Effect of Nb2O5/ZnO Addition on Microwave Properties of (Zr0.8Sn0.2)TiO4 Ceramics

The effects of Nb₂O₅ and ZnO addition on the dielectric properties, especially the quality factor, of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics were investigated in terms of the sintered density acquired by the zinc. For ZST ceramics with 2 mol% added ZnO, the relative density of the samples decreased with >0.5 mol% addition of Nb₂O₅. On the other hand, for samples with 6 mol% added ZnO, the relative density remained >97%, even when the amount of Nb₂O₅ was increased to 2.0 mol%. When >0.5 mol% Nb₂O₅ was added, both the quality factor and the dielectric constant exhibited similar trends with sintered density. The ZST ceramics with 6 mol% added ZnO, especially, still manifested a quality factor >40 000 and a dielectric constant of 37, even when the amount of Nb₂O₅ was increased, values that are not explainable by the previously suggested electronic defect model.     

Differences between Zirconium Hydroxide (Zr(OH)4·nH2O) and Hydrous Zirconia (ZrO2·nH2O)

Based on cause and effect rather than experiment data, zirconium hydroxide (Zr(OH)₄· n H₂O) is always considered as hydrous zirconia (ZrO₂· n H₂O). With the aid of nitric acid, XPS, XRD, XPF, and TGA, some differences between them have been confirmed. It is found that in contrast to zirconium hydroxide (the binding energy of zirconium = 183.6 eV), hydrous zirconia does not dissolve in nitric acid. The chemical properties of zirconium (181.8 eV) in hydrous zirconia are similar to those in zirconia (182.2 eV for the binding energy of zirconium), and the weight loss of hydrous zirconia is 21.5%, different from 32.19%, the weight loss of zirconium hydroxide. According to the experimental data, a structure for hydrous zirconia is proposed and then the different phenomena are explained.     

Reactive Hot Pressing of ZrB2–SiC Composites

A ZrB₂–SiC composite was prepared from a mixture of zirconium, silicon, and B₄C via reactive hot pressing. The three-point bending strength was 506 ± 43 MPa, and the fracture toughness was 4.0 MPa·m^1/2. The microstructure of the composite was observed via scanning electron microscopy; the in-situ -formed ZrB₂ and SiC were found in agglomerates with a size that was in the particle-size ranges of the zirconium and silicon starting powders, respectively. A model of the microstructure formation mechanism of the composite was proposed, to explain the features of the phase distributions. It is considered that, in the reactive hot-pressing process, the B and C atoms in B₄C will diffuse into the Zr and Si sites and form ZrB₂ and SiC in situ , respectively. Because the diffusion of Zr and Si atoms is slow, the microstructure (phase distributions) of the obtained composite shows the features of the zirconium and silicon starting powders.     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Structure of Commensurate and Incommensurate Ordered Phases in the System ZrTiO4–Zr5Ti7O24

The low-temperature, Zr–Ti-ordered, form of zirconium titanate has been investigated using high-resolution transmission electron microscopy in order to characterize the incommensurate structure of phases with compositions ZrTiO₄ to near Zr₅Ti₇O₂₄. Electron diffraction reveals that compositions with Zr : Ti between 5 : 7 and 1 : 1 have incommensurate superstructures, and phases close to 1 : 1 are commensurate with an a -axis repeat 2X that of the disordered structure. High-resolution images show that the a -doubling in ZrTiO₄ corresponds to a new structure, one that consists of two Zr-rich, distorted octahedral layers alternated with two Ti-rich octahedral layers. The incommensurate compositions are composed of blocks of the 1 : 1 structure intercalated with blocks of the commensurate 5 : 7 structure, the latter having a tripled a -repeat and a ZTTZTT sequence of cation layers. The intercalation can be described as an "interface-modulated" structure resulting from the quasiperiodic insertion of (100) faults with displacement vector R =–1/3 a_ord in the ordered 5 : 7 phase. Although their spacing is variable, the faults are uniformly distributed in such a way as to produce incommensurate satellite reflections. The findings regarding the structure of ordered compositions in the ZrTiO₄–Zr₅Ti₇O₂₄ system provide an improved framework for understanding the effects of ordering on the properties of zirconium titanate dielectric ceramics.     

Low-Temperature Sol–Gel Synthesis of NaZr2P3O12

The NZP family of new low-expansion materials has attracted wide interest for its potential in advanced technological applications. NaZr₂P₃O₁₂, which is the parent composition of this family, has been synthesized by the solution sol-gel method using special precursor solutions, which led to its formation (although poorly crystalline) at temperatures as low as 120°C. The lowest temperature of formation of a single phase of NaZr₂P₃O₁₂ with a high degree of crystallinity was found to be 600°C.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Effect of Fe and Cr Addition on the Sintering Behavior of ZrB2 Produced by Self-Propagating High-Temperature Synthesis

An investigation of the sintering behavior of ZrB₂ powder with Fe and Cr (0 to 20 wt%) addition was conducted. It was observed that Fe addition helps to enhance the density of ZrB₂ only up to 10 wt%. Further addition of Fe degrades the sintering by segregation of Fe-rich phases. Formation of a eutectic phase containing a Fe:Zr ratio of 92.57:7.43 was also found in Fe-added samples. The addition of Cr to a ZrB₂ matrix was found to result in swelling of the samples, leading to several cracks.     

Thermal Expansion of Solid Solutions in the ZrTiO4—In2O3—M2O5 (M = Sb, Ta) System

Axial and dilatometric thermal expansions and phase transformations were studied for solid solutions having the α-PbO₂ structure in the ZrTiO₄—In₂O₃—M₂O₅ (M = Sb, Ta) system with nominal formulas of Zr _x Ti _y In _z Sb _z O₄ and Zr _x Ti _y In _z Ta _z O₄ where x + y + 2 z = 2. With increased substitution of z , the cell volume increased, the difference in the b parameters at room temperature between those quenched from 1400° and 1000°C decreased, and the thermal expansion decreased. The axial thermal expansion of ZrTi _y In _z · Ta _z O₄ with z = 0.3 was almost identical with that of HfTiO₄, and those with z = 0.4 and z = 0.45 were smaller than that of HfTiO₄. Unit-cell volumes of these compound were compared with those of single oxides to make it clear that the unit-cell volume of ZrTiO₄ was small anomalously and to distinguish the normal and abnormal substitution systems. These results were explained by the working hypothesis proposed for these compounds.     

Thermal Expansion Anisotropy and Acoustic Emission of NaZr2P3O12 Family Ceramics

Most members of the NaZr₂P₃O₁₂ (NZP) family possess low, near zero, overall thermal expansion coefficients. However, they also exhibit anisotropy of axial thermal expansion. Some compounds have opposite anisotropy; for example, the a parameter of CaZr₄P₆O₂₄ contracts on heating and that of SrZr₄P₆O₂₄ expands, while the c parameter expands for the Ca compound and contracts for the Sr compound. The anisotropy of the axial thermal expansion of these materials is believed to induce microcracking. The acoustic emission method was employed here to detect microcracking in ceramics due to the axial thermal expansion anisotropy. Acoustic signals were observed during cooling of the Ca and Sr compounds from 500°C, and Na and K compounds from 600°C. On the other hand, no acoustic emission signal is detected in Ca_0.5Sr_0.5Zr₄P₆O₂₄ ceramics, in which the lattice parameters a and c remain nearly unchanged in the temperature range of room temperature to 500°C. Thus, a direct correlation between microcracking of ceramics and their anisotropic axial thermal expansion coefficients was established by employing acoustic emission monitoring techniques.     

Crystal Chemistry and Stabilization in Air of Brannerite, UTi2O6

Brannerite, UTi₂O₆, can be formed only under low oxygen pressures by dry ceramic processing techniques, but the substitution of ∼0.2 and 0.3 formula units (fu) of Ca or Gd, respectively, for U allows the stabilization of the phase in air. The Ca/Gd in brannerite provides charge compensation for some U to exist in valence states >+4, as found by X-ray absorption spectroscopy of the U L _III-edge. The maximum solubilities of Ca and trivalent rare earths in the air-fired samples, 0.3 and 0.5 fu, respectively, correspond to U having an average valence of +5. Ca and Gd had maximum solubilities of 0.2 and 0.45 fu, respectively, in argon-fired samples. An absorption band at 1448 nm in both air- and Ar-fired U-brannerite doped with Ca and Gd was observed using diffuse-reflectance spectroscopy and attributed to an electronic transition of U⁵⁺. A similar band was observed in an annealed natural brannerite, which contained Ca, rare earths, and Th, although the band was present at ∼1520 nm in the unannealed, X-ray amorphous sample. In synthetic ThTi₂O₆ (thorutite, having the brannerite structure), the solubility of Ca was undetectable and that of rare earths <0.1 fu. Other ionic substitutions in synthetic brannerites involved Hf, Pu, La, and Y for U, (Gd + Nb) for U + Ti, and Fe in the Ti site.     

Effect of Praseodymium and Uranium Addition on the Jc of Ba2 YCu3O7

Ba₂YCu₃O₇ ceramics doped with either Pr or U at 0.000 17 to 1.7 wt% levels have been prepared. For each sample J _c (magn) has been measured with a vibrating sample magnetometer. No improvement in J _c was found for either dopant and it is concluded that neither provides the clusters necessary to produce suitable pinning sites.     

Ferromagnetism and Magnetoresistance in La0.67Sr0.33Fe0.07Mn0.93O3

A bulk ceramic sample La_0.67Sr_0.33Fe_0.07Mn_0.93O₃ (LSFMO) with a rhombohedral structure has been prepared from a coprecipitated carbonate precursor in this study. Ferromagnetism and a negative, isotropic magnetoresistance (MR) as large as 11% have been observed in a ceramic sample of LSFMO. There are two resistivity transition peaks on the resistivity versus temperature curves. The resistivity peak and MR have been related to the ferromagnetic state in LSFMO.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

Reactions During the Processing of U3O8—Al Cermet Fuels

The cermet fuel (U₃O₈ dispersed in Al) being considered for use in the Savannah River Site Reactors is thermodynamically unstable because of the potential for an exothermic metallothermic reduction reaction. This paper describes work performed to quantify the extent of reaction during powder metallurgy (P/M) processing of the U₃O₈—Al cermet fuel, and to determine the effect of partial reduction to U₄O₉ on the metallothermic reduction reaction. During the fabrication of the U₃O₈—Al cermet fuel by the P/M technique, a significant portion of the U₃O₈ is reduced to U₄O₉. The reaction between U₄O₉ and Al is also exothermic; however, the maximum heat released by the reaction is substantially less than that released for the U₃O₈—Al reaction, approximately 335 J (80 cal) per gram of oxide reacted compared to 940 J (225 cal). Metallothermic reduction reactions for U₃O₈/U₄O₉/Al mixtures do not occur at the normal reactor operating temperature, ∼ 370 K (∼ 100°C) or at temperatures below the melting point of aluminum, 930 K (660°C).