Journal of the American Ceramic Society

The role of water vapor in crystallite growth and the tetragonal‐to‐monoclinic phase transformation of ZrO₂ was studied using three specially prepared samples: an ultrafine powder of monoclinic ZrO₂ obtained by hydrolysis of ZrOCI₂, an aggregated powder of tetragonal ZrO₂ obtained by thermal decomposition of Zr(OH)₄ under reduced pressure, and an ultrafine powder of tetragonal ZrO₂ obtained by thermal decomposition of zirconyl acetate dispersed in caramel. The samples were heat‐treated up to 1000°C in dry and wet atmospheres saturated with water vapor at 90°C. It was found that water vapor markedly accelerated crystallite growth for both monoclinic and tetragonal ZrO₂ and facilitated the tetragonal‐to‐monoclinic phase transformation. Water vapor increases surface diffusion and thus enhances crystallite growth and decreases surface energy, which leads to stabilization of the tetragonal phase.

The solidification point of calcia which has been distributed by the Commission on High‐Temperature and Solid‐State Chemistry of the International Union of Pure and Applied Chemistry was measured by digital pyrometry using an arc imaging furnace. The solidification point was determined to be 2899°C with ±3°C as the standard deviation for this sample. The reliability of this solidification point was determined to be within ±17°C based upon discussions of the measuring system and the material.

The effect of stable crack extension on fracture toughness test results was determined using single‐edge precracked beam specimens. Crack growth stability was examined theoretically for bars loaded in three‐point bending under displacement control. The calculations took into account the stiffness of both the specimen and the loading system. The results indicated that the stiffness of the testing system played a major role in crack growth stability. Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions. Hot‐pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension. The fracture toughness values from the unstable tests were 10% higher than those from the stable tests. This difference, albeit small, is systematic and is not considered to be due to material or specimen‐to‐specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughness values.

As for plants, far‐red (FR) light with wavelength from 700 nm to 740 nm is critical for processes of photosynthesis and photomorphogenesis. Light‐controlled development depends on light to control cell differentiation, structural and functional changes, and finally converge into the formation of tissues and organs. Phosphor converted FR emission under LED excitation is a cost‐effective and high‐efficient way to provide artificial FR light source. With the aim to develop an efficient FR phosphor that can promote the plant growth, a series of gadolinium yttrium gallium garnet (GYGAG) transparent ceramic phosphors co‐doped with Mn²⁺ and Si⁴⁺ have been fabricated via chemical co‐precipitation method, followed sintered in O₂ and hot isostatic pressing in this work. Under UV excitation, the phosphor exhibited two bright and broadband red emission spectra due to Mn²⁺: ⁴T₁ → ⁶A₁ spin‐forbidden transition, and one of which located in the right FR region. And then, Ce³⁺ ions were co‐doped as the activator to enhance the absorption at blue light region and the emission of Mn²⁺. It turns out that the emission band of GYGAG transparent ceramic phosphors matches well with the absorption band of phytochrome P_FR, which means they are promising to be applied in plant cultivation light‐emitting diodes (LEDs) for modulating plant growth. Besides, the thermal stability of this material was investigated systematically, and an energy transferring model involves defects was also proposed to explain the phenomenon of abnormal temperature quenching.

The oxidation of MoSi₂ in air at atmospheric pressure was studied by electron diffraction, X‐ray diffraction, and thermogravimetric analyses. The oxidation process occurs in two parts: (1) formation of MoO₃ and SiO₂ at temperatures below the boiling point of MoO₃, and (2) formation of Mo₅Si₃ and SiO₂ at higher temperatures. Evidence is presented which indicates that oxygen permeation through a silica layer, which may be of a mixed crystalline‐glassy nature, controls reaction rate at high temperatures and that Mo₅Si₃ is present directly beneath the protective oxide. The activation energy for oxidation of MoSi₂ above 1200°C was calculated as 81.3 kcal mole⁻¹.

In this study, we aimed to examine the effect of dopant type and concentration on the ionic conductivity of ceria‐based electrolytes. Ceria electrolytes doped with samarium (SDC), gadolinium (GDC), neodymium (NDC), and lanthanum (LDC) for solid oxide fuel cells were prepared through the polyol process. Acetate compounds of cerium and dopants were used as starting materials, and triethylene glycol was used as a solvent. Prepared powders and pellets were characterized by TG/DTA, XRD, FTIR, SEM, EIS, and EDS techniques. The results of the TG/DTA and XRD indicated that a single‐phase fluorite structure formed at the relatively low calcination temperature of 500°C. The relative densities of the pellets were higher than 90% and these finding were supported by the SEM images. The lattice parameters of the samples increased with the dopant concentration. According to the electrochemical analysis results, the samples with maximum conductivity values were SDC‐20, GDC‐15, NDC‐15, and LDC‐15. The results of the impedance spectroscopy revealed that the SDC‐20 sample exhibited the highest ionic conductivity with a value of 4.29 × 10⁻² S/cm at 800°C in air.

Binary Sb₂O₃‐GeO₂ glasses containing 45 mol% Sb₂O₃ and ternary Sb₂O₃‐B₂O₃‐GeO₂ glasses containing 50 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high‐temperature viscosities were estimated visually. Small amounts of Sb₂O₃ (∼10 mol%) appear to perturb neither the Ge‐O‐Ge network nor those B‐O‐Ge networks with small B/Ge ratios (∼0.2). The B‐O‐Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb₂O₃. Amounts of Sb₂O₃ >10 mol% appear to depolymerize the Ge‐O‐Ge and Ge‐O‐B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb₂O₃‐B₂O₃‐GeO₂ glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.

The infrared absorption spectra of fused B₂O₃ and of a series of soda borate glasses are presented. These spectra were obtained using vacuum‐pressed briquettes of the powdered glass and powdered KBr. The spectrum of fused B₂O₃ shows quite definitely that this glass does not consist of a completely continuous triangularly coordinative network. It is shown that hydrogen bonds play an important part in the atomic arrangement of the glasses of zero or low soda content. The B₂O₃ glass apparently consists of complexes of an approximate unit (B₉O₁₄)‐ held together by hydrogen bonds. One in nine borons is tetrahedrally coordinated. The glasses of low soda content are similar. The spectra for soda concentrations greater than 15% did not permit the determination of the atomic arrangement with exactitude, but it is shown to be quite different from that found in glasses with 10% Na₂O or less.

Compositions in the series Na_1+xZr₂P3‐_xSi_xO₁₂ and Na_1+4zZr₂‐_zP₃O₁₂, where x = 0≤x≤1 and z=0≤z≤1/2, have thermal expansions that can be controlled (by selecting the composition) to values in the range <10×10_‐7 in a temperature regime of a few hundred degrees. The lowest a values were obtained for x ∼0.33 and z∼0.125.

The thermal expansion of the skeletal framework was essentially zero for NaZr₂(PO₄)₃‐type compounds; the interstitialion, e.g., Na⁺, was primarily responsible for the total thermal expansion. The composition dependence of the thermal expansion is discussed in terms of the amounts, crystallographic sites, and ionic radii of the interstitial ions. The mechanism which results in low thermal expansion was clarified, particularly for KZr₂(PO₄)₃, in which a larger ion is substituted for Na⁺, and NbZr(PO₄)₃, which does not contain Na⁺. Polycrystalline ceramics formed from these crystals might be useful as thermal‐shock‐resistant materials.