UDC 666.221.6 ON THE POSSIBILITY OF PRECISION CONTROL OF THE LINEAR THERMAL EXPANSION COEFFICIENT OF TRANSPARENT LITHIUM-ALUMINUM- SILICATE SITALS NEAR ZERO VALUES V. N. Sigaev, 1, 3 V. I. Savinkov, 1 G. Yu. Shakhgil’dyan, 1 A. S. Naumov, 1 S. V. Lotarev, 1 N. N. Klimenko, 1 N. V. Golubev, 1 and M. Yu. Presnyakov 2 Translated from Steklo i Keramika, No. 12, pp. 11 – 16, December, 2019. It is shown that the linear thermal expansion coefficient (CLTE) for sitalls based on the lithium aluminosilicate system can be precisely controlled near values close to zero in the temperature range from –80 to +500°C. It was found that heat-treatment for several hours at temperatures corresponding to the stage of nucleation of a crystalline phase makes it possible, after a lengthy incubation period, to change the sign of the CLTE of sitall from positive to negative, while preserving its sign constancy in the entire temperature interval near zero va- lues of the CLTE. Key words: crystallization, sitall, transparent sitall, low linear thermal expansion coefficient, lithium alumi- nosilicate system, eucryptite. Sitalls based on the lithium aluminosilicate (LAS) sys- tem are actively used in different industries. Ordinarily, they form the base of cook tops, protective screens for fireplaces, and substrates for 3D printers, and in engineering they are ir- replaceable as telescope mirrors and laser gyro housings [1 – 3]. These materials are of interest because they afford the possibility of combining optical transparency with high-heat resistance, which is achieved by low values of the thermal linear expansion coefficient (CLTE). As technology advances, the requirements of the em- ployed sitalls become more stringent. This concerns mostly the possibilities of precise matching of the CLTE material in a specified temperature range. New technologies, such as deep-UV photolithography, require the presence of substrates with practically no dimensional changes of the components in a specified temperature range. The requirements of laser gyros are likewise continuously increasing and correspond- ingly the requirements of optical and thermo-mechanical properties of sitalls are increasing. In addition, often, the CLTE must be kept constant when crossing from a region of negative into a region of positive temperatures [4]. Also of special interest is the possibility of direct laser formation of integrated optical components in materials with close-to-zero CLTE, which will make it possible to create integrated de- vices protected from changes of their geometric dimensions associated with sharp changes of the ambient environmental conditions [5, 6]. In the general case the CLTEs of sitalls exhibit nonlinear temperature dependence, and CLTEs can be tweaked by opti- mizing the phase composition and establishing precise ratios between the volume fractions of the amorphous and crystal- line phases. It is especially difficult to find these ratios when the CLTEs must be minimized at values close to zero. Previous studies in this field by means of small-angle neutron scattering [7, 8] have shown that at the initial stages of sitallization of LAS glass amorphous phase separation followed by the formation of nuclei of crystallization of b-eucryptite solid solutions occurs in a definite range of compositions. However, even though much research has been done in this field the quest for glass compositions and their sitallization conditions is, as a rule, of an empirical nature. In the present work our task is to determine the relation- ships between the conditions of heat-treatment, the structural changes initiated by them in the LAS glass, and the CLTEs of sitalls in a wide temperature range in order to look for a possibility of precisely varying the thermal expansion of sitalls in a range of values close to zero. Glass and Ceramics, Vol. 76, Nos. 11 – 12, March, 2020 (Russian Original, Nos. 11 – 12, November – December, 2019) 446 0361-7610/20/1112-0446 © 2020 Springer Science+Business Media, LLC 1 D. I. Mendeleev University of Chemical Technology of Russia, Russia, Moscow. 2 Russian Nuclear Center Kurchatov Institute, Russia, Moscow. 3 E-mail: vlad.sigaev@gmail.com. DOI 10.1007/s10717-020-00220-9