The System Bi2O3-SiO2

The Bi₂O₃-rich side of the system Bi₂O₃-SiO₂ was studied with powder X-ray diffraction and differential thermal analysis. In the composition 6Bi₂O₃. x SiO₂, the metastable γ phase (bcc) was observed to exist over the range of 0 < x ≤ 1. In most of the compositions studied, metastable phases of water-quenched melts transformed into another metastable phase before reaching stable phases. A modification of the phase diagram is proposed.     

Preferential X-ray line Broadening and Thermal Behavior of γ-Fe2O3

Preferential X-ray line broadenings in γ-Fe₂o₃ samples prepared from γ-FeOOH, α-FeOOH, N₂H₅Fe(N₂H₃-COO)₃-H₂O, FeOOCH₃, and colloidal Fe₃O₄ are compared. Isotropic size and small crystallites are the origin of the uniform and enhanced X-ray line broadening in samples derived from hydrazinate and colloidal Fe₃O₄. Nonuniform line broadening in ex-α-FeOOH and ex-γ-FeOOH is due to an elongated crystallite shape and the presence of stacking faults, respectively. The thermal behavior of samples with low crystallite size and uniform line broadening is characterized by an exothermal recrystallization process simultaneous to the phase transformation γ-Fe₂0₃→α-Fe₂O₃.     

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.     

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 of SrY2O4

Crystals of SrY₂O₄ (space group Pnam ) were examined by high-temperature powder X-ray diffractometry to determine the changes in unit-cell dimensions with temperature. The individual cell dimensions linearly increased with increasing temperature up to 1473 K. The expansion coefficients (K⁻¹) were 1.263(8) × 10⁻⁵ along the a- axis, 7.46(6) × 10⁻⁶ along the b- axis, and 9.93(10) × 10⁻⁶ along the c- axis. The coefficient of mean linear expansion was 1.001(8) × 10⁻⁵ K⁻¹.     

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.     

Negative Thermal Expansion in (HfMg)(WO4)3

A single-phase material (HfMg)(WO₄)₃ with an orthorhombic structure, A₂ (WO₄)₃-type tungstate, has been successfully prepared for the first time by the calcination of HfO₂, MgO, and WO₃, substituting Hf⁴⁺ and Mg²⁺ for A³⁺ cations in A₂(WO₄)₃. The new material shows a negative thermal expansion coefficient of approximately −2 ppm/°C from room temperature to 800°C. The mechanism of negative thermal expansion is assumed to be the same as that of Sc₂(WO₄)₃.     

Thermal Expansion of Pb3O4

Thermal expansion of Pb₃O₄ was investigated by high-temperature X-ray diffraction. The coefficient in the a ₀ direction is 14.6×10⁻⁶/°C. Expansion in the c₀ direction is 32% greater, with a coefficient of 19.3×10⁻⁶/°C. Coefficients of expansion are linear from 25° to 490°C and are comparable with those of tetragonal and orthorhombic PbO.     

Effect of Ionic Substitution on the Thermal Expansion of ZrTiO4

New oxide compounds with α-PbO₂ structure have been synthesized by solid-state reactions. These are derived from ZrTiO₄ and HfTiO₄ by a different kind of ionic substitution. The thermal expansion behavior of these phases was investigated by means of a dilatometer and an X-ray heating diffractometer. These measurements revealed rather low expansion for some of the Zr(Me³⁺/Me⁵⁺)O₄, solid solutions. This behavior is attributed to their high expansion anisotropy, which leads to extended formation of microcracks.     

Thermal Expansion of Nb2O5

The high-temperature thermal expansion of monoclinic Nb₂O₅ was studied with X-ray and dilatometric techniques. The X-ray axial thermal expansion was anisotropic; the mean coefficients in the a, b , and c directions, respectively, were 5.3, 0, and 5.9×10⁻⁶°C^-1 from room temperature to 1000°C. It is proposed that this anisotropy causes microcrack formation in sintered polycrystalline samples. The bulk linear thermal expansion of both sintered and hot-pressed samples was determined with a dilatometer from room temperature to 1200°C. A hysteresis effect between heating and cooling data observed for sintered samples was attributed to the occurrence and recombination of internal microcracks.     

Effect of the c/a Ratio on the Domain Structure in (Pb1-x,Cax)TiO3

The strain-induced ferroelectric domain structure in tetragonal (Pb_1-x,Ca_x)TiO₃ is absent when x ≥ 0.39, because the c / a ratio approaches unity with increasing x . For compositions where the c / a ratio is close to unity, the lack of strain allows a domain structure different from the strain-induced domains. These domains, referred to here as random domains, have domain walls that do not lie on a particular crystallographic plane and are nonplanar. A simple model is presented to predict the conditions under which the ferroelectric strain-induced domain structure is absent.     

Thermal Expansion of MgFe2O4, FeO, and MgO·2FeO

The thermal expansion of magnesium ferrite, wüstite, and magnesio-wüstite has been measured up to 1000°C. Good agreement has been found with published data obtained at lower temperatures, but the reported extrapolations have been shown to be low. The thermal expansion of MgFe₂O₄ has been interpreted to support Gilleo's hypothesis that the super-exchange energy for Fe³⁺-O^2--Fe³⁺ linkages is independent of the Fe³⁺-O^2- distance. This conclusion is supported by the fact that the energy per linkage is the same as that for Fe³⁺-O^2-Fe³⁺ linkages in a variety of crystal structures.     

Low-Temperature Synthesis of NiFe2O4 by a Hydrothermal Method

Nickel ferrite (NiFe₂O₄) nanoparticles were successfully synthesized via a hydrothermal process and characterized by X-ray diffraction and transmission electron microscope techniques. The effects of reaction temperature, holding time, and RH ratio (isopropyl alcohol/water) were discussed. The NiFe₂O₄ nanoparticles could be obtained at 60°C within 3 h. The crystallization of the spinel ferrites was promoted by the increase in reaction temperature, holding time, and RH ratio.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

Thermal Expansion and Athermal Phase Transition of Y4AI2O9 Ceramics

Thermal expansion of Y₄A1₄O₉ ceramics prepared at 1600° and 1800°C was measured from room temperature to 1500°C in air. Volume changes at the phase transition of Y₄A1₂O₉, along with thermal hysteresis, were observed around 1400°C. The volume of the high-temperature phase was about 0.5% lower than that of the low-temperature phase. The hysteresis width for the sample prepared at 1600°C was 56°C, wider than that (14°C) for the sample prepared at 1800°C. The averages of phase transition start temperatures on heating and on cooling for these samples were, however, almost the same at 1377°C. The phase transitions did not occur at fixed temperatures, and the proportions of the high-temperature phase and the low-temperature phase did not change with time as long as the temperature remained constant ( athermal character). The sample prepared at 1800°C also showed another thermal hysteresis behavior from room temperature to about 1000°C.     

Thermal Expansion Behavior of the Si3N4-Whisker-Reinforced Soda-Borosilicate Glass Matrix Composite

Thermal expansion behaviors of a Si₃N₄-whisker-reinforced sodaborosilicate glass matrix composite are studied. An abrupt increase of the coefficient of thermal expansion is observed and is attributed to formation of crystobalite in the sodaborosilicate glass matrix. This thermal expansion behavior is discussed with special reference to the phase transformation of the crystobalite.