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Alumina : Sintering and Optical Properties

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Alumina : Sintering and Optical Properties Alumina : sintering and optical properties Citation for published version (APA): Peelen, J. G. J. (1977). Alumina : sintering and optical properties. Technische Hogeschool Eindhoven. https://doi.org/10.6100/IR4212 DOI: 10.6100/IR4212 Document status and date: Published: 01/01/1977 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 26. Sep. 2021 ALUMINA: SINTERING AND OPTICAL PROPERTIES PROEFSCHRIFT TER VERKRlJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL EINDHOVEN, OP GEZAG VAN DE RECTOR MAGNIFICUS, PROF. DR. P. VAN DER LEEDEN, VQOR EEN COMMISSIE AANGEWEZEN DOOR HET COLLEGE VAN DEKANEN IN HET OPEN· BAAR TE VERDEDIGEN OP DINSDAG 17 MEl 1977 TE 16.00 UUR DOOR JAN GERARD JACOB PEELEN GEBOREN TE RENKUM Dl1' PROEPSCHRlFT IS GOEDGEKEURD DOOR DE PROMOTOREN PROF. JR. A.L. STUIJTS EN DR. R. METSELAAR Alln Beitsche Aan Evelien, Marc en Janine Aan mijn oudas DANKWOORD lIe! onderzoek beschreven in dit procfschrift is uitgevoerd op hct Natuurkundig Laboratorium van de N.Y. Philips' Gioeibmpenfabrieken te Eindhoven. lk ben de dirccti.;; van dit iabocatoriul1l erkentelijk voor de mij geboden gelegcnhcid om het onder7.oek in dele vorm te publiceren, Gaarne wil ik mijn dank betuigen aan allen, die bij de uitvocring van de experimcn­ ten betrokken zijn geweest en aan de lotstandkoming van dit procfschrift hebben bijgcdragcn, Ecn belangrijk deel van het ondcrzock is geda~n in samenwcrking met Dr. R. Metselaar. Yoor dCl.e samenwe!'king ben ik hem vcc1 dank verschuldigd. MijI\ dank gaat verder uit naar mijn coHcga's, die het manuscript van dlt proefschrift krilisch hcbbcn doorgelezen. CONTENTS I. INTRODUCTION 1.1 Properties of alumina . 1.2 Application of alumina as a lamp envelope 2 1.3 The present investigation 2 REFERENCES . 4 2. SINrERJNG OF ALUMlNA AND THE lNFLUENCE OF DOPES 5 2.1 Short introduction to the sintering process . 5 .2.2 Sintering of alumina, Review of the literature 8 2.2.1 lnfluence of impurities. 8 2.2.2 Influence of grain boundaries. 'I 2.2.3 Influence of additives on sintering. Defect structure. 11 2.2.4 Influence of additives on the microstructure. , . 12 2.2.5 Conclusion ............. 14 2,3 Influence of MgO on the evolution of the microstructure 14 2.3.1 Introduction . 14 2.3.2 Experimental prooedures . 16 2.3.3 ReSults . , . , . , 18 2.3.3.1 Influence of MgO on the dens.ity 18 2.3.3.2 lnfIuence of MgO on the grain size 19 2.3.3.3 lnl1uence of extra addition ofeaO and Y103 24 23.3.4 Auger spectroscopy 26 2.3.4 Discussion . 29 2.3,5 Conclusion. 31 REFERENCES . 32 3. HOT PRESSING OF ALUMINA 37 3.1 Introduction to hot pressing 37 3.2 Continuous hot pressing 38 3.3 Results and discussion 40 3.3.1 Influence of the atmosphere 41 3.3.2 Influence of the hot-pressing parameters 43 3.3.3 Influence of powder properties . 45 3.3.4 The mic\"ostructure of hot-pressed alumina 46 3.3.5 Crystallographic texture of the grains 49 3.4 Conclusion. 52 REFERENCES . 53 4. OPTICAL PROPERTIES OF ALUMINA 55 4.1 General introduction . 55 4.1.1 Review of the literature 56 4.1.2 Factors contributing to transmission losses 57 4.2 The in-ljne trans)ni~siQn 59 4.2.1 Scattering theory. 59 4.2.2 Calculation of the in-line tf<lmmission 63 4.2.2.1 Pores with a fixed radius 63 4.2.2.2 Porc siLC diSlribution 68 4.2.2.3 Inl1ucIlcc of the pore size distribution on the scattering coefficient . 69 4.2.3 Measurements of the in-line transmission 71 4.2.3.1 IntlL!cnce ,)f the pore siLc . 73 4.2.3.2 Influence of the MgO content 75 4.2.3.3 Int1uence of extra addition of CaO and Y1 0 3 77 4.2.4 Dekrminalion of microstructural paramotcrs [rom the measured transmission spectra. 78 4.2.5 Conclusion. 83 4.3 Transparent and lranslucent alumina 83 4.}.1 Introduction , 8} 4.3.2 Theoretical c()nsiderations 85 433 Experimental part 86 4.3.4 Rc~ults and disC\lssion 89 4.3.S Conclusion, 92 REFERENCES 94 SUMMARY 97 SAMENV ATTlN(~ 99 L INTRODUCTION 1.1 Properties of alumina In ceramic literature the term "alumina" is used rather loosely to denote L aluminous material of all types taken collectively; 2. the anhydrous and hydrous aluminium oxides taken indiscriminately; 3. the calcined or substantially water-free aluminium oxides, without distinguishing the phases present, and 4. corundum Dr alpha alumina, specifically. Gitzen I) in his standard review on alumina uses the term in the sense of the second definition. In the present work we wHl use the term in the SenSe of the fourth defini· tion. Many other phases like gamma, delta, cIa, kappa, chi, rho, theta alumina, all corresponding to the molecular formula Al2 0 3 , are described in the literature. They are all transition phases and some of them are of doubtful existence or the differences are mainly baSed on somewhat subtle X-ray diffraction differences. Beta alumina is not a monotropic form but it is a mixed oxide containing alkali or alkaline earth atoms. More information on these compounds can be found in the literature 1,2). Alpha alumina, the only thermally stable oxide of aluminium, has the corundum structure. This structure may be described as a slightly distorted hexagonal close­ packing of oxygen ions. The aluminium ions occupy two-thirds of the octahedral inlerstices, while one third is empty. This structure is extremely stable. The formation energy of alumina is about 400 kcal/mol, only exceeded by the oxides of some rarc earth metals, like La. Consequences are an extremely low vapour pressure and a high melting point (2045 °C). The deviation from the stoichiometric composition of alumina at normal atmospheric conditi(ms is < 5 x 10'$ even at 1600 °c ~). Alumina has a unique combination of useful electrical, mechanical and chemical jJroperties. Electrically it possesses high resistivity (around 10 17 nem at room temperature), good dielectric strength and a low dielectriC loSS factor at high frequencies. These electrical pr<Jpertjes were already applied early in the technical development of sintered alumina as insulators for spark plugs. The electrical con­ ductivity is very sensitive to the purity of the oxide. Mechanically, alumina possesses great hardness, resistan~e to abrasive wear and dimensional stability. The strength properties of ~i!1tered alumina are strongly influenced by the microstructure: porosity, grain size, pore size, second phases. In the application of alumina as cutting tools use is made of its special mechanical properties. Finally, alumina i~ inert against attack from most chcmicals and can be used in severe 2 environments. Even sodium does not provoke any appreciable reaction. Alumina maintains these characteristics to high temperatures: the working temperature can be \lP to 1500 °c. I.! Application of alumina as a lamp envelope The combination of the above·mentioned properties makes sintered alumina a very suitable material to be used as a lamp envelope in the high·pressure sodium lamp. This lamp is based on the phenomenon that the n{)[mally narrow spectrum of sodium light can be broadened to cover most of the visible spectrum, if the sodium pressure and temperature inside the lamp are increased 4).The lamp produces a golden.white light with a very high efficiency. The material of the discharge tube has to resist the ,mack from ~odium vapour at 1250 QC, and of course the light loss in the visible part of the spectrum should be as low as possible. To meet the latter requirement a material of very low porosity has to be used. In the fifties t.he problem in sintcring alumina waS the effect. of discontinuous grain growth. The somewhat larger grains with more strongly curved boundaries tend to grow very fast, so that pores become enclosed in the grains and are unable to diffuse out.
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