G. Ya. Akimov

Properties of ceramic manganite (La0.65Sr0.35)1 − xMn1 + xO3 ± Δ (x = 0, 0.1, 0.2) sintered at a temperature of 1500°C

It has been shown for the first time that the structure of (La0.65Sr0.35)1 − xMn1 + xO3 ± Δ (x = 0, 0.1, 0.2) ceramics sintered at 1500°C substantially depends on the content of superstoichiometric Mn. It has been found that the average grain size increases from 10 to >100 μm with increasing x from 0 to 0.2. It has been revealed that grains of the ceramic samples sintered from powders with excess Mn have an internal nanoscale layered structure. A correlation has been revealed between the size and structure of grains and and the magnetoresistive properties.

Effect of the sintering temperature of ceramic manganites La 0.7 Mn 1.3 O 3 ± Δ on their grain sizes, magnetic and electrical properties

The effect of the sintering temperature from 1070 to 1670 K of ceramic samples of lanthanum manganite La0.7Mn1.3O3 on their grain size, structure, magnetic and resistive properties has been studied. An increase in the sintering temperature to 1270 K is shown to lead to an insignificant increase in the grain size and an increase in the density, fraction of the ferromagnetic phase in a grain, and colossal magnetoresistance. The ceramics sintering at temperatures higher than 1470 K is found to sharply increase the grain size; simultaneously, the grain takes a layered structure. The grain growth at these temperatures is established to be accompanied by manganese precipitation at the grain boundaries and likely in the grain interior. The increase in the sintering temperature is accompanied by appearance of a magnetically phase heterogeneity and a decrease in the Curie temperature and magnitude of the colossal magnetoresistance effect.

Coercive force of nanocrystalline manganites

Samples of La0.7Mn1.3O3±Δ and (La0.65Sr0.35)0.8Mn1.2O3±Δ with particle sizes ranging from 6to200nm are obtained using cold isostatic pressing. The coercive force of the experimental samples is determined from the field dependences of the resistance and dynamic magnetic susceptibility. It is determined by two methods that the contribution of the surface layer to the magnetic characteristics of manganites is composition dependent. It is shown experimentally for the first time that the coercive force in manganites reaches its highest values with particle size of the order of 70nm for both compositions and vanishes completely for lanthanum manganite with ∼6nm particles as a result of reaching a superparamagnetic state.

Influence of the composition of the tetragonal phase in the surface layers of zirconia-based ceramics on their strength

The strength of partially stabilized zirconia-based ceramics is analyzed as a function of the porosity, the grain size, and the degree of tetragonality of the tetragonal phase. It is found that the strength of the studied ceramics, unlike conventional materials, is virtually independent of the porosity and the average grain size and is determined primarily by the content of the easily transformed tetragonal phase.

Influence of the size effect on the electrical conductivity of partially stabilized zirconia

The volume component of the electrical conductivity of bulk ceramics of the partially stabilized zirconia (ZrO2)0.94(Y2O3)0.05(Al2O3)0.01 is found to increase by a factor of 1.7 with the grain size decreasing from 600–800 to 200–300 nm. The observed effect is explained by the action of the pressure produced by surface tension forces, which shifts equilibrium toward the point of the polymorphic transition to the cubic phase.

Evolution of the phase composition and physicomechanical properties of ZrO2 + 4 mol % Y2O3 ceramics

The evolution of the phase composition and physicomechanical properties of ZrO2 + 4 mol % Y2O3 ceramics subjected to hot isostatic pressing and subsequent calcining in air is investigated. It is found that hot isostatic pressing results in the formation of an easily transformed phase Tet with a degree of tetragonality c/a=1.035, which determines high fracture toughness. After calcining in air, the phase Tet decomposes to form a nontransformed phase T′ with a degree of tetragonality c/a=1.005, which determines low fracture toughness.

Properties of the ceramic manganite (La0.65Ca0.35)1 − xMn1 + xO3 ± Δ (x = 0.2) sintered at temperatures of 1000–1450°C

It has been shown that the ceramics (La0.65Ca0.35)1 − xMn1 + xO3 ± Δ (x = 0.2) sintered at temperatures up to 1450°C is formed as a composite material consisting of manganite and manganese oxide grains. It has been found that, at a sintering temperature of 1450°C, the manganite grain size abruptly increases, which is accompanied by the formation of a nanometer-sized layered structure. It has been revealed for the first time that the temperature dependence of the magnetoresistance of the ceramic manganite with this structure is characteristic of single crystals.

Crystallite size and magnetic properties of La0.7Mn1.3O3 ± Δ

The results of investigation into nanocrystalline lanthanum manganites La0.7Mn1.3O3 ± Δ produced by repeated cold isostatic pressing of a charge material are reported. A powder compact with a crystallite size of 5–7 nm exhibits no magnetic properties, unlike a coarse-grained (20 nm) powder compact.

Specific physical properties of nanocrystalline (La0.65Sr0.35)0.8Mn1.2O3 ± Δ samples obtained by cold isostatic pressing

Single-phase powders of manganites (La0.65Sr0.35)0.8Mn1.2O3 ± Δ with average crystallite sizes of 30, 50, and 500 nm were produced by co-precipitation. The samples studied were obtained by cold isostatic pressing of powders at a pressure of 1 GPa without subsequent sintering. It is shown that the size of particles has a significant effect on the electromagnetic properties of the manganite samples. As the crystallite size decreases, the electrical resistance and coercive force increase and the tunneling magnetoresistance of the samples and the Curie temperature decrease.

On Structural Features Responsible for the Mechanical Properties of Ceramics from Zirconia Partially Stabilized with Y3+ and Mg2+ Ions

It is shown that Y-PSZ ceramics mainly fracture by an intercrystalline mechanism and the grain size fluctuates from 0.2 to 0.8 μm, whereas Mg-PSZ ceramics mainly fracture by a transcrystalline mechanism and the grain size fluctuates from 30 to 80 μm. The high level of mechanical properties is provided by compressive stresses that appear in the transformation of the β′-phase into the a-phase. In the Y-PSZ ceramics the transformation concerns individual grains, and in the Mg-PSZ ceramics lenticular segregations of the β′-phase are positioned in the γ-phase. The transformation is induced by tensile stresses of the elastic field of a crack, i.e., can be classified as transformation toughening.