Rapid Rate Sintering of Yttria Transparent Ceramics

This paper reports on the influence of rapid rate sintering (RRS) on densification and microstructure evolution of yttria transparent ceramics by using vacuum sintering. The presence of temperature gradient has been confirmed during the RRS process. The higher the heating rate (HR), the larger the temperature gradient in the samples would be. By using RRS, e.g., HR = 40°C/min, the samples could be densified very fast to a relative density of 99.6%. However, these samples could not be further densified, due to the presence of difference in densification caused by a heating rate‐induced temperature gradient. By using a two‐step RRS with an intermediate‐temperature thermal treatment, this problem has been successfully addressed. The intermediate‐temperature treatment allowed for the particle neck growth, so that effective thermal conductivity of the compacts was increased greatly. Therefore, the temperature gradient and differentiate densification were effectively prevented. Samples sintered using the two‐step RRS process could be fully densified and excellent in‐line optical transmittance was achieved. It is believed this strategy is applicable to other transparent ceramics, as well as other engineering ceramics.     

Elastic Moduli of Refractory Spinels

Elastic moduli of spinel phases in the system Mg(Al, Cr, Fe)₂O₁ were determined from sonic analyses of porous, polycrystalline specimens prepared by hot-pressing. A special iterative least-squares technique (ILS) and standard curve-fitting techniques were used to obtain moduli at zero porosity by extrapolation. A minimal standard error of estimate was achieved in all cases by using an exponential form for the porosity dependence and the ILS technique. Moduli were checked for self-consistency. Extrapolated moduli for MgAl₂O₁ agreed well with respective moduli calculated from single-crystal elastic constants and Birch's law. Longitudinal and shear sound velocities decreased with increasing phase density in the solid-solution systems and systematics indicate that elastic moduli of arbitrary solid solutions can be estimated to lt; ± 10% from the phase density.     

Effective Elastic Moduli of Porous Ceramic Materials

This paper compares the applicability of a few theoretical models for determining effective elastic moduli, using published experimental data on ceramic materials in a porosity range of 0–40% and on a cellular material with a porosity of about 90%. As the experimental data for the effective Poisson's ratio involve a large scatter, a set of numerical experiments using the finite element method was carried out to obtain the variation of the effective Poisson's ratio with porosity. These variations show that the effective Poisson's ratio approaches 0.25 with increasing porosity, irrespective of the material Poisson's ratio. The effect of pore shapes on the effective elastic moduli and the Poisson's ratio has also been analyzed using FEM.     

透明弹性腻子的研制

介绍了透明弹性腻子的研制、参考配方、生产工艺、检测方法、施工方法等。     

Densification of Yttria Transparent Ceramics: The Utilization of Activated Sintering

Highly transparent 0.5 at.% Tm:Y₂O₃ ceramics were prepared by using solid‐state reaction combined with vacuum sintering method, with ZrO₂ and Al₂O₃ as sintering aids. Doping amount of ZrO₂ was fixed at 1 at.%, while the effect of Al₂O₃ on densification, microstructure evolution, and transmittance of the Y₂O₃ ceramics was carefully studied. It was found that the addition of Al₂O₃ was very effective in improving densification of Y₂O₃, due to the formation of an Al‐rich eutectic phase Y₄Al₂O₉ (YAM) during the sintering process. As the content of Al₂O₃ was increased from 0 to 81.8 wt ppm, porosity of the ceramics was decreased and transmittance was increased. However, when the content of Al₂O₃ was increased to 137 wt ppm, a secondary phase began to segregate at grain junctions. Further increase in the amount of Al₂O₃ led to an increase in both amount and size of the secondary phase. At the optimized content of Al₂O₃ with 81.8 wt ppm, the Tm:Y₂O₃ ceramics sintered at 1860°C for 13 h exhibited an in‐line transmittance of 83.0% at 2000 nm and 76.5% at 600 nm. It is expected that this finding can be readily applied to other transparent ceramics.     

Fabrication of Transparent Yttria Ceramics at Low Temperature Using Carbonate-Derived Powder

A new preparation method for a highly sinterable yttria powder has been developed, the resultant powder characterized, and its sinterability studied. As a precursor of the yttria powder, a fine and needle-shaped yttrium carbonate was prepared by a precipitation and aging method. A fine and low-agglomeration yttria powder was obtained by calcining the carbonate precursor at 1100°C. The primary crystallites measured ∼0.1 μm and weakly agglomerated to a size of ∼0.3 μm. The powder had a very high sinterability, so that the powder compact could be sintered to transparency by normal sintering under vacuum without additives at low temperatures, over 1600°C. The sintered transparent ceramics exhibited a homogeneous microstructure with no abnormal grain growth.     

Elastic Moduli of Silica Gels Prepared with Tetraethoxysilane

The elastic moduli of several organometallic silica gels were measured using pulse-superposition interferometry (PSP). A series of gels were prepared under acid-catalyzed conditions where the molar ratio of water to tetraethoxysilane (TEOS) in the sol was varied (4:1, 12:1, 16:1, and 24:1). It was found that the elastic moduli varied systematically with the hydrolysis conditions used to prepare the gels, as well as with the relative humidity used to equilibrate the gels before and during the PSP measurement. The experimental PSP data for the gels were evaluated using a composite model (matrix plus pores) and then the elastic moduli of the gel matrix were determined by applying a correction for porosity based on a self-consistent-scheme (SCS) approximation. The results show that the elastic moduli of the gel matrix also exhibit a systematic dependence upon the preparation conditions, but they always fall below the corresponding values for fused silica. Even when excess water Was used to hydrolyze the TEOS, this did not lead to a fully polymerized silica matrix structure. It is suggested that the lowered matrix moduli values are due to the presence of residual hydroxy and ethoxy groups which are internal to a polymeric structure.     

Relation of Single-Crystal Elastic Constants to Polycrystalline Isotropic Elastic Moduli of MgO

The three elastic compliance coefficients of synthetic periclase were determined in the kilocycles per second frequency range by a resonance method. Polycrystalline elastic moduli on dense-formed MgO were measured. The calculated isotropic elastic moduli for polycrystalline MgO obtained from the single-crystal constants are in good agreement with experimental values measured on the dense MgO at the theoretical density point. The measured Young's modulus and the shear modulus of polycrystalline MgO are found to be 30.50 and 12.90 × 10¹¹ dynes per cm² respectively. The results of the present investigation are compared with the earlier work in the megacycles per second frequency range. A theoretical analysis is made to establish the validity of the present values. Schemes of averaging the single crystalline elastic constants for polycrystalline behavior are reviewed for crystals of cubic symmetry.     

Elastic Moduli and Hardness of Cubic Silicon Nitride

The bulk modulus B ₀= 290(5) GPa and its first pressure derivative B '₀= 4.9(6) were obtained for c -Si₃N₄ from volume versus pressure dependence. Measurements were performed under quasi-hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X-ray powder diffraction. This combined with nanoindentation measurements determined the shear modulus G ₀ of c -Si₃N₄ to be 148(16) GPa. The Vickers microhardness H _V(0.5) for dense, oxygen-free c -Si₃N₄ was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c -Si₃N₄ exceed those of the hexagonal counterparts, α- and β-phases.     

Elastic Moduli of Glasses by a Dynamic Method

The elastic moduli and speed of sound of twenty-three optical glasses, a series of Na₂O₃, CaO₃, SiO₃, and B₂O₃ glasses, and some specimens of fused silica were measured by a dynamic method. Each specimen, in the shape of a prismatic bar, was vibrated in four different ways: longitudinally, flexurally on the wide side, flexurally on the thin side, and torsionally. Good agreement was found on Young's modulus as determined by the first three methods. The torsional mode was used to obtain the shear modulus. Statically and dynamically determined values of Young's modulus for six sample glasses also showed good agreement.     

Fabrication of Transparent Yttria Ceramics by the Low-Temperature Synthesis of Yttrium Hydroxide

Thin flakes of yttrium hydroxide agglomerated in a manner resembling houses of cards with aging at 10°C. The agglomerate then dissociated into fine yttria particles with calcination at >800°C. The particle size of the calcined powder increased appreciably as the calcination temperature increased. The shrinkage curve indicated similar densification behavior among undoped yttria powders calcined at 800°–1000°C, despite considerable particle growth as the calcination temperature increased. Increasing the calcination temperature to >1000°C shifted the shrinkage curve appreciably to the high-temperature region. Sulfate-ion-doped yttria particles had round edges, irrespective of calcination temperature, in contrast to the sharp edges of the undoped yttria particles. A calcination temperature of <1000°C resulted in skeleton yttria particles, which exhibited poor sinterability. At a calcination temperature >1000°C, the skeleton particles dissociated into monodispersed particles that densified easily. When the calcination temperature was >1000°C and the average particle sizes were similar, the undoped and sulfate-ion-doped yttria showed similar densification rates. The transparency of the sintered yttria ceramics was dependent on both the calcination temperature and sulfate-ion doping: that is, sulfate-ion doping and calcining at 1100°C were both necessary conditions for the fabrication of a transparent body.     

Temperature Dependence of the Elastic Moduli of Monoclinic Zirconia

We have used Brillouin scattering to measure the temperature dependence of the sound velocities of the longitudinal and transverse vibrational modes in various directions in small single crystals of monoclinic ZrO₂. Using these velocities and the Christoffel equation, we have calculated the 13 elastic stiffness moduli between room temperature and the monoclinic—tetragonal transformation temperature.     

Microhardness and Elastic Moduli of Si-Y-A1-O-N Glasses

Glasses containing up to 15 at.% N were prepared and tested. The glasses were translucent to transparent, very hard, and had elastic moduli as high as 186 GPa.     

Empirical Dependence of Elastic Moduli on Porosity for Ceramic Materials

Twenty-seven sets of elastic modulus-porosity data spanning ranges > 20% in porosity were fitted by linear, exponential, Hasselman, and 2/3-power relations. The fits were tested for goodness of fit, for extrapolation to zero-porosity moduli, and for consistency in determining zero-porosity Poisson's ratios and bulk moduli. On the whole, the linear relation appears to give a superior fit. This result is seen as supportive of self-consistent theoretical calculations of moduli in porous media. Nonlinear data sets are attributed to a tendency for the spheroidicity of pores to increase with porosity.     

Adiabatic Elastic Moduli of Vitreous Calcium Alurninates to 3.5 Kilobars

"Pulse superposition" ultrasonic interferometry was used to measure elastic wave velocities and related elastic parameters of four vitreous calcium aluminate specimens at pressures up to 3.5 kbars at 25°C. The values obtained for the bulk ( K₂ ) and shear (μ) moduli and for Poisson's ratio (σ) were much higher than those reported for silicate glasses. The bulk moduli for the calcium aluminates, however, fit quite well with the relation between bulk modulus and specific volume per ion pair suggested by Soga and Anderson for 29 glasses. The ( dv_p/dP ) value was highest (8.9×10⁻³ km/(s.kbars)) for the SiO₂-doped vacuum-melted calcium aluminate and lowest (6.8×10⁻³ km/(s.kbars)) for the BaO-doped air-melted specimen. The BaO doping seemed to cause anomalous behavior in the shear wave velocity vs pressure relation; the ( dv₃/dP ) value for the BaO-doped air-melted specimen was −4.1 ×10⁻³ km/(s.kbars). The differences in the compressibility at 1 bar and the rate of change of compressibility with pressure appear to be related to composition and melt conditions. The ( dK₃/dP ) values ranged from 4.0 to 4.7 and the ( dσ/dP ) values from 6.8 to 7.5 ×10⁻⁴/kbars.     

Effect of Initial Particle and Agglomerate Size on the Elastic Moduli of Porous Yttria (Y2O3)

The present study shows the effect of porosity on the dynamic elastic moduli of partially dense yttria (Y₂O₃). This article reveals that the sound velocities and the elastic moduli of the porous samples depend not only on density, but also upon the reciprocal of the initial particle and agglomerate size. These findings explain some of the variation in the elastic moduli found especially, but not only, for highly porous yttria.