No abstract

No abstract

ESR study of BaTiO 3 ferroelectrics doped with cerium or with niobium and strontium, both types of samples being doped with manganese, was performed at room temperature. In the samples of the first type the most intensive line with g-factor 1.9612 was shown to originate from paramagnetic center Ti. The small intensity line was related to Fe 3+ -V O center, which can be unavoidable impurity in BaTiO 3 . In the samples doped with strontium, the six lines intensive spectrum, which corresponds to hyperfine structure of Mn 2+, was observed. The intensity of the spectrum was shown to decrease with strontium concentration increase. This effect was supposed to be related to the decrease of the grain sizes that results in migration of manganese and, possibly, niobium ions. The influence of these impurities, of their charge states and positions in crystalline lattice of BaTiO 3 on the PTCR effect is discussed.

Physicochemical research is conducted to examine the phase formation under dynamic and static annealing of MgO-Al 2 O 3 -SiO 2 tempered glass (composition range adjacent to stoichiometric cordierite). Thirty glass compositions are examined in this system and the range is determined over which single-phase ceramic materials with the lowest possible thermal expansion coefficient are synthesized under specific crystallization conditions.Melt crystallization provides materials based on alumina compounds, including mullite (3Al 2 O 3 ⋅ 2SiO 2 ), forsterite (2MgO ⋅ SiO 2 ), and cordierite (2MgO ⋅ 2Al 2 O 3 ⋅ 4SiO 2 ) with uniform chemical and phase compositions and controlled content of impurity phases. Two processing options may be used: (i) forming of parts from molten glass and their subsequent annealing (T ≥ 1000°C) for crystallization and (ii) production of granulated glass, its grinding, forming, and annealing using conventional ceramic processing.Single-phase α-cordierite with low thermal linear expansion coefficient (TLEC) and high thermal resistance was produced by cooling a cordierite melt at a rate of 1.5 °C/sec to 900-1100°C with subsequent annealing at these temperatures for four to five days [1]. To promote the crystallization of cordierite and decrease the volume content of the glass phase, oxide additions are used to form crystallization nuclei in glass: 3-5 wt.% ZrO 2 [2], 0.1-15 wt.% Nb 2 O 5 , Ta 2 O 5 [3], ≤10 wt.% TiO 2 [4]. This leads to a 25% decrease in annealing time, and the content of crystalline cordierite reaches 98-99 vol.%.The thermal linear expansion coefficients of samples representing stoichiometric cordierite ceramics and cordierite-based solid solutions are 2.0 ⋅ 10 -6 °C -1 over a range between 25 and 1000°C [5]. A substantial amount of the glass phase that had no admixtures of alkaline or alkaline-earth metals (ratios 1 : 1 : 4, 1 : 1 : 5, and 1 : 1 : 6 containing to 81% Si 2 ) increased the thermal expansion coefficient from 2.0 to 3 ⋅ 10 -6 °C -1 . The high content of crystalline cordierite (95-97%) resulted from arc melting of a cordierite charge consisting of serpentinite, quartz sand, and alumina [6]. When glass was heated at a rate of 10 °C/min, differential thermal analysis (DTA) revealed an exothermic effect of cordierite crystallization with a peak at 1000°C. There are no data on the thermal expansion of crystallized cordierite samples. In contrast to conventional negative attitude to alkaline oxide admixtures, it was established in [7] that about 7.56 wt.% K 2 O introduced into cordierite glass in accordance with the formula 4MgO ×

Refractory packing masses are being used more and more in the manufacture of linings for heat assemblies. One of the important properties of packing masses is the constancy of their volume or the very slight increase in volume during the preparatory heat treatment and later service. A slight increase in the volume of the packing lining on heating produces compressive stresses which have a favorable effect on the strength of the lining and prevent cracking.The present article reports some results of studies of refractory packing masses based on chamotte containing kaolin or milled pyrophillite as the finely dispersed component.In the preparation of the packing masses we used the following: chamotte (TU 14-8-58-72) containing 42%* fractions of 2.5-1.0 mm; 22%, 1.0-0.4 ram; 27%, 0.4-0.063 mm; and 9?0, < 0.063 mm; finely milled Glukhovetsk kaolin; and finely milled pyrophillite from the Mozyr'-Ovrunsk Deposits. As the bonding agent, we used water glass (WG) of density 1.4 g/cm 3 (GOST 13078-67) and an aluminochromophosphate bonding (ACPB) of density 1.6 g/cm a (TU 6-18-166-73).We thought it would be interesting to replace the kaolin by the pyrophillite since heat treatment even at comparatively low temperatures (1150-1300~ causes the pyrophillite to decompose into mullite and an active silicon oxide; this improves the sintering qualities of the refractory lining and increases its strength.We prepared four compositions of a chamotte packing mass (Nos. 1-4). All the masses contained 75% chamotte and 25% pyrophillite (Nos. 1 and 3) or of kaolin (Nos. 2, 4). To mass Nos. 1 and 2 we added 15% water glass and to masses 3 and 4, 15% ACPB.In the preparation of masses we added to the preliminarily mixed dry powders the appropriate bonding agent and the mixture was incorporated to a uniform state. All the specimens were pressed under a pressure of 25 MPa. For the compressive strength tests we used specimens in the form of cylinders, diameter and height 20 ram, previously dried at 100~ and fired for 2 h at various temperatures. The open porosity of the specimens was 23-25%. The change in the length of the specimens after heat treatment was determined from the change in length of beams measuring 8 x 8 x 50 mm. The thermal-shock resistance was determined using a heat-cycle method (800~ --water) for specimens of diameter and height 30 mm and also using a method which involved heating hollow cylinders in rings of external diameter 50 and internal 25 mm and depth 20 mm [1].