Crystal Structure and Thermoelectric Properties of YAl3C3

The crystal structure of YAl₃C₃ was refined from laboratory X-ray powder diffraction data (Cu K α₁) using the Rietveld method. The crystal structure is hexagonal (space group P 6₃ mc , Z =2) with lattice dimensions a =0.342157(4) nm, c =1.72820(1) nm, and V =0.175217(3) nm³. The final reliability indices were R _wp=9.94% ( R _wp/ R _e=1.18), R _p=7.36%, R _B=1.77%, and R _F=1.03%. The compound shows an intergrowth structure with electroconductive [YC₂] thin slabs separated by Al₄C₃-type [AlC] layers. This material had thermoelectric properties superior to those of the layered carbides Zr₂[Al_3.56Si_0.44]C₅, Zr₂Al₃C₄, and Zr₃Al₃C₅ in the temperature range of 500– 1073 K, with a maximal power-factor value of 1.96 × 10⁻⁴ W·(m·K²)⁻¹ at 1073 K.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Defect Structure and Dielectric Properties of Nd2O3-Modified BaTiO3

The defect structure and dielectric properties of BaTiO₃ with 1 to 10 mol% Nd₂O₃ additions were studied. The results indicated that neodymium occupies the barium site and charge compensation takes place by creation of titanium vacancies. The dependence of inverse electric susceptibility, spontaneous polarization, and specific heat on temperature for samples containing more than 2 mol% of Nd₂O₃ were characteristic of a diffuse transformation resulting from a disordering of defects. The addition of Nd2O3 leads to a very drastic sfiift in the Curie temperature ( Tc ) of BaTiO₃; 3 mel% Nd₂O₃ addition moves Tc below room temperature.     

Ordered Structure and Dielectric Properties of Lanthanum-Substituted Ba(Mg1/3Ta2/3)O3

Both 1:2 and 1:1 ordered structures form in the perovskite solid solutions of La-substituted BMT Ba_{1− x} La _x (Mg_{(1+ x )/3}Ta_{(2− x )/3})O₃, sintered at 1600°C. The 1:2 ordered structure exists in the composition range 0.0 ≤ x ≤ 0.12, while that of 1:1 ordered structure exists in a wider composition range 0.04 ≤ x ≤ 1.0. Two ordered phases coexist in 0.04 ≤ x ≤ 0.12. High-resolution micrographs indicate that 1:2 and 1:1 ordered domains coexist in one grain. The ordering parameter of 1:2 phase decreases with x , yet that of 1:1 phase increases with x . Both increase with soak time. Variations in ordering are discussed in terms of cation occupancy and crystal chemistry. The quality factor increases with x , reaches a maximum, then decreases with x . The dielectric constant increases with x first, and levels off.     

Dielectric Properties of Explosively Compacted BaTiO3

Polycrystalline BaTiO₃ was explosively compacted to nearly theoretical density, and its dielectric and piezoelectric properties were investigated. The compacted bodies did not exhibit dielectric hysteresis, a Curie point, or piezoelectric properties; their dielectric constant and dissipation factor increased linearly with temperature. The dielectric constant was much lower than those of similar dense bodies obtained by conventional sintering. These results may be interpreted in terms of weak intergranular bonding and increased numbers of defects generated in the shocked material. When explosively compacted bodies were sintered, normal ferroelectric and piezoelectric properties and improved microstructures were obtained.     

Polymorphism and Dielectric Properties of Nb-Doped BaTiO3

A working subsolidus phase diagram for the system BaTiO₃–Ba₅Nb₄O₁₅ has been determined by firing sol–gel-synthesized samples over a range of temperatures. The main difference from previous diagrams is the greater extent of the Nb-doped BaTiO₃ cubic solid solutions, BaTi_{1−5 x} Nb_{4 x} O₃, at lower temperatures with x extending to 0.09 at 900°C, but only 0.05 at 1400°C. Electrical property measurements show that compositions with large x ( x ≥0.0025) are highly insulating for pellets sintered at 1300°C in air, followed by a slow cool. Compositions with low x , however, exhibit a residual semiconducting grain core and are not fully reoxidized readily. Composition dependence of the dielectric properties shows a continuous and smooth transition from classic ferroelectric behavior with pure BaTiO₃ to normal dielectric response with a temperature-independent relative permittivity of approximately 22–24 for x >∼0.08. At intermediate compositions, ranges of both relaxor ferroelectric and quasi-ferroelectric behavior are observed. Possible reasons for an observed anomalous increase in value of the permittivity at the ferroelectric transition temperature at low x , which is superposed on an overall decrease in permittivity with increasing x , are discussed.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Ordering Structure and Dielectric Properties of Undoped and La/Na-Doped Pb(Mg1/3Nb2/3)O3

Transmission electron microscopy was used to study the ordered domain structures in undoped and La/Na-doped Pb(Mg_1/3Nb_2/3)O₃ (PMN), where the compositions of the doped samples were specifically chosen so as to elucidate the ordering mechanism. The results showed that the Mg²⁺ ions and Nb⁵⁺ ions are short-range ordered on the B-site sublattice in undoped PMN, with a domain size of 2 to 5 nm. This short-range ordering gives rise to B-site composition fluctuations occurring on a nanometer scale, and it is this compositional inhomogeneity which is believed to be responsible for the diffuse phase transition behavior. Donor doping with La₂O₃ can compensate for the local charge imbalance resulting from the short-range order and thus enhances the degree of ordering. Acceptor doping with Na₂O, however, increases the charge effect, and hence ordering is suppressed. The effect of Na doping and La doping on the dielectric properties of PMN is also discussed.     

Crystal Structure of the Microwave Dielectric Resonator Ba2Ti9O20

A single-crystal X-ray study of dibarium nonatitanate, Ba₂Ti₉O₂₀, yielded the triclinic space group P 1 with a =0.7471(1), b= 1.4081(2), c= 1.4344(2) nm, α=89.94(2)°, β= 79.43(2)°, γ= 84.45(2)°, V = 1.476 nm³ Z = 4, and D_x= 4.61 Mg/m³. A refinement of atomic coordinates and isotropic thermal parameters led to a residual of 0.03. The structure consists of hexagonally closest-packed layers of Ba and O atoms in the sequence (hch)₃. All Ti atoms reside in octahedral interstices of this closest packing. The various Ti coordination octahedra share only edges and corners with each other. One-half of the Ba atoms is twelve-coordinated by oxygen atoms, the other half is eleven-coordinated.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

Crystal Structure of Zr2Al3C4

The crystal structure of Zr₂Al₃C₄ was refined by the Rietveld method from conventional X-ray powder diffraction data. The structure was hexagonal (space group P 6₃ mc , Z =2) with a =0.334680(6) nm, c =2.22394(3) nm, and V =0.215731(6) nm³, being isomorphous with that of U₂Al₃C₄. The final reliability indices were R _wp=8.57%, R _p=6.06%, and S =1.32. The crystal showed an intergrowth structure with NaCl-type ZrC slabs separated by Al₄C₃-type Al₃C₂ layers.     

Microwave Dielectric Properties of Hexagonal 12R-Ba3LaNb3O12 Ceramics

The dielectric properties of dense ceramics of the n =0 member of a newly identified homologous series Ba_{3+ n} LaNb₃Ti _n O_{12+3 n} , where n =0, 1, and 2, are reported. Single-phase powders can be obtained from the mixed-oxide route at 1350°C and dense ceramics (>97% of the theoretical X-ray density) with uniform microstructures (3–5 μm) can be obtained by sintering in air at 1500°C. The ceramics are excellent dc insulators with a band gap >2.6 eV that resonate at microwave frequencies with a relative permittivity, ɛ_r∼44, a quality factor, Q × f _r, of ∼9000 at f _r∼5.5 GHz and a temperature coefficient of resonant frequency, TC_f,∼−100 ppm/K.     

Phase Structure and Dielectric Properties of Bi2O3-ZnO-Nb2O5-Based Dielectric Ceramics

Phase structures and dielectric properties of the compounds with formulas BixZn_2/3Nb_4/3O_4+3x/2 (group M), Bi_xZn_8/3-x Nb_4/3O_6+x/2 (group V), and Bi_xZn_2-2x/3Nb_2-x/3O₇ (group W) have been investigated. Initial results indicate that a cubic pyrochlore structure is the predominant phase of these compound. Most of the measured ceramic specimens exhibit dielectric properties suitable as temperatures-stable and temperature-compensating dielectrics in the capacitor industry. The values of the dielectric constant K are 80-160, while those of the temperature coefficient are–500 to + 160 ppm/°C. The composition limits of the single pyrochlore phase are determined mainly by Bi₂O₃ additives.     

Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3-La(Mg2/3Nb1/3)O3 System

A complete range of perovskite solid solutions can be formed in the (1 − x )Ba(Mg_1/3Nb_2/3)O₃- x La(Mg_2/3Nb_1/3)O₃ (BMN-LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.     

Structure and Microwave Dielectric Properties of (Zn1−xCox)TiO3 Ceramics

Dielectric ceramics in the system (Zn_{1− x} Co _x )TiO₃ ( x = 0–1) were synthesized by the solid-state reaction route. The phase distribution, microstructure, and dielectric properties were characterized by using powder X-ray diffraction analysis, electron microscopy, and microwave measurement techniques. Three phase composition regions were identified in the specimens sintered at 1150°C; [spinel + rutile] at 0 ≤ x ≤ 0.5, [spinel + ilmenite + rutile] at 0.5 < x ≤ 0.7, and [ilmenite] phase at 0.7 < x ≤ 1. For the 0 ≤ x ≤ 0.5 region, the amount of Ti-rich precipitates incorporated into the spinel phase decreased with the Co content at 0 ≤ x ≤ 0.5, with a concomitant increase of the rutile phase. The ilmenite phase appeared for high Co content. The microwave dielectric properties depended on the phase composition and volume according to the three phase regions, where the relative amount of rutile to the spinel or ilmenite determined the dielectric properties. The dielectric constant as a function of Co addition was modeled with a Maxwell mixing rule. An optimum phase distribution was determined in this system with dielectric constant of 25, a Q * f 70 000 GHz, and a low temperature coefficient of the resonant frequency.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.