Low-Temperature Synthesis of Calcium Hexaboride Nanoparticles Via Magnesiothermic Reduction in Molten Salt

Journal of the Ceramic Society of Japan 125 [12] 866-871 2017 Full paper

Low-temperature synthesis of calcium hexaboride nanoparticles via magnesiothermic reduction in molten salt

Ke BAO, Liangxu LIN, Hong CHANG and Shaowei ZHANG³

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK

Calcium hexaboride (CaB6) nanoparticles were prepared via low temperature magnesiothermic reduction of CaO and B2O3 in molten NaCl, KCl or CaCl2. The effects of salt type, Mg amount, and firing temperature and time on the reaction extents were examined, and the responsible reaction mechanisms were discussed. Under an identical firing condition, CaCl2 facilitated the overall synthesis more effectively than the other two salts. In the case of using 20 mol % excessive Mg, phase-pure CaB6 nanoparticles of ³50 nm were formed in CaCl2 after 6 h at 800°C. The “dissolution-precipitation” mechanism is believed to be responsible for the molten salt synthesis of high quality nanosized CaB6 particles at such a low temperature. ©2017 The Ceramic Society of Japan. All rights reserved.

Key-words : Calcium hexaboride, Molten salt synthesis, Reduction, Boron oxide, Calcium oxide

[Received May 3, 2017; Accepted September 20, 2017]

cursors. The effects of salt type, Mg amount, and firing temper- 1. Introduction ature and time on the CaB6 formation were investigated, based on Calcium hexaboride (CaB6) is a divalent alkaline-earth cubic which, the synthesis conditions optimized, and the responsible hexaboride characterized by low density (2.45 g/cm3), high melt- mechanisms underpinning proposed. ing point (2235°C), high hardness/Young’s modulus [27 GPa (Hv)/379 GPa], high electrical conductivity, as well as good 2. Experimental procedures chemical stability. In addition, it has several other unique prop- 2.1 Sample preparation erties, including low work function, stable specific resistance, Chemical reagent grade CaO (99.9%), B2O3 (99.98%) and Mg low thermal expansion coefficient (within certain temperature (²99%) powders were used as the main raw materials. Salts ranges), high-temperature weak ferromagnetism, and high neu- used to form molten reaction media were KCl (²99%), NaCl 1)4) tron absorbability. These excellent properties make CaB6 a (²99%) and CaCl2 (²96%). Except CaCl2 from Thermo Fisher highly attractive candidate material for many important applica- Scientific (Geel, Belgium), all were supplied by Sigma-Aldrich tions, e.g., as a boron-source in manufacturing boron-alloyed (Gillingham, UK). steel, a deoxidizing agent for production of oxygen-free copper, 0.003 mol CaO and 0.009 mol B2O3 were pre-mixed with the an antioxidant for carbon-containing refractories, an abrasive or stoichiometric [corresponding to the overall Reaction (1), i.e., wear-resistant material, a neutron absorbent, and a ferromagnet- 0.03 mol] or nonstoichiometric amounts (1020 mol % extra) of ism material.5)11) Mg, in a mortar and pestle (250 ml), before being further com- The techniques/methodologies used to date for the CaB6 bined with 15 g of KCl, NaCl or CaCl2. Each resultant powder powder synthesis include mainly, direct elemental reaction,12) mixture was placed in a covered alumina crucible (L82 mm © borothermal or borocarbide reduction,13)19) mechanochemical W40 mm © H24 mm) which was heated in an alumina tube (I/D treatment or combustion assisted metallothermic reduction,3),20) 60 mm) furnace in flowing argon (50 ml/min) at 3 °C/min to a solution-based processing,21)23) molten salt electrolysis,10),24),25) target temperature between 800 and 1000°C and held for 48h floating zone method,26) and aluminum flux method.4) Unfortu- before furnace-cooling to room temperature. nately, these techniques suffer from at least one of the following CaO þ 3B O þ 10Mg ¼ CaB þ 10MgO ð1Þ drawbacks: use of expensive and/or hazardous raw materials/ 2 3 6 precursors (e.g., elemental boron, calcium and NaBH4), require- Salts and the byproduct MgO remained in the reacted mass ment of specialty equipment/reaction vessels and high process- were removed via repeated-washing with hot distilled water and ing temperature/pressure, and/or long processing time, heavy 2 h additional leaching with 1 M HCl. The resultant purified agglomeration and poor purity of final products, and high overall product powder was collected via centrifugation and then rinsed production cost. with deionized water several times (until no Cl¹ was detectable Recently, the present authors have successfully synthesized by AgNO3 in the centrifugal liquid) before overnight oven-drying ZrB2, LaB6 and TiB2 ultrafine powders via magnesiothermic at 80°C. reduction of oxide-based raw materials in molten chloride salts.27)29) In this work, such a molten salt synthesis (MSS) 2.2 Sample characterization technique was further extended and modified, aiming to provide Powder X-ray diffraction (XRD) analysis (X-ray diffractom- an alternative approach to low temperature synthesis of high- eter, D8 Advance, Bruker, Germany) was performed to identify quality CaB6 nanoparticles from cost-effective oxide-based pre- crystalline phases in samples. XRD spectra were collected at 40 mA and 40 kV using Cu K¡ radiation ( = 1.5418 ¡), at a scan ³ Corresponding author: S. Zhang; E-mail: [email protected] rate of 2° (2ª)/min with a step size of 0.03°. ICDD cards used for

phase identification are CaB6 (35-741) and Mg3B2O6 (38-1475). formation. This was due to the evaporation loss of Mg at this A scanning electron microscope (SEM, Nova 600; FEI, Hillsboro, relatively high temperature,32)34) as similarly observed and dis- OR, USA) and a transmission electron microscope (TEM, JEM cussed in our previous MSS studies,27)29) and further verified 2100; JEOL, Tokyo, Japan) were used to examine microstructures below. and phase morphologies of samples, and the linked energy- dispersive X-ray spectroscopy (EDS; Oxford Instrument, Oxford, 3.3 CaB6 formation using excessive Mg UK) was used for semi-quantitative analysis of elemental com- To address the issue of Mg evaporation loss mentioned above, positions of phases in samples. the effect of using more Mg than the stoichiometric amount, on the CaB6 formation was further investigated. Figure 3,asan 3. Results and discussion example, illustrates phase evolution in samples after 4 h firing in 3.1 CaB6 formation in different salts CaCl2 at 1000°C, with excessive amount of Mg. Use of 10 mol % Figure 1 shows XRD patterns of samples of stoichiometric more Mg [Figs. 3(a)3(b)] resulted in substantial decrease in composition after 4 h firing in different salts at 850°C. In the Mg3B2O6 and increase in CaB6, revealing quite positive effects case of using NaCl or KCl [Figs. 1(a) and 1(b)], the intermediate from the Mg compensation. Upon using 20 mol % excessive Mg3B2O6 appeared as the primary phase (see Section 3.6 below Mg [Fig. 3(c)], Mg3B2O6 disappeared completely, and only CaB6 for detailed discussion on its formation), along with a little CaB6, was formed, confirming the completion of the magnesiothemric indicating the overall low extents of the magnesiothermic reduc- reduction and CaB6 formation reaction. tion and CaB6 formation in either of these two salts. Mg3B2O6 was also identified with CaB6 in the case of using CaCl2 [Fig. 1(c)], 3.4 Effect of firing time on CaB6 formation however, the former decreased whereas the latter increased sub- Figures 46 illustrate phase formation in samples with differ- stantially, compared to the cases of using the other two salts, ent amounts of Mg, after firing in CaCl2 at 800°C for differ- revealing the enhanced reaction extents, and more importantly, the ent times. In samples of stoichiometric composition (i.e., with best effect of CaCl2 among the three salts in accelerating the 0 mol % excessive Mg) (Fig. 4), CaB6 slightly increased, whereas magnesiothermic reduction as well as CaB6 formation. Mg3B2O6 slightly decreased, upon increasing the firing time from As confirmed by our recent studies on MSS of other borides,27)29) the solubility of a reactant in the molten salt medium is key in the MSS process. The best accelerating-effect of molten CaCl2 demonstrated by Fig. 1 and described above was attrib- utable mainly to the greater solubility values of CaO and Mg in it than in the other two salts,30),31) which facilitated the magnesiothermic reduction and other reactions involved in CaB6 formation (see Section 3.6 below).

3.2 CaB6 formation at different temperatures Given in Fig. 2 are XRD patterns of samples of stoichiometric composition after 4 h firing in CaCl2 at different temperatures. CaB6 already started to form evidently at 800°C [Fig. 2(a)], but much Mg3B2O6 still remained. Increasing the temperature to 900°C [Fig. 2(b)] led to considerable decrease in Mg3B2O6 and increase in CaB6, suggesting much improved reaction extents. However, further increasing the temperature to 1000°C [Fig. 2(c)] adversely caused the increase in Mg3B2O6 but decrease in CaB6, Fig. 2. XRD patterns of samples of stoichiometric composition after 4 h i.e., reduced extents of the magnesiothermic reduction and CaB6 firing in CaCl2 at (a) 800, (b) 900, and (c) 1000°C.

Fig. 1. XRD patterns of samples of stoichiometric composition after 4 h Fig. 3. XRD patterns of samples using (a) 0, (b) 10, and (c) 20 mol % ﬁring at 850°C in (a) NaCl, (b) KCl, and (c) CaCl2. excessive Mg, after 4 h ﬁring in CaCl2 at 1000°C.

867 JCS-Japan Bao et al.: Low-temperature synthesis of calcium hexaboride nanoparticles via magnesiothermic reduction in molten salt

samples fired with 15 mol % excessive Mg [Fig. 5], Mg3B2O6 decreased and CaB6 increased more evidently with increasing the time. After 8 h, CaB6 was formed as the primary phase and only small amounts of Mg3B2O6 remained [Fig. 5(c)]. These results indicated that the increase in firing time had more significant effects on the CaB6 formation in samples using excessive Mg than in samples using the stoichiometric amount of Mg. Never- theless, phase-pure CaB6 was still not obtained under either of these conditions, which was addressed by further optimizing the firing conditions. As shown in Fig. 6, upon further increasing the excessive amount of Mg to 20 mol %, the firing time showed much more sensitive effect on the reaction extents. For instance, by increasing the firing time from 4 h by just 1 h [Figs. 6(a) and 6(b)], CaB6 increased and Mg3B2O6 decreased considerably. Furthermore, after another 1 h, Mg3B2O6 completely disappeared, and only Fig. 4. XRD patterns of samples using stoichiometric amount of Mg phase-pure CaB6 was formed [Fig. 6(c)]. (i.e., 0 mol % excessive Mg) after firing in CaCl2 at 800°C for (a) 4, (b) 6, or (c) 8 h. 3.5 Microstructure of CaB6 product powder As demonstrated in Figs. 3(c) and 6(c) and discussed above, when 20 mol % excessive Mg was used with CaCl2, phase-pure CaB6 could be prepared after either 4 h firing at 1000°C or 6 h at 800°C. Figure 7 presents SEM images and EDS of product powders synthesized under these two conditions. In the sample resultant from 6 h firing at 800°C [Fig. 7(a)], nanosized particles <100 nm were formed, though they were loosely agglomerated together. By contrast, in the sample resultant from 4 h firing at 1000°C [Fig. 7(b)], nanosized particles (<100 nm) coexisted with submicron-sized particles (up to 500 nm), indicating significant grain growth at this temperature, despite the shorter firing time. The size change with the grain growth also suggested that the product particle’s size could be controlled at least to certain extents by using the present MSS method. Only Ca and B, along with a trace of O contamination, were detected by EDS [Figs. 7(c) and 7(d)] in the samples fired under both conditions, this, in addition to the XRD results in Figs. 3(c) and 6(c), further fi Fig. 5. XRD patterns of samples using 15 mol % excessive Mg after con rmed the formation of essentially phase-pure CaB6. TEM firing in CaCl2 at 800°C for (a) 4, (b) 6, or (c) 8 h. examination further revealed that CaB6 particles resultant from 6h firing at 800°C had an average size of ³50 nm [Fig. 8(a)]. Moreover, EDS only detected Ca and B in these particles, along with tiny O contamination [Fig. 8(b)], revealing additionally their high purity.

3.6 Further discussion and reaction mechanisms As mentioned earlier, the overall synthesis process of CaB6 can be indicated by Reaction (1). Nevertheless, the actual formation process should be involved with several individual steps. Based on reaction extents and phase formations in different samples fired under different conditions (Figs. 18), and previous MSS studies on other borides,27)29) the main reactions involved in each individual step can be detailed as follows. At testing temperatures (8001000°C), CaCl2 initially melted, forming a desirable molten salt medium in which both Mg and CaO partially dissolved,30),31) and subsequently reacted, forming aCaMg binary liquid [Ca in this liquid is indicated by (Ca)] and MgO [Reaction (2)].35),36) Fig. 6. XRD patterns of samples using 20 mol % excessive Mg after ð Þþ ð Þ¼ð Þþ ð Þ firing in CaCl2 at 800°C for (a) 4, (b) 5, or (c) 6 h. CaO dissolved Mg dissolved Ca MgO 2 On the other hand, the dissolved Mg also diffused through the molten salt medium to the liquid (dissolved or undissolved in 4 to 6 h [Figs. 4(a) and 4(b)]. On further extending the time to CaCl2)B2O3 and then reduced it to B [Reaction (3)]. 8 h, however, the former only marginally increased and the latter 3Mg ðdissolvedÞþB O ð1Þ¼2B þ 3MgO ð3Þ marginally decreased [Fig. 4(c)]. On the other hand, in the 2 3

868 Journal of the Ceramic Society of Japan 125 [12] 866-871 2017 JCS-Japan

Fig. 7. SEM images (a, b) and corresponding EDS results (c, d) of CaB6 powders resultant from (a, c) 6 h ﬁring at 800°C and (b, d) 4 h ﬁring at 1000°C, in CaCl2.

Fig. 8. TEM and corresponding EDS of CaB6 nanoparticles resultant from 6 h ﬁring in CaCl2 at 800°C (the small C and Cu peaks arose from the carbon ﬁlm-Cu TEM grid).

Before all of the B2O3 was consumed, the unreduced B2O3 Mg B O þ 3Mg ðdissolvedÞ¼2B þ 6MgO ð5Þ would react with the MgO from Reactions (2) and (3) forming the 3 2 6 27)29) intermediate Mg3B2O6 [Reaction (4)] (Figs. 16). Finally, B resultant from Reactions (3) and (5) reacted with (Ca) from Reaction (2), in molten CaCl2, forming the desired B2O3 ðunreducedÞþ3MgO ¼ Mg3B2O6 ð4Þ CaB6 [Reaction (6)]. With improving/optimizing the synthesis conditions, e.g., ðCaÞþ6B ¼ CaB ð6Þ increasing ﬁring temperature or time, and using excessive Mg 6 37) (Figs. 16), the formed Mg3B2O6 would be further reduced by According to the CaOB2O3 binary phase diagram, unreact- Mg dissolved in CaCl2, producing more B and releasing MgO ed CaO could also react with unreduced B2O3, forming inter- [Reaction (5)]. mediate calcium borates such as Ca3B2O6 [Reaction (7)].

869 JCS-Japan Bao et al.: Low-temperature synthesis of calcium hexaboride nanoparticles via magnesiothermic reduction in molten salt

signiﬁcant grain growth occurred in this case, resulting in a mixture of nanosized (<100 nm) and submicron-sized (up to 500 nm) particles. The “dissolution-precipitation” mechanism is con- sidered to be responsible for the overall MSS process and low temperature formation of nanosized CaB6 particles.

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