Production of transparent ceramics (review) - Springer Link

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Glass and Ceramics. Vol. 52, Nos. l -2, 1995. Science for Ceramics ..... ceramics," Trudy MKhTI Ira. Mendeleeva, Issue 18, 78-89. (1981). 6. R. L. CoNeĀ ...
Glass and Ceramics

Vol. 52, Nos. l - 2 , 1995

Science for Ceramics M a n u f a c t u r e UDC 666.3:535.345.1.002.2

PRODUCTION OF TRANSPARENT CERAMICS (REVIEW) A. V. B e l y a k o v a and A. N. S u k h o z h a k ~ Translated from Steklo i Keramika, Nos. 1 - 2, pp. 14 - 20, January - February, 1995. A phenomenological description of the processes occurring at different stages of manufacturing transparent ceramics from preparing the powder to firing is given from the standpoint of the irreversible and nonequilibrium processes developed by Prigogine and his school. Principles for choosing compacting additions and techniques which impede powder aggregation and local compaction in drying and firing, and the formation of closed intercrystalline and intracrystalline pores in sintering are considered.

ramics, water vapor, nitrogen, and carbon can penetrate closed pores. This makes closed pores the most dangerous factor in production of transparent ceramics. In this connection, the prerequisites for their appearance should be excluded at all manufacturing stages. Crystal boundaries should be thin and optically perfect. The presence of a second phase on the crystal boundaries which has different optical properties from the main crystalline phase leads to reflection and refraction of light and makes the ceramic less transparent. For this reason, transparent ceramics are obtained from raw materials of h i ~ purity and the amount of additive is chosen so that they completely dissolve in the solid solution with the main phase. The crystals in ceramics should be optically perfect. The main requirement is the absence of optical defects, namely, pores, inclusions of the second solid phase, aggregate boundaries, and dislocations. In ceramics from optically anisotropic crystals (crystal systems other than cubic), an additional scattering of liglat arises on the boundaries because of their arbitrary crystallographic orientation [1]. Some investigators think that the size of crystals in transparent ceramics should be minimal in order to reduce the probability that pores will be captured by growing crystals and to shorten the diffusion path for removal of intracrystalline pores [2]. However, this increases the lengh of the boundaries and thus the light scattering on them. These considerations lead us to the requirements to be observed by manufacturers of transparent ceramics. It is necessary to exclude at all manufacturing stages the causes of the formation of intracrystalline pores and evolution of the second phase on crystal boundaries and inside crystals. Transparent ceramics are commonly produced with the use of special compacting additives [5]. The additives accelerate diffusion processes, ensure removal of intracrystalline and intercrystalline pores, and improve the structure of crys-

Transparent ceramics (transparent polycrystalline materials) have a lower transmittance than monocrystals, but are more heat resistant and sometimes even stronger. This makes them a less expensive material, especially when the articles have complex shapes [1, 2], and explains the growing interest in such materials. To be transparent, the ceramic should be poreless and have optically perfect crystal boundaries and crystals. The surface of a pore is a boundary between phases with sharply different optical properties, and therefore, it intensely reflects and refracts light. A large number of pores makes ceramics opaque. Pores may be intercrystalline or intracrystalline. The elimination of intracrystalline pores, even if they are submicron in size, is a much longer process than elimination of closed intercrystalline pores. Intercrystalline pores occur at crystal boundaries which are sinks of vacancies, and this makes their removal easier. Intracrystalline pores may acquire equilibrium faceting [3], which makes their removal more difficult. The impurities that form a gaseous phase during heat treatment and create excessive pressure inside closed pores also preserve the intracrystalline porosity. Compacting additions are concentrated on the surface of the pores and accelerate the diffusion processes that lead to their equilibrium faceting. An elevated concentration of the compacting additive, i.e., magnesium oxide, was found on the surface of the residual closed pores of translucent ceramics from aluminum oxide [4]. Magnesium oxide has a much h i ~ e r saturated vapor pressure than aluminum oxide, and therefore, when fired in oxygen, the former creates excessive pressure in the pores. In the process of transformation of hydroxides into oxides and burning of the temporary technological binder from oxide ceD. I. Mendeleev Russian Chemical Technological University, Moscow, Russia.

14 0361-7610/95/0102-0014512.50 9 1995 Plenum PuNishing Corporation

Production of Transparent Ceramics

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tal boundaries and crystals. The amount of additives is chosen so that they will not evolve as a new solid phase, but will enter the solid solution together with the main crystalline phase. In other words, the concentration of the additive should be less than its ultimate solubility in the main phase at the firing temperature. The additives should impede crystal growth, i.e., slow down movement of their boundaries [6, 7]. In this case, the probability that a pore will be separated from the boundary and transformed into an intracrystalline pore is reduced. Moreover, the additives should accelerate surface diffusion and thus stimulate elimination of pores situated along crystal boundaries. The additives are chosen using defect formation reactions [8, 9]. Let us consi'der the principles for choosing additives for production of transparent ceramics frofn oxides. If diffusion in crystals occurs by the mechanism of vacancies, which is typical for oxides, the vacancies should be both in the cation sublattice (metal sublattice) and in the oxygen sublattice. At sintering temperatures, oxygen is more volatile in oxides than in metals. As a result, oxygen vacancies Vo" are formed in the oxygen sublattice, especially in boundary layers of the crystal, i.e., MeO --+ Me~e + Vo" + I/2 0 2 + 2e'.

(1)

An increase in the concentration of oxygen vacancies [Vo"] in accordance with Eq. (3) leads to a reduction in the concentration of vacancies in the cation sublattice by the Schottky equation

[V~e]

MeO

0