The influence of Zr/Ti content on the morphotropic phase boundary in the PZT-PZN system

A quantitative structural investigation was carried out on (1-y)PbZrxTi1-xO3-yPbZn(1/3)Nb(2/3)O(3) where y=0.1 and 0.2 ((1-y)PZT-yPZN). High resolution XRD data have been used for quantitative phase analysis. The nominal compositions were prepared by a two-step low temperature calcining solid-state method. The sintered samples show an average grain size of 1-2 mu m. It is demonstrated that the increase in the concentration of PZN leads to the shift of the morphotropic phase boundary (MPB) of PZT towards the PbZrO3 end member. In the present work, an effort has been made to quantitatively determine the MPB phase contents and to regain the coexistence of tetragonal and monoclinic phases by varying the value of x(i.e. Zr/Ti ratio). The width of the MPB becomes considerably larger for y=0.10 and 0.20 as compared to pure PZT. This is attributed to the considerably lower grain size of our samples resulting from the adopted preparation method. (C) 2010 Elsevier B.V. All rights reserved

Apparatus to measure high-temperature thermal conductivity and thermoelectric power of small specimens

A thermal conductivity apparatus based on the parallel heat-flow technique has been fabricated to measure the thermal conductivity of small specimens. The steady-state method is used to measure samples of 1x1x10 $mm^{3}$ dimension in the temperature range of room temperature-700K. The details of instrument fabrication, the method of calibration,and typical measurements on test samples are described. The apparatus can also be used to measure the Seebeck coefficient in the same temperature range. As an example we report the thermal properties of $CrSi_{2}$, which is a potential candidate for high-temperature thermoelectric applications

Improved ductility and oxidation resistance in Nb and Al co-substituted MoSi2MoSi_2

Polycrystalline samples of $(Mo_{1-x}Nb_x)(Si_{1-y}Al_y)_2$ with x = 0.0-0.03 and y = 0.0, 0.04 were prepared by arc melting followed by densification by hot pressing. The mechanical and oxidation behaviors of high density polycrystalline pellets were evaluated. Thermogravimetric analysis shows lower oxidation in $(Mo_{1-x}Nb_x)(Si_{0.96}Al_{0.04})_2$ samples compared to pure $MoSi_2$. X-ray diffraction analysis of the oxide scale revealed the presence of $\alpha-Al_2O_3$ peaks. Mechanical characterizations carried out on (Mo_{0.99}Nb_{0.01})(Si_{0.96}Al__{0.04})_2 composition show a marked improvement compared to that of unsubstituted $MoSi_2$. Micro-hardness measurements reveal a lowering of the room temperature hardness and stiffness compared to pure $MoSi_2$. Plastic deformation under compression is observed at room temperature with a yield stress of 500 MPa. The $K_{1c}$ value increased from $3.33 (\pm3) MPa m^{0.5}$ to $4.05 (\pm0.3) MPa m^{0.5}$ with the fracture mode being predominantly transgranular

Role of milling parameters and impurity on the thermoelectric properties of mechanically alloyed chromium silicide

Mechanical alloying of Cr–Si powders (1:2 molar ratio) was carried out under different milling conditions using stainless steel milling media. In addition to formation of nanocrystalline $CrSi_2$ phase, depending on the milling duration, speed and use of dispersant, X-ray diffraction revealed presence of CrSi phase and EDAX showed iron contamination in varying amounts. The amount of contamination and the secondary phase are found to be linearly proportional to the input impact energy during milling. The nanocrystalline powders are seen to be thermally stable in air up to 900 K. SEM of the hot pressed powders reveal an equiaxed microstructure with grain size depending on the milling duration. Electrical resistivity \rho, Seebeck coefficient (S) and thermal conductivity (K) were measured up to 700 K. It is seen that the thermoelectric figure of merit (ZT) is high when the amount of secondary phases is minimum. The maximum ZT observed in the samples is $\sim 0.2$ at 600K which is comparable to other high temperature thermoelectric materials in the temperature range studied

Role of crystal structure on the thermal expansion of Ln_2W_3O_1_2 (Ln =La, Nd, Dy, Y, Er and Yb)

The negative thermal expansion material Y_2W_3O_1_2 belongs to Ln_2W_3O_1_2 family of compositions. The thermal expansion behavior of Ln_2W_3O_1_2 (Ln = La, Nd, Dy, Y, Er and Yb) members synthesized by the solid-state reaction have been studied and correlated to their crystal structure. The lighter rare earth tungstates (Ln = La, Nd and Dy) crystallize in monoclinic structure (C2/c) whereas the heavy rare earth tungstates (Ln = Y, Er and Yb) form the trihydrate orthorhombic Ln_2W_3O_1_2·3H_2O at room temperature and above 400 K transforms to unhydrated orthorhombic structure (Pnca). The hot pressed (1273 K and 25 MPa) ceramic pellets have been studied for thermal expansion property by dilatometry and high temperature X-ray diffraction. The heavy rare earth tungstates show a large initial expansion up to 400 K, followed by a thermal contraction. The light rare earth tungstates, on the other hand, show thermal expansion. The difference in the thermal expansion behavior in Ln_2W_3O_1_2 series is attributed to the difference in the structural features. The heavy rare earth tungstates have corner sharing of $LnO_6$ octahedra with $WO_4$ tetrahedra, where the now well established mechanism of transverse vibrations operate. The light rare earth tungstates have edge sharing of $LnO_8$ polyhedra where in such a mechanism is absent

Effect of nominal copper concentration in yttrium barium copper oxide (YBa_2Cu_{3\pmx}O_{7-\delta} ) on the superconducting transition

Variation of [Cu] in the nominal compn. YBa_2Cu_{3\pmx}O_{7-\delta} markedly influences the superconducting transition. The transition occurs at the highest temp. with the lowest width, when $x = 0.25