figure 4.9 Ignition temperature of ZrO2 – B2O3 – Mg reactant mixture containing 40% excess Mg and B2O3, which was ball milled for 7.5 hours and then ignited in the furnace preheated to 900 0C under argon atmosphere.
It can be clearly seen from the temperature vs. time graph that, adding excess amounts of reactants increased the ignition temperature from 282 0C
65 to 314 0C. The XRD patterns of this sample after ignition in furnace are given in Figure 4.10.
1. ZrO2 3 2. MgO
3. ZrB2
4. Mg3B2O6 2
(a.u.) 3
2
Intensity 3 3 3 3 3 2 2 3 3 1 4 4 44
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.10 XRD pattern of the sample containing 40% excess Mg and B2O3, ball milled for 7.5 hours and then ignited in the furnace under argon atmosphere for ZrB2 production.
The XRD patterns indicate that ZrB2 was produced with the formation of minor phase, Mg3B2O6. A very large amount of ZrO2 was reduced to ZrB2.
Although, a great amount of ZrO2 was reduced to ZrB2, a very small amount of ZrO2 still remained unreacted.
After ignition in furnace, the products of volume combustion synthesis were subjected to leaching process that was carried out in 1 M HCl/water solution at room temperature for 15 hours in order to obtain pure ZrB2 and
66 remove both MgO and Mg3B2O6. The XRD patterns of this sample before and after leaching process are given in Figure 4.11.
1. ZrO2 3
2. MgO
3. ZrB2 3 4. Mg3B2O6
3 3 3 3 3 (a.u.)
1 3 3
y 1 (b) 1 141 44 44 1 3 2 3 In ten sit 2 3 3 33 3 3 3 2 (a) 1 4 2 4 44
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.11 XRD patterns for ZrB2 production (a) Sample containing 40% excess Mg and B2O3, ball milled for 7.5 hours and ignited in the furnace under argon atmosphere, (b) Sample in (a) leached in 1 M HCl/water solution for 15 hours.
Figure 4.11 (b) indicates that after leaching, MgO was completely removed from the final product, which was now composed of ZrB2, ZrO2 and
Mg3B2O6. Again, ZrO2 could not be reduced completely and Mg3B2O6 could not be removed from the final product. Mg3B2O6 particles may have been entrapped between ZrB2 particles and due to this entrapment its surface could not have been in contact with HCl/water solution. Mg3B2O6 particles could have been removed by leaching process if they had been freed by an
67 extra milling operation. Wet ball milling seemed to be the best solution for this problem. After ignition in furnace, milling the product in ethanol medium would liberate the Mg3B2O6 particles and this liberation would lead to the dissolution of Mg3B2O6 particles. Therefore, the same sample was ball milled in ethanol medium for 7 hours after ignition in furnace. Before applying leaching, powder was subjected to ball milling operation twice, hence; leaching time was shortened to 2.5 hours in order to avoid further dissolution of ZrB2 particles. Meanwhile the concentration of HCl/water solution was increased to 5 M due to shortening of leaching time. Leaching was again conducted at room temperature. The XRD patterns of this sample after ignition in furnace and after leaching operation are presented in Figure
4.12.
1. ZrO2 2. MgO 3
3. ZrB2
4. Mg3B2O6
3 (a.u.)
33 3 3 3 3 1 3 (b) 11 11 1 1 Intensity 3 2 3 2 3 3 (a) 3 33 3 3 2 1114 24 44
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.12 XRD patterns for ZrB2 production (a) Sample containing 40% excess Mg and B2O3, ball milled for 7.5 hours and ignited in the furnace under argon atmosphere, (b) Sample in (a) ball milled for 7 hours in ethanol medium and then leached in 5 M HCl/water solution for 2.5 hours.
68 As can be seen from the XRD patterns, MgO and Mg3B2O6 were completely removed from the final product. Applying 7 hours of wet ball milling in ethanol medium after ignition in furnace, reduced the particle size and liberated Mg3B2O6 particles. This resulted in easier leaching of them. This final experiment gave the best results for the formation of ZrB2 by volume combustion synthesis. ZrO2 is a very stable oxide and this makes reduction very difficult. Therefore, in all volume combustion synthesis experiments, complete reduction of ZrO2 could not be achieved. In all of them, after ignition in furnace there was some ZrO2 that remained unreacted.
Furthermore, ZrO2 was insoluble in 1 M and 5 M HCl/water solutions. This insolubility prevented the removal of ZrO2 during leaching process. SEM micrographs of the powder mixture after ball milling of 7.5 hours, the product after ignition in furnace and the final product after leaching are given in Figure 4.13 (a), (b) and (c), respectively.
69
(a) (b)
(c)
Figure 4.13 SEM micrographs for ZrB2 production (a) Sample ball milled for 7.5 hours, (b) Sample in (a) ignited in the furnace preheated to 900 0C under argon atmosphere, (c) Sample in (b) ball milled for 7 hours in ethanol medium and then leached in 5 M HCl/water solution for 2.5 hours.
Figure 4.13 (a) shows the perfect mixing of the starting powder mixture. The product of leaching had a porous structure; which may have formed due to the leaching of MgO. The final product consisted of ZrB2 and ZrO2 particles which were smaller than 1 micrometer.
When all the presented results are considered, it can be summarized that the production of ZrB2 by direct volume combustion synthesis under argon atmosphere was not possible because of the high ignition temperature. ZrB2 formation may have occurred in air probably due to the heat released
70 during the oxidation of Mg. Milling the reactants before ignition in furnace for 7.5 hours decreased the ignition temperature which made the reduction reaction possible under argon atmosphere, however; complete reduction of
ZrO2 was impossible since it was a very stable oxide. Using excess Mg and
B2O3 did not change the results considerably. Incomplete reduction of ZrO2 was also observed by Khanra et al. [79, 80] during the production of ZrB2 from ZrO2 – H3BO3 – Mg system by SHS. The formation of Zr2ON2 was prevented when the reaction took place under argon atmosphere. The removal of minor phase, Mg3B2O6, became possible by leaching after 7 hours of wet ball milling operation in ethanol medium.
4.1.2 Mechanochemical Process (MCP)
For the formation of ZrB2 by mechanochemical synthesis, first of all, ball to powder ratio was decided. It was taken as 1/15 (weight of powder/weight of stainless steel balls). This value was determined according to the maximum feed volume that can be charged into the nominal volume of the grinding jar. During performing experiments, the stainless steel jar was filled with argon gas to avoid the oxidation of Mg. In the preliminary experiments, the planetary ball mill was operated for 30 hours at 300 revolutions per minute
(rpm). At each half an hour it stopped automatically, waited for 1 minute and moved in the reverse direction. Also, in every 5 hour period, it was stopped and a very small amount of the sample was taken for XRD analysis.
In these experiments, 8 balls with a diameter of 20 millimeters were used.
The starting powder mixture was prepared stoichiometrically according to
Reaction 4.1. The XRD patterns of this experiment are given in Figure 4.14.
Each pattern belongs to the 5 hours intervals starting from the 15th hour.
71 1. ZrO2 2. MgO 3. ZrB2 2 6. Fe 7. Mg 3 2 3 1 1 (d) 1 3 1 1 2 6 1 3 3 3 2 2 2
3 2 (a.u.)
1 1 3 1 3 (c) 1 2 6 3 1 2 1 3 1 2 1 1 1 2 1 2 1 3 1 1 1 1 6 1 1 1 2 (b) 11 3 3 2 1 1 1 2 Intensity 11 7 1 1 1 11 7 (a) 11 7 1 111 1 7 1 1 1 1 7 7 1 7 1 1 1 1
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.14 XRD patterns for ZrB2 production (a) Sample ball milled for 15 hours, (b) Sample ball milled for 20 hours, (c) Sample ball milled for 25 hours, (d) Sample ball milled for 30 hours.
Figure 4.14 (a) shows that until the 15th hour, the system contained only the reactants which were Mg, ZrO2 and B2O3. Due to its amorphous structure,
B2O3 can not be seen in Figure 4.14 (a). Looking through Figures 4.14 (a), (b) and (c), the sample which was ball milled for 20 hours or more, contained
ZrO2, ZrB2, MgO and Fe. The remaining ZrO2 shows that the reaction did not occur completely, however; the powder mixture did not contain any elemental Mg; all Mg was in the form of MgO. This result gave the idea that
Mg may have been oxidized before the formation of ZrB2. The reason of this oxidation may have been the opening of the jar every 5 hour for taking sample. Because of the applied mechanical energy, Mg became very active and this very active Mg may have been oxidized when it came into contact with air during opening of the jar. The peaks of formed ZrB2 had very low
72 intensities and the peaks of ZrB2 and MgO which are at the angles 430 – 440, respectively could not be decomposed. Extremely small particle size of formed ZrB2 may have been the reason why the decomposition could not take place. Iron (Fe) in the powder mixture came from the wear of stainless steel jar and balls.
It was thought that opening of the jar in every 5 hours during ball milling operation may have hindered the formation of ZrB2. Therefore, the experiment was repeated continuously. During the whole experiment, the jar was never opened and the powder mixture did not come into contact with air. All the experimental parameters were kept constant but 10% excess amount of Mg and B2O3 were added to increase the efficiency and to reduce
ZrO2 completely. The XRD pattern of this experiment is given in Figure 4.15.
1. ZrO2 3 2. MgO
3. ZrB2 6. Fe
(a.u.) 3
2 3 Intensity 3 3 3 3 2 3 3 2 1 1 2 6 1
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.15 XRD pattern of the sample ball milled continuously for 30 hours for ZrB2 production.
73 As can be seen from Figure 4.15, at the end of the milling operation the reaction took place. The formed ZrB2 peaks were more intense compared to the previous experiment. Besides, they were distinguished from the peaks of MgO easily. In addition to the major phases, ZrB2 and MgO, unreacted
ZrO2 and Fe due to the wear of stainless steel jar and balls were also present in the final product.
When the ball milling operation was not interrupted, it was observed that the reaction took place, which means Mg should not come into contact with air and oxidize before the reaction takes place. These results create a conflict with the obtained results in the formation of TiB2. During the production of
TiB2, interrupting the milling does not affect the results [86]. This result shows that a different mechanism may exist in the formation of ZrB2 by mechanochemical process not observed in the case of TiB2.
As mentioned before, the mechanochemical processing products consisted of ZrB2, MgO, ZrO2 and Fe. In order to produce pure ZrB2, the products were leached in 1 M HCl/water solution at room temperature for 30 minutes. The slurry density was taken as 1/100 (g/ml) and the leaching was conducted at room temperature. The XRD patterns of the sample that was ball milled for 30 hours continuously and the leached product of this sample are given in Figure 4.16.
74 1. ZrO2 3 2. MgO
3. ZrB2 6. Fe 3
(a.u.) 3 33 3 3 3 3 (b) 11 1 1
3 Intensity 3
3 3 2 3 3 3 (a) 3 2 3 1 2 6 1 2
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.16 XRD patterns for ZrB2 production (a) Sample containing 10% excess Mg and B2O3, ball milled for 30 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
It can be seen from Figure 4.16 (b) that MgO and Fe were completely removed from the powder, but ZrO2 could not be removed completely. It was considered that by increasing the amounts of Mg and B2O3, it could be possible to achieve complete reduction of ZrO2. Therefore, after leaching operation, pure ZrB2 could have been obtained.
In order to reduce ZrO2 completely, both the milling time and the excess amounts of Mg and B2O3 were increased. Excess Mg and B2O3 were raised from 10% to 30%. Milling time was extended from 30 hours to 40 hours.
After the milling operation, the product was leached in 1 M HCl/water solution at room temperature for 30 minutes. The slurry density was again
1/100 (g/ml). The XRD patterns of the sample that was ball milled for 40
75 hours continuously and the leached product of this sample are given in
Figure 4.17.
1. ZrO2 2. MgO 3 3. ZrB2 6. Fe
3 (a.u.) 33 3 3 3 3 3 (b) 11 1 1
Intensity 3
3
3 2 3 3 3 3 3 (a) 2 6 223 1
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.17 XRD patterns for ZrB2 production (a) Sample containing 30% excess Mg and B2O3 ball milled for 40 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
From Figure 4.17, it can be easily claimed that there is not any difference between the two experiments. The leached products of both experiments are given in Figure 4.18. Setoudeh and Welham [81] synthesized ZrB2 by mechanochemical process and they also observed incomplete reduction of
ZrO2. They suggested using excess Mg and B2O3 for complete conversion, however; as can be seen from Figure 4.18, in spite of increasing both milling time and excess amounts of Mg and B2O3, some ZrO2 still remained unreacted.
76 1. ZrO2 3 3. ZrB2
3
3 3
(a.u.) 3 3 3 3 3 (b) 1 1 1
3 Intensity 3
3 3 3 3 3 3 3 (a) 1 1 1
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.18 XRD patterns for ZrB2 production (a) Leached product of the sample containing 30% excess Mg and B2O3 ball milled for 40 hours, (b) Leached product of the sample containing 10% excess Mg and B2O3 ball milled for 30 hours.
SEM micrographs of the sample containing 30% excess Mg and B2O3 and ball milled for 40 hours and the leached product of this sample are given in
Figure 4.19 (a) and (b), respectively.
77
(a) (b)
Figure 4.19 SEM micrographs for ZrB2 production (a) Sample containing 30% excess Mg and B2O3 ball milled for 40 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
As can be observed from Figure 4.19 (a), the product, which consisted of
ZrB2, MgO and very small amount of ZrO2, was composed of very small particles. Due to the ball milling, agglomerations were observed. SEM micrograph of the leached product is given in Figure 4.19 (b) and from this micrograph average grain size was calculated as 0.235±0.045 micrometer.
It can be concluded from the results obtained that 30 hours of ball milling was sufficient for the formation of ZrB2; complete reduction could not be accomplished, however. Increasing both milling time and excess amounts of
Mg and B2O3 did not result in complete reduction of ZrO2. Apart from this, minor phases were not observed among mechanochemical process products and the final product was composed of ZrB2 and ZrO2 after leaching process.
78 4.2 Formation of Lanthanum Hexaboride
4.2.1 Volume Combustion Synthesis (VCS)
Determination of the ignition temperature was the first step for the formation of lanthanum hexaboride (LaB6) by volume combustion synthesis using magnesiothermic reduction. The production of LaB6 was done according to Reaction 4.3.
La2O3 + 6B2O3 + 21Mg = 2LaB6 + 21MgO (4.3)
The reactants were mixed stoichiometrically in a ceramic mortar and placed in the pot furnace. The pot furnace was preheated to 1000 0C. The experiment was conducted under argon atmosphere so as to avoid the oxidation of Mg and the formation of unwanted minor phases. The temperature vs. time graph of this experiment is presented in Figure 4.20.
79 900
800 695 oC
700
600 C) o 500
400
300
( Temperature 200
100
0 0 50 100 150 200 250 300 350 Time (sec)
Figure 4.20 Ignition temperature of La2O3 – B2O3 – Mg reactant mixture which was ignited in the furnace preheated to 1000 0C under argon atmosphere.
Ignition temperature was determined as 695 0C. The XRD pattern of this sample is given in Figure 4.21.
80 1. LaB6 2. MgO 2
3. Mg3B2O6
4. LaBO3 1
2 (a.u.)
1
1 1
Intensity 1 1 1 1 2 1 3 2 3 3 3 1 2 4 4 3 3 4 3 4 4 3 4 4
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.21 XRD pattern of the sample ignited in the furnace preheated to 1000 0C under argon atmosphere for LaB6 production.
By examining the XRD pattern, it is easily seen that the reaction took place.
As expected, the product contained LaB6 and MgO. In addition to these,
Mg3B2O6 (magnesium borate) and LaBO3 (lanthanum borate) were also present in the product. LaBO3 may have formed from the reaction between
La2O3 and B2O3 during the reduction of La2O3 by Mg.
After ignition in the furnace, the products obtained from volume combustion synthesis were subjected to leaching process in order to eliminate unwanted minor phases. The slurry density was 1/100 (g/ml) as in
ZrB2 experiments. The leaching operation was conducted with 1 M
HCl/water solution at room temperature for 15 hours. The XRD patterns of
81 the ignited sample in the furnace and the leached product of this sample are presented in Figure 4.22.
1. LaB6 2. MgO 1
3. Mg3B2O6 1
4. LaBO3 1 1 1 1 1 1 1
(a.u.) 1 3 3 (b) 3 3 3 33 3
1 2 Intensity 1 2 1 1 1 1 1 1 2 1 4 3 3332 3 1 2 (a) 3 4 4 4 3 344 4
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.22 XRD patterns for LaB6 production (a) Sample ignited in the furnace preheated to 1000 0C, (b) Sample in (a) leached in 1 M HCl/water solution for 15 hours.
As can be noticed from Figure 4.22 (b), MgO and LaBO3 were completely removed from the final product, however; dissolution of Mg3B2O6 in the leach solution could not be attained.
It was thought that Mg3B2O6 may have been entrapped between LaB6 particles. Therefore, the liberation of this minor phase may lead to the dissolution of it during leaching. This liberation can be achieved by wet ball milling. By the application of mechanical energy, the particle size decreases,
82 the specific surface area of particles increases and as a result the entrapped particles can get in contact with the leach solution. Hence, the removal of the minor phase may be accomplished. On the basis of these ideas, experiment was repeated with the same parameters, but after ignition in furnace the product was ball milled in ethanol medium for 7 hours. Then, the wet ball milled sample was leached in 1 M HCl/water solution at room temperature for 15 hours. The XRD patterns of the ignited sample and the leached product of this sample are presented in Figure 4.23.
1. LaB 6 1 2. MgO
3. Mg3B2O6
4. LaBO3 1
1 1 1
(a.u.) 1 1 1 1 1
(b) 1 Intensity 1 2 1 2 1 1 1 1 11 2 33 1 (a) 4 4 3 4 2 33 3 4 4 4 3 1 4 2
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.23 XRD patterns for LaB6 production (a) Sample ignited in the furnace preheated to 1000 0C, (b) Sample in (a) ball milled for 7 hours in ethanol medium and then leached in 1 M HCl/water solution for 15 hours.
As shown in Figure 4.23 (b), pure LaB6 was obtained after leaching operation. This result strengthens the idea about entrapment of minor phases between LaB6 particles. Due to ball milling, size reduction occurs and
83 this makes leaching of Mg3B2O6 particles easier. Both the rate of dissolution and solution – powder contact increases by grinding the particles into smaller size. Another possibility was the dissolution of Mg3B2O6 in the ethanol medium. This is not possible, however; as Mg3B2O6 is known to be insoluble in ethanol from a previous study [86]. SEM micrographs of the sample ignited in the furnace preheated to 1000 0C and the leached product of this sample are shown in Figure 4.24.
(a) (b)
Figure 4.24 SEM micrographs for LaB6 production (a) Sample ignited in the furnace preheated to 1000 0C, (b) Sample in (a) ball milled for 7 hours in ethanol medium and then leached in 1 M HCl/water solution for 15 hours.
The volume combustion synthesis product, which is shown in Figure 4.24
(a) consists of 10 – 20 micrometer particles and has a porous structure.
Figure 4.24 (b) represents the pure LaB6. By the effect of both wet ball milling and leaching, the agglomerations in Figure 4.24 (a) were broken apart. The removal of MgO and the minor phases by leaching, freed the
LaB6 particles which were generally smaller than 1 micrometer.
84 To sum up, the production of LaB6 by volume combustion synthesis occurred along with the formation of the minor phases; Mg3B2O6, LaBO3.
After leaching process, MgO and LaBO3 were removed but Mg3B2O6 could not be eliminated. The final product contained the desired hexaboride and
Mg3B2O6.
Applying 7 hours of wet ball milling in ethanol medium after volume combustion synthesis, reduced the particle size and liberated the minor phase particles. This resulted in easier leaching of Mg3B2O6 particles. As a consequence, pure LaB6 was obtained. The molar yield of pure LaB6 obtained by volume combustion synthesis was calculated as 71.5%.
4.2.2 Mechanochemical Process (MCP)
In the production of LaB6 by mechanochemical process, ball to powder ratio was taken as 1/15 (weight of powder/weight of stainless steel balls). The stainless steel grinding jar was filled with argon gas before the experiments in order to avoid oxidation. The planetary ball mill was operated for 30 hours at 300 revolutions per minute (rpm). At each half an hour it stopped automatically, waited for 1 minute and moved in the reverse direction. 8 stainless steel balls with 20 millimeters of diameter were used to mill the powder. The starting powder mixture was prepared stoichiometrically according to Reaction 4.3. The XRD pattern of this experiment is given in
Figure 4.25.
85 1. LaB6 1 2. MgO
1 (a.u.) 1 1
1 1 2 1 1 Intensity 1 2 1 2
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.25 XRD pattern of the sample ball milled for 30 hours for LaB6 production.
The XRD analysis shows that the reaction occurred successfully. 30 hours of ball milling was sufficient to produce the desired hexaboride. In the case of mechanochemical synthesis, LaB6 and MgO formed without the formation of any other minor phase. In order to remove MgO and to obtain pure LaB6, the sample was leached for 30 minutes in 1 M HCl/water solution at room temperature. The XRD pattern of the final product and 30 hours of ball milled sample are presented in Figure 4.26.
86 1. LaB6
2. MgO 1
1
1 1 1 1 1 1 1
(a.u.) 1 (b)
1
Intensity 1 1 1
1 1 1 2 1 2 1 1 (a) 2
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.26 XRD patterns for LaB6 production (a) Sample ball milled for 30 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
The XRD pattern given in the Figure 4.26 (b) indicates that MgO was successfully removed from the powder and pure LaB6 was produced by mechanochemical process. SEM micrographs of the products before and after leaching process are given in Figure 4.27 (a) and (b), respectively.
87
(a) (b)
Figure 4.27 SEM micrographs for LaB6 production (a) Sample ball milled for 30 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
SEM micrograph given in Figure 4.27 (a) shows that the product obtained at the end of ball milling was composed of MgO and LaB6 and it had very small particles. Due to the milling, some agglomerations were observed.
The SEM micrograph of the leached product of this sample is shown in
Figure 4.27 (b). When it is compared with ZrB2, it can be observed that particle sizes are very similar to each other. The produced LaB6 had an average particle size of 0.5 micrometer.
Based on the results stated above, pure LaB6 was obtained by mechanochemical process very easily. 30 hours of ball milling was enough to reduce La2O3 and to form LaB6. MgO was successfully eliminated from the mechanochemical process product after 30 minutes of leaching process and pure LaB6 was obtained finally. The molar yield of LaB6 obtained by ball milling for 30 hours and leaching for 30 minutes in 1 M HCl solution was calculated as 90.4%. This value is much higher than the molar yield of volume combustion product.
88 4.3 Formation of Cerium Hexaboride
4.3.1 Volume Combustion Synthesis (VCS)
The experiments performed for the formation of cerium hexaboride (CeB6) by volume combustion synthesis by magnesiothermic reduction were started with the determination of the ignition temperature. The production of CeB6 was done according to Reaction 4.4.
CeO2 + 3B2O3 + 11Mg = CeB6 + 11MgO (4.4)
The stoichiometric amounts of the reactants were mixed in a ceramic mortar and placed in a graphite crucible. The crucible was then loaded in a pot furnace which was preheated to 1000 0C. The experiment was conducted under argon atmosphere in order to avoid the oxidation of Mg and the formation of unwanted minor phases. The temperature vs. time graph of this experiment is given in Figure 4.28.
89 1000 900 719 oC 800 700 C) o (
600 500 400 300 Temperature 200 100 0 0 50 100 150 200 250 300 350
Time (sec.)
Figure 4.28 Ignition temperature of CeO2 – B2O3 – Mg reactant mixture which was ignited in the furnace preheated to 1000 0C under argon atmosphere.
The ignition temperature was determined as 719 0C from Figure 4.28. The
XRD pattern of the product of this experiment is given in Figure 4.29 (a).
After ignition in the furnace, the products were leached in 1 M HCl/water solution for 15 hours at room temperature. The slurry density was taken as
1/100 (g/ml) as in the previous experiments. The leached product is presented in Figure 4.29 (b).
90 1. CeB6 1 2. MgO
3. Mg3B2O6
1
1 1 1 1
(a.u.) 1 1 y 1 1 (b) 3 3 333 3 3 3 3 Intensit 2 2 1 1 1 1 1 1 1 1 11 2 (a) 3 23 33 3 3 33 2
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.29 XRD patterns for CeB6 production (a) Sample ignited in the furnace preheated to 1000 0C, (b) Sample in (a) leached in 1 M HCl/water solution for 15 hours.
Figure 4.29 (a) implies that CeB6 and MgO were produced with the formation of a minor phase, Mg3B2O6. As described previously, Mg3B2O6 occurs due to the reaction between unreacted B2O3 and formed MgO. As can be seen from Figure 4.29 (b), after leaching process, MgO was completely removed, however; Mg3B2O6 remained in the final product. In the production of LaB6, Mg3B2O6 dissolved in the leach solution after the application of wet ball milling. Due to this reason, following the ignition, volume combustion synthesis products were wet ball milled and then leached in 1 M HCl/water solution for 15 hours at room temperature. The
XRD patterns and the SEM micrographs of the ignited sample and the leached product of this sample are shown in Figure 4.30 and Figure 4.31, respectively.
91 1. CeB6 1 2. MgO 1 3. Mg3B2O6
1 1 1 1 (a.u.)
1
y 1 1 (b) 1
Intensit 2 1 2 1 1 1 (a) 23 3 1 1 1 11 1 22 3 3 3 333 3
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.30 XRD patterns for CeB6 production (a) Sample ignited in the furnace preheated to 1000 0C, (b) Sample in (a) ball milled for 7 hours in ethanol medium and then leached in 1 M HCl/water solution for 15 hours.
(a) (b)
Figure 4.31 SEM micrographs for CeB6 production (a) Sample ignited in the furnace preheated to 1000 0C, (b) Sample in (a) ball milled for 7 hours in ethanol medium and then leached in 1 M HCl/water solution for 15 hours.
92 Figure 4.30 (a) represents the ignition products. The ignition product included CeB6, MgO and Mg3B2O6, as expected. The application of mechanical energy made the leaching easier and as a consequence Mg3B2O6 was completely removed from the product as shown in Figure 4.30 (b). The final product was pure CeB6.
By observing SEM micrographs, it was discovered that the volume combustion product shown in Figure 4.31 (a) was composed of 20 – 30 micrometer particles and had a porous structure. The leached product of this sample is presented in Figure 4.31 (b). From this micrograph, it can be stated that by the removal of MgO and Mg3B2O6, agglomerations were broken apart and CeB6 particles were freed. The size of CeB6 particles was smaller than 1 micrometer.
The production of CeB6 by volume combustion synthesis is very similar to the production of LaB6. Based on the results presented in Figure 4.30, pure
CeB6 can be formed by the following procedure: (i) Ignition of the reactants under argon atmosphere, (ii) Ball milling of ignition product in ethanol medium and (iii) Leaching of milled product in 1 M HCl/water solution for
15 hours. The molar yield of pure CeB6 obtained by volume combustion synthesis following this procedure was calculated as 68.6%.
4.3.2 Mechanochemical Process (MCP)
The production of CeB6 by volume combustion synthesis was very similar to LaB6 formation. Hence, in the production of CeB6 by mechanochemical processing, the same experimental parameters were used as in LaB6
93 formation. A stoichiometric sample was prepared according to Reaction 4.4.
The ball to powder ratio was taken as 1/15 and 30 hours of ball milling was applied at 300 rpm with 8 stainless steel balls of 20 millimeter diameter.
Before starting the milling operation, stainless steel jar was filled with argon gas. During 30 hours, planetary ball mill changed its rotation in the reverse direction at each 30 minutes. The XRD pattern of this sample is given in
Figure 4.32 (a). Following the ball milling, the sample was leached in 1 M
HCl/water solution for 30 minutes. The XRD pattern of the leached product is given in Figure 4.32 (b). Along with the XRD patterns, SEM micrographs of the products before and after leaching process are presented in Figure
4.33 (a) and (b), respectively.
1. CeB 6 1 2. MgO 3. Fe 1 1
1 1 (a .u.) 1 1 1 y 1 (b) 1
Intensit 1 1 1 1 2 1 1 1 (a) 1 3 2 1 1 2
10 20 30 40 50 60 70 80 2θ (deg.)
Figure 4.32 XRD patterns for CeB6 production (a) Sample ball milled for 30 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
94
(a) (b)
Figure 4.33 SEM micrographs for CeB6 production (a) Sample ball milled for 30 hours, (b) Sample in (a) leached in 1 M HCl/water solution for 30 minutes.
As can be observed from Figure 4.32 (a), at the end of 30 hours, Reaction 4.4 had taken place and CeB6 and MgO had formed. MgO was removed after 30 minutes of leaching in 1 M HCl/water solution at room temperature and pure CeB6 was obtained, as shown in Figure 4.32 (b).
SEM micrographs indicate that the mechanochemical synthesis product had a porous structure and consisted of very small particles. Besides, some agglomerations were also observed as in the case of other metal borides.
The leached product of this sample is presented in Figure 4.33 (b). By examining this micrograph, it can be claimed that agglomerations were broken apart and the final product consisted of very small particles. The produced CeB6 had an average particle size of 0.45 micrometer.
According to the mechanochemical process experiments, it can be concluded that 30 hours of ball milling was adequate to form CeB6 and
MgO. MgO was successfully removed from the mechanochemical process
95 product after 30 minutes of leaching process and pure CeB6 was obtained.
The molar yield of pure CeB6 obtained by mechanochemical process was calculated as 84.4%.
96
CHAPTER 5
CONCLUSION
In this study, formation of zirconium diboride (ZrB2), lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) by magnesiothermic reduction using volume combustion synthesis (VCS) and mechanochemical processing (MCP) were investigated.
The production of ZrB2 by direct volume combustion synthesis under argon atmosphere was not possible probably because of high ignition temperature; however, in air atmosphere the ignition of the reactants occurred at 758 0C. The formation of ZrB2 occurred in air possibly due to the heat released during oxidation of Mg. Ball milling the reactants before ignition in furnace for 7.5 hours decreased the ignition temperature to 282
0C and this made the reduction reaction possible under argon atmosphere.
Ignition under argon atmosphere prevented the formation of minor phase
Zr2ON2 but the formation of Mg3B2O6 could not be prevented. Besides, complete reduction of ZrO2 could not be achieved. In order to reduce ZrO2 completely, 40% excess amounts of Mg and B2O3 were added to the starting powder mixture. This increased the ignition temperature to 314 0C and a very large amount of ZrO2 was reduced to ZrB2. Although, a great amount of ZrO2 was reduced to ZrB2, a very small amount of ZrO2 still remained
97 unreacted. Complete reduction of ZrO2 was not possible. After leaching this product in 1 M HCl/water solution at room temperature for 15 hours, MgO was completely removed. However, ZrO2 and Mg3B2O6 could not be removed. Elimination of Mg3B2O6 from the reaction product could be achieved by first ball milling the product for 7 hours in ethanol and then leaching in 5 M HCl/water solution for 2.5 hours. This resulted in easier leaching of Mg3B2O6 and the final product was composed of ZrB2 and ZrO2.
30 hours of milling time was sufficient to produce ZrB2 by mechanochemical processing; however, ZrO2, again, could not be reduced completely.
Therefore, both milling time (from 30 hours to 40 hours) and excess amounts Mg and B2O3 (from 10% to 30%) were increased. This did not change the results very much and some ZrO2 remained unreacted. The advantage of mechanochemical process over volume combustion was the absence of minor phases in the obtained product. Only Fe was detected in mechanochemical process product as an unexpected phase. It occurred due to the wear of stainless steel balls and jar and it was removed during leaching operation very easily. The mechanochemical process product was leached in 1 M HCl/water solution at room temperature for 30 minutes and
MgO was completely removed. The final product consisted of ZrB2 and
ZrO2.
Formation of LaB6 and CeB6 by volume combustion synthesis and mechanochemical process were very similar to each other. In the production of LaB6 by volume combustion synthesis, the ignition temperature was determined as 695 0C. In the case of CeB6, the ignition temperature was 719
0C. LaB6 formation by volume combustion synthesis was accompanied with
98 the formation of minor phases, Mg3B2O6 and LaBO3. Mg3B2O6 was the only minor phase that formed during the production of CeB6. For the removal of
Mg3B2O6 phase, the volume combustion synthesis products were ball milled for 7 hours in ethanol medium and then leached in 1 M HCl/water solution at room temperature for 15 hours. After leaching, MgO and Mg3B2O6 were removed and pure LaB6 or CeB6 was obtained.
Pure LaB6 or CeB6 could be obtained by mechanochemical process very easily. 30 hours of ball milling was sufficient to produce the desired hexaboride. After leaching in 1 M HCl/water solution at room temperature for 30 minutes, MgO was completely removed and pure LaB6 or CeB6 was obtained.
Future work suggestions:
1) A pilot plant can be constructed for the batch production of these
metal borides by both volume combustion synthesis and
mechanochemical process. The economical feasibility of both
processes can be investigated in this pilot plant.
2) Different experimental parameters can be studied to achieve
complete reduction of ZrO2.
3) Properties and applications of ZrO2 – ZrB2 composites produced by
volume combustion synthesis and mechanochemical process can be
investigated.
4) Production of other metal borides such as molybdenum boride
(MoB), vanadium boride (VB2), iron boride (Fe2B) and chromium
boride (CrB2) by both methods can be studied.
99
REFERENCES
[1] “The Economics of Boron”, 10th Edition, Roskill Information Services Ltd., London, (2002).
[2] National Boron Research Institute of Turkey, “Application of Boron Minerals and Compounds”, http://www.boren.gov.tr/en/expin2.htm, Last date accessed: 27.09.2007.
[3] E. L. Muetterties, “The Chemistry of Boron and Its Compunds”, John Wiley & Sons, Inc., New York, (1967).
[4] National Boron Research Institute of Turkey, “Description of Boron Minerals, World Boron Reserves”, http://www.boren.gov.tr/en/expin1.htm, Last date accessed: 27.09.2007.
[5] R. M. Adams, editor, “Boron, Metallo – Boron Compounds and Boranes”, Interscience Publishers, New York, (1964).
[6] R. J. Brotherton and H. Steinberg, editors, “Progress in Boron Chemistry Volume 2”, Pergamon Press, New York, (1970).
[7] I. E. Campbell and E. M. Sherwood, editors, “High – Temperature Materials and Technology”, John Wiley & Sons, Inc., New York, (1967).
[8] C. H. Wen, T. M. Wu and W. C. J. Wei, “Oxidation Kinetics of LaB6 in Oxygen Rich Conditions”, Journal of European Ceramic Society, Vol. 24, No. 10-11, pp. 3235-3243, (2004).
100 [9] H. Zhang, Q. Zhang, J. Tang and L. C. Qin, “Single – Crystalline CeB6 Nanowires”, Journal of the American Chemical Society, Vol. 127, No. 22, pp. 8002-8003, (2005).
[10] T. Lundström, “Structure, Defects and Properties of Some Refractory Borides”, Pure and Applied Chemistry, Vol. 57, No. 10, pp. 1383-1390, (1985).
[11] WebElements, “Calcium: thermal properties and temperatures”, http://www.webelements.com/webelements/elements/text/Ca/heat.html, Last date accessed: 27.09.2007.
[12] WebElements, “Lanthanum: thermal properties and temperatures”, http://www.webelements.com/webelements/elements/text/La/heat.html, Last date accessed: 27.09.2007.
[13] WebElements, “Lanthanum: thermal properties and temperatures”, http://www.webelements.com/webelements/elements/text/Ce/heat.html, Last date accessed: 27.09.2007.
[14] R. W. Johnson and A.H. Daane, “The Lanthanum – Boron System”, Journal of Physical Chemistry, Vol. 65, No. 6, pp. 909-915, (1961).
[15] P. K. Liao, K.E. Spear and M. E. Schlesinger, “The B-Ce (Boron-Cerium) System”, Journal of Phase Equilibria, Vol. 18, No. 3, pp. 280-283, (1997).
[16] W. A. Sanders and H. B. Probst, “Hardness of Five Borides at 1625 0C”, Journal of the American Ceramic Society, Vol. 49, No. 4, pp. 231-232, (1966).
[17] C. A. Brookes, “Plastic Deformation and Anisotropy in the Hardness of Diamond”, Nature, Vol. 228, pp. 660-661, (1970).
[18] L. Brewer, D. L. Sawyer, D. H. Templeton and C. H. Dauben, “A Study of the Refractory Borides”, Journal of the American Ceramic Society, Vol. 34, No. 6, pp. 173-179, (1951).
101 [19] W. G. Fahrenholtz, G. E. Hilmas, I. G. Talmy and J. A. Zaykoski, “Refractory Diborides of Zirconium and Hafnium”, Journal of the American Ceramic Society, Vol. 90, No. 5, pp. 1347-1364, (2007).
[20] C. Monticelli, F. Zucchi, A. Pagnoni and M. D. Colle, “Corrosion of A Zirconium Diboride/Silicon Carbide Composite in Aqueous Solutions”, Electrochimica Acta, Vol. 50, No. 16-17, pp. 3461-3469, (2005).
[21] L. Bsenko and T. Lundström, “The High Temperature Hardness of ZrB2 and HfB2”, Journal of the Less Common Metals, Vol. 34, No. 2, pp. 273-278, (1974).
[22] T. A. Parthasarathy, R. A. Rapp, M. Opeka and R. J. Kerans, “A Model for the Oxidation of ZrB2, HfB2 and TiB2”, Acta Materialia, Vol. 55, No. 17, pp. 5999-6010, (2007).
[23] M. Brach, V. Medri and A. Bellosi, “Corrosion of Pressureless Sintered ZrB2-MoSi2 Composite in H2SO4 Aqueous Solution”, Journal of the European Ceramic Society, Vol. 27, No. 2-3, pp. 1357-1360, (2007).
[24] F. Monteverde, S. Guicciardi and A. Bellosi, “Advances in Microstructure and Mechanical Properties of Zirconium Diboride Based Ceramics”, Materials Science and Engineering A, Vol. 346, No. 1-2, pp. 310- 319, (2003).
[25] J. J. Melendez-Martinez, A. Dominguez-Rodriguez, F. Monteverde, C. Melandri and G. de Portu, “Characterization and High Temperature Mechanical Properties of Zirconium Boride Based Ceramics”, Journal of the European Ceramic Society, Vol. 22, No. 14-15, pp. 2543-2549, (2002).
[26] M. M. Opeka, I. G. Talmy, E. J. Wuchina, J. A. Zaykoski and S. J. Causey, “Mechanical, Thermal and Oxidation Properties of Refractory Hafnium and Zirconium Compounds”, Journal of the European Ceramic Society, Vol. 19, No. 13-14, pp. 2405-2414, (1999).
102 [27] A. L. Chamberlain, W. G. Fahrenholtz and G. E. Hilmas, “Pressureless Sintering of Zirconium Diboride”, Journal of the American Ceramic Society, Vol. 89, No. 2, pp. 450-456, (2006).
[28] J. Inagaki, Y. Sakai, N. Uekawa, T. Kojima and K. Kakegawa, “Synthesis and Evaluation of Zr0.5Ti0.5B2 Solid Solution”, Materials Research Bulletin, Vol. 42, No. 6, pp. 1019-1027, (2007).
[29] N. L. Okamoto, M. Kusakari, K. Tanaka, H. Inui and M. Yamaguchi, “Temperature Dependence of Thermal Expansion and Elastic Constants of Single Crystals of ZrB2 and the Suitability of ZrB2 as a Substrate for GaN Film”, Journal of Applied Physics, Vol. 93, No. 1, pp. 88-93, (2003).
[30] M. L. Muolo, E. Ferrera, R. Novakovic and A. Passerone, “Wettability of Zirconium Diboride Ceramics by Ag, Cu and Their Alloys with Zr”, Scripta Materialia, Vol. 48, No. 2, pp. 191-196, (2003).
[31] R. Voytovych, A. Koltsov, F. Hodaj and N. Eustathopoulos, “Reactive vs Non – Reactive Wetting of ZrB2 by Azeotropic Au – Ni”, Acta Materialia, Vol. 55, No. 18, pp. 6316-6321, (2007).
[32] S. C. Zhang, G. E. Hilmas and W. G. Fahrenholtz, “Presureless Densification of Zirconium Diboride with Boron Carbide Additions”, Journal of the American Ceramic Society, Vol. 89, No. 5, pp. 1544-1550, (2006).
[33] F. Monteverde and A. Bellosi, “Effect of the Addition of Silicon Nitride on Sintering Behavior and Microstructure of Zirconium Diboride”, Scripta Materialia, Vol. 46, No. 3, pp. 223-228, (2002).
[34] F. Monteverde, A. Bellosi and S. Guicciardi, “Processing and Properties of Zirconium Diboride Based Composites”, Journal of the European Ceramic Society, Vol. 22, No. 3, pp. 279-288, (2002).
[35] W. G. Fahrenholtz, “The ZrB2 Volatility Diagram”, Journal of the American Ceramic Society, Vol. 89, No. 12, pp. 3509-3512, (2005).
103 [36] M. M. Opeka, I. G. Talmy and J. A. Zaykoski, “Oxidation Based Materials Selection for 2000 0C + Hypersonic Aerosurfaces: Theoretical Considerations and Historical Experience”, Journal of Materials Science, Vol. 39, No. 19, pp. 5887-5904, (2005).
[37] A. L. Chamberlain, W. G. Fahrenholtz, G. E. Hilmas and D. T. Ellerby, “High Strength Zirconium Diboride Based Ceramics”, Journal of the American Ceramic Society, Vol. 87, No. 6, pp. 1170-1172, (2004).
[38] S. Norasetthekul, P. T. Eubank, W. L. Bradley, B. Bozkurt and B. Stucker, “Use of Zirconium Diboride – Copper as an Electrode in Plasma Applications”, Journal of Materials Science, Vol. 34 No. 6, pp. 1261-1270, (1999).
[39] M. Ürgen, A. F. Çakır, O. L. Eryılmaz and C. Mitterer, “Corrosion of Zirconium Boride and Zirconium Boron Nitride Coated Steels”, Surface and Coatings Technology, Vol. 71, No. 1, pp. 60-66, (1995).
[40] M. I. Ignatov, A. V. Bogach, S. V. Demishev, V. V. Glushkov, A. V. Levchenko, Y. B. Paderno, N. V. Shitsevalova and N. E. Sluchanko, “Anomalous Charge Transport in CeB6”, Journal of Solid State Chemistry, Vol. 179, No. 9, pp. 2805-2808, (2006).
[41] N. Ogita, S. Nagai, N. Okamoto, M. Ugawata, F. Iga, M. Sera, J. Akimitsu and S. Kunii, “Raman Scattering investigation of RB6 (R = Ca, La, Ce, Pr, Sm, Gd, Dy and Yb)”, Physical Review B, Vol. 68, No. 22, pp. 224305- 224313, (2003).
[42] B. Post, D. Moskowitz and F. W. Glaser, “Borides of Rare Earth Metals”, Journal of the American Chemical Society, Vol. 78, No. 9, pp. 1800-1802, (1956).
[43] S. Kimura, H. Okamura, T. Nanba, M. Ikezawa, S. Kunii, F. Iga, N. Shimizu and T. Takabatake, “Optical Spectra of RBx (R = rare – earth, x = 4, 6, 12)”, Journal of Electron Spectroscopy and Related Phenomena, Vol. 101- 103, pp. 761-764, (1999).
104 [44] W. Waldhauser, C. Mitterer, J. Laimer and H. Störi, “Sputtered Thermionic Hexaboride Coatings”, Surface and Coatings Technology, Vol. 98, No. 1-3, pp. 1315-1323, (1998).
[45] S. S. Kher and J. T. Spencer, “Chemical Vapor Deposition of Metal Borides: 7. the Relatively Low Temperature Formation of Crystalline Lanthanum Hexaboride Thin Films from Boron Hydride Cluster Compounds by Chemical Vapor Deposition”, Journal of Physics and Chemistry of Solids, Vol. 59, No. 8, pp. 1343-1351, (1998).
[46] D. M. Goebel, Y. Hirooka and T. A. Sketchley, “Large Area Lanthanum Hexaboride Electron Emitter”, Review of Scientific Instruments, Vol. 56, No. 9, pp. 1717-1722, (1985).
[47] S. Zheng, G. Min, Z. Zou, H. Yu and J. Han, “Synthesis of Calcium Hexaboride Powder via the Reaction of Calcium Carbonate with Boron Carbide and Carbon”, Journal of the American Ceramic Society, Vol. 84, No. 11, pp. 2725-2727, (2001).
[48] J. Xu, Y. Zhao and C. Zou, “Self Catalyst Growth of LaB6 Nanowires and Nanotubes”, Chemical Physics Letters, Vol. 423, No. 1-3, pp. 138-142, (2006).
[49] H. Zhang, Q. Zhang, J. Tang and L. C. Qin, “Single – Crystalline LaB6 Nanowires”, Journal of the American Chemical Society, Vol. 127, No. 9, pp. 2862-2863, (2005).
[50] C. Chen, T. Aizawa, N. Iyi, A. Sato and S. Otani, “Structural Refinement and Thermal Expansion of Hexaborides”, Journal of Alloys and Compounds, Vol. 366, No. 1-2, pp. L6-L8, (2004).
[51] K. Flachbart, M. Reiffers, S. Molokac, A. Belling, J. Bischof, E. Konovalova and Y. Paderno, “Thermal conductivity of LaB6: The Role of Phonons”, Physica B: Condensed Matter, Vol. 263-264, pp. 749-751, (1999).
[52] I. Barin, “Thermochemical Data of Pure Substances”, VCH, New York, (1989).
105 [53] E. Storms and B. Mueller, “Phase Relationship, Vaporization and Thermodynamic Properties of the Lanthanum – Boron System”, The Journal of Physical Chemistry, Vol. 82, No. 1, pp. 51-59, (1978).
[54] C. Chen, L. T. Zhang and W. C. Zhou, “Characterization of LaB6 – ZrB2 Eutectic Composite Grown by the Floating Zone Method”, Journal of Crystal Growth, Vol. 191, No. 4, pp. 873-878, (1998).
[55] C. Chen, L. T. Zhang, W. C. Zhou and M. Q. Li, “High Temperature Oxidation of LaB6 – ZrB2 Eutectic in Situ Composite”, Acta Materialia, Vol. 47, No. 6, pp. 1945-1952, (1999).
[56] C. Y. Zou, Y. M. Zhao and J. Q. Xu, “Synthesis of Single Crystalline CeB6 Nanowires”, Journal of Crystal Growth, Vol. 291, No. 1, pp. 112-116, (2006).
[57] Applied Physics Technologies, “Refractory Materials and Electron Sources Web Brochure”, http://www.a-p-tech.com/Webbrochure.pdf, Last date accessed: 27.09.2007.
[58] R. Gao, G. Min, H. Yu, S. Zheng, Q. Lu, J. Han and W. Wang, “Fabrication and Oxidation Behavior of LaB6 – ZrB2 composites”, Ceramics International, Vol. 31, No. 1, pp. 15-19, (2005).
[59] K. C. Patil, S. T. Aruna and Tanu Mimani, “Combustion Synthesis: An Update”, Current Opinion in Solid State and Materials Science, Vol. 6, No. 6, pp. 507-512, (2002).
[60] A. Varma and A. S. Mukasyan, “Combustion Synthesis of Advanced Materials: Fundamentals and Applications”, Korean Journal of Chemical Engineering, Vol. 21, No. 2, pp. 527-536, (2004).
[61] A. S. Rogachev, A. S. Mukasyan and A. Varma, “Volume Combustion Modes in Heterogeneous Reaction System”, Journal of Materials Synthesis and Processing, Vol. 10, No. 1, pp. 31-36, (2002).
106 [62] K. C. Patil, S. T. Aruna and S. Ekambaram, “Combustion Synthesis”, Current Opinion in Solid State and Materials Science, Vol. 2, No. 2, pp. 158- 165, (1997).
[63] C. Suryanarayana, “Mechanical Alloying and Milling”, Marcel Dekker, New York, (2004).
[64] A. W. Weimer, “Carbide, Nitride and Boride Materials Synthesis and Processing”, Chapman and Hall, London, (1997).
[65] E. T. Turkdogan, “Physical Chemistry of High Temperature Technology”, Academic Press, Inc., New York, (1980).
[66] A. Makino and C. K. Law, “SHS Combustion Characteristics of Several Ceramics and Intermetallic Compounds”, Journal of the American Ceramic Society, Vol. 77, No. 3, pp. 778-786, (1994).
[67] H. S. Cooper, “Refractory Metal Borides”, U. S. Patent No. 2678870, (1954).
[68] I. E. Campbell, C. F. Powell, D. H. Nowicki and B. W. Gonser, “The Vapor Phase Deposition of Refractory Materials I. General Conditions and Apparatus”, Journal of the Electrochemical Society, Vol. 96, pp. 318-333, (1949).
[69] J. J. Gebhardt and R. F. Cree, “Vapor Deposited Borides of Group IVA Metals”, Journal of the American Ceramic Society, Vol. 48, No. 5, pp. 262- 267, (1965).
[70] A. Wang and G. Male, “Experimental investigation of the Zr – B – Cl – H CVD System”, Journal of the European Ceramic Society, Vol. 11, No. 3, pp. 241-251, (1993).
[71] L. Chen, Y. Gu, Z. Yang, L. Shi, J. Ma and Y. Qian, “Preparation and Some Properties of Nanocrystalline ZrB2 Powders”, Scripta Materialia, Vol. 50, No. 7, pp. 959-961, (2004).
107 [72] H. Zhao, Y. He and Z. Jin, “Preparation of Zirconium Boride Powder”, Journal of the American Ceramic Society, Vol. 78, No. 9, pp. 2534-2536, (1995).
[73] S. K. Mishra, S. Das and L. C. Pathak, “Defect Structures in Zirconium Diboride Powder Prepared by Self Propagated High Temperature Synthesis”, Materials Science and Engineering A, Vol. 364, No. 1-2, pp. 249- 255, (2004).
[74] S. K. Mishra, R. K. P. Rupa, S. K. Das and V. Shcherbakov, “Effect of Titanium Diluent on the Fabrication of Al2O3 – ZrB2 Composite by SHS Dynamic Compaction”, Composites Science and Technology, Vol. 67, No. 7- 8, pp. 1734-1739, (2007).
[75] S. K. Mishra, S. K. Das, P. Ramachandrarao, D. Y. Belov and S. Mamyan, “Synthesis of Zirconium Diboride – Alumina Composite by the Self Propagating High Temperature Synthesis Process”, Metallurgical and Materials Transactions A, Vol. 34, No. 9, pp. 1979-1983, (2003).
[76] S. K. Mishra, S.K. Das and P. Ramachandrarao, “Self Propagating High Temperature Synthesis of a Zirconium Diboride – Alumina Composite: A Dynamic X – ray Diffraction Study”, Philosophical Magazine Letters, Vol. 84, No. 1, pp. 41-46, (2004).
[77] V. Sundaram, K. V. Logan and R. F. Speyer, “Reaction Path in the Formation of Titanium Diboride by a Magnesium thermite Process”, British Ceramic Society Transactions, Vol. 56, pp. 111-120, (1995).
[78] W. Weimin, F. Zhengyi, W. Hao and Y. Runzhang, “Chemistry Reaction Processes during Combustion Synthesis of B2O3 – TiO2 – Mg System”, Journal of Materials Processing Technology, Vol. 128, No. 1-3, pp. 162-168, (2002).
[79] A. K. Khanra, “Reaction Chemistry during Self Propagating High Temperature Synthesis (SHS) of H3BO3 – ZrO2 – Mg System”, Materials Research Bulletin, Vol. 42, No. 12, pp. 2224-2229, (2007).
108 [80] A. K. Khanra, L. C. Pathak and M. M. Godkhindi, “Double SHS of ZrB2 Powder”, Journal of Materials Processing Technology, (2007) (in press).
[81] N. Setoudeh and N. J. Welham, “Formation of Zirconium Diboride (ZrB2) by Room Temperature Mechanochemical Reaction between ZrO2, B2O3 and Mg”, Journal of Alloys and Compounds, Vol. 420, No. 1-2, pp. 225- 228, (2006).
[82] W. Wenyuan, X. Jingyu, P. Kewu and T. Ganfeng, “High Temperature Chemical Reaction of La2O3 in H3BO3 – C System”, Journal of Rare Earths, Vol. 25, No. 3, pp. 282-285, (2007).
[83] L. Y. Markovskii, N. V. Vekshina, E. T. Bezruk, G. E. Sukhareva and T. K. Voevodskaya, “A Magnesium – Thermic Method for the Preparation of Metal Borides”, Powder Metallurgy and Metal Ceramics, Vol. 8, No. 5, pp. 350-354, (1969).
[84] “Retsch Ball Mills Brochure”, Retsch GmbH, Haan, (2007).
[85] Retsch GmbH, “Planetary Ball Mill PM 100 Versions and Accessories”, http://www.retsch.com/products/milling/ball-mills/pm-100/versions- accessories/, Last date accessed: 27.09.2007.
[86] E. Bilgi, “Production of Titanium Diboride”, M. S. Thesis, METU, Ankara, (2007).
109