The thermophysical properties of LiKSO4 crystals were studied around the high-temperature phase, atTc2=943 K. A Heraeus (DSC) technique was used to measure the specific heat,cp, while the thermal conductivity,K, was measured by the linear heat flow steady-state method. The measured parameters showed an anomaly in the temperature dependence of bothcp andK. Anisotropy in the thermal conductivity coefficient was also observed in the different crystallographic axes. 相似文献
We have used the method of high-temperature series expansions to investigate the critical point properties of a continuous-spin Ising model and g04d Euclidean field theory. We have computed through tenth order the hightemperature series expansions for the magnetization, susceptibility, second derivative of the susceptibility, and the second moment of the spin-spin correlation function on eight different lattices. Our analysis of these series is made using integral and Padé approximants. In three dimensions we find that hyperscaling fails for sufficiently Ising-like systems; the strong coupling limit of g043 depends on how the ultraviolet cutoff is removed. The level contours of the renormalized coupling constant for this model in theg0, correlation-length plane exhibit a saddle point. If the ultraviolet cutoff is removed beforeg0 , the usual field theory results and the renormalization-group fixed point with hyperscaling is obtained. If the order of these limits is reversed, the Ising model limit where hyperscaling fails and the field theory is trivial is obtained. In four dimensions, we find that hyperscaling fails completely; g044 is trivial for all g0 when the ultraviolet cutoff is removed.Work supported in part by the U.S. Department of Energy. 相似文献
The dissociation constants of monochlorophenols (2-, 3-, 4-chlorophenols) were examined using direct UV-vis spectroscopy at temperatures from 25 to 175°C and at saturated vapor pressures in a high-temperature, high-pressure cell. The dissociation constant, Ka increased under experimental temperatures in the order: 2-chlorophenol, 3-chlorophenol, and 4-chlorophenol. The dissociation constant of 4-chlorophenol increased with increasing temperature under experimental conditions, while those of 2- and 3-chlorophenol reached maximum values at around 125°C, and then decreased with further increases in temperature. The slope of (log K)/ (1/T) was nonconstant and positive, that is, endothermic, below 150°C. The data on dissociation constants were analyzed by simultaneous regression to yield a five-term equation that described the Van't Hoff isobar. The magnitude of enthalpy H increased at 25°C in the order: 3-chlorophenol, 4-chlorophenol, and 2-chlorophenol. The decrease in enthalpy at the absolute temperature was larger for 3-chlorophenol than for either 2- or 4-chlorophenol. Considering the equilibrium constant Kb for the isocoulombic reaction of monochlorophenol with OH–, the nearly linear relationship of log Kbvs. 1/T for temperatures between 25 and 175°C indicates that the Cp values for this isocoulombic reaction are low. 相似文献
Differential thermal analysis was carried out on the self-propagating high-temperature synthesis reaction 3TiO2+4Al+3C→3TiC+2Al2O3. The results allow the ignition temperature of the reaction to be estimated and the reaction mechanism to be identified. The ignition temperature was 900°C and the results suggest that the reaction proceeds by an initial reaction between titania and aluminium (3TiO2+4Al→3Ti+2Al2O3) and the titanium formed reacts with the carbon (Ti+C→TiC). 相似文献
Mathematical models that have been proposed for creep in ceramics are described. Emphasis is on models involving grain boundary motion (sliding or flow). In Lifshitz models the crystalline grains elongate with strain; the elongation results from diffusion, slip, or solution and precipitation. In Rachinger models the grains do not elongate during creep. The sliding strain can be accommodated by viscous flow of a glassy phase at the grain boundaries, or if there is no boundary glass by diffusion or slip in superplastic models. Sliding of a glass-free boundary can result in cavitation, cracking, or formation of boundary dislocations or triple point folds.
Most models of ceramic creep at high temperatures predict a steady state (stage II) creep rate that depends on the applied stress, grain size, and temperature. A general equation for the creep rate as a function of these factors, as well as the elastic modulus and a diffusion coefficient, is used to compare models. The models give different exponents for the functional dependence of creep rate on grain size and strain and different temperature dependencies. These differences are compared in tables, and the main mechanistic features of the models are described in the text.
The purpose of this review is to describe creep models rather than to compare them with experimental results or to select the most applicable models. There are few critical experimental tests that allow selection of the most accurate models; such experiments are suggested as the next step in choosing between the models for specific experimental results. 相似文献
Sol-gel processing methods offer novel pathways for tailoring glasses. Amongst the issues that have received the least attention are the effects of the curing temperature on the behavior and photophysics of a dopant molecule sequestered within a sol-gel-derived xerogel. Of particular interest to our group are the effects of processing variables on the ability of a dopant molecule, that is sequestered within a xerogel glass, to be accessed by an analyte and the distribution of the dopant sites within the xerogel. The thermal stability of the luminophore tris(4,7-diphenyl-1,10-phenanthroline) ruthenium (II) ([Ru(dpp)3]2+) provides a convenient way to address these issues and develop an understanding of how one might best exploit curing temperature to construct improved chemical sensors. This paper focuses on quantifying how the film curing temperature affects the spectroscopy and O2 quenching of ([Ru(dpp)3]2+) sequestered within sol-gel-derived xerogel thin films. Our quenching data on films once they have been cured demonstrate that there is a dramatic increase in the sensitivity of the ([Ru(dpp)3]2+) molecules to O2 quenching when the films have been cured at elevated temperatures. This arises primarily because there are two main types of ([Ru(dpp)3]2+) microenvironments within the glass and higher temperature curing leads to an increase in the bimolecular quenching rate between O2 and ([Ru(dpp)3]2+). This is accomplished as follows. Below a curing temperature of 100–150°C, 15% of the xerogel-doped ([Ru(dpp)3]2+) molecules are not accessed to any detectable degree by the O2 molecules during the ([Ru(dpp)3]2+) excited-state luminescence lifetime. However, as the xerogel is cured at or above 150°C, residual silanol-bound waters (or other impurities) dissociate from the xerogel and those ([Ru(dpp)3]2+) molecules that were initially inaccessible become accessible to O2. The dissociation of these water molecules, plus other events, also causes the originally inaccessible ([Ru(dpp)3]2+) population to ultimately exhibit a quenching rate that is greater than the fraction of initially accessible ([Ru(dpp)3]2+) molecules that were formed under ambient curing conditions. 相似文献
Magnetic flux noise is generated by any conductor in equilibrium with a bath as a result of random fluctuating currents. A physical model of this flux noise is proposed, based on allowable current patterns in the conductor, which we describe as natural current modes. This model gives insight into the spatial characteristics of the magnetic noise which is encountered in a variety of magnetic measurements and imaging modalities such as magnetic resonance imaging (MRI). 相似文献
In the present study high-temperature X-ray diffraction, thermal analysis, and mass-spectrometry have been employed for investigation of samples in the La1?xSrxFeO3?δ family (0 ≤ x ≤ 1), the materials being solid solutions having perovskite structure. It has been shown that the loss of oxygen by the samples on heating to 1200°C in air (0 ≤ x ≤ 1) or in vacuum (x < 0.75) does not result in structural rearrangement of the solid solutions, but causes an increase in the lattice parameters. Heating of the compositions with x ≥ 0.75 in vacuum affords phases with ordered vacancies. The observed structural evolutions (growth of the unit cell parameter and vacancy ordering) are reversible, and on re-oxidation (on cooling in air or additional heating of the “vacuum” samples in air atmosphere) the original parameters of the oxides are recovered. The amount of oxygen evolved on heating increases in vacuum or in helium atmosphere, as compared to air, and also grows with rising strontium content, but under experimental conditions does not reach the maximum possible value (δ = x/2). 相似文献