Strength, Fracture Mode, and Thermal Stress Resistance of HfB2 and ZrB2

Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 1800°C and 120,000 psi. Bend strengths were measured on the fully dense materials from 25° to 1400° C in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700° and 1000°C and vary from 39,000 to 68,000 psi for HfB₂ and 30,000 to 56,000 psi for ZrB₂. The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds.     

Change of pore properties during carbonization of coking coal

Porosimetry, sorption and density measurements are reported on two caking bituminous coals, West Virginia Jewel No. 2 medium volatile and a Pennsylvania Pittsburgh seam high volatile C, for final carbonization temperatures between 400 and 1000°C. Samples were not confined and heating rates of 3 and 8.2°/min were employed. The medium volatile samples exhibit pronounced maxima in pore volume, pore surface area and porosity between 600 and 800°C. These temperatures are considerably greater than the characteristic temperature and the temperature at which maximum dilation occurs. The high volatile C coal does not exhibit well defined maxima. Results are interpreted in terms of pore development mechanisms. A mathematical model for pore development is proposed and shown to correlate satisfactorily, the pore volume and surface area measurements.     

Theoretical approach to the temperature dependence of local flow in polymer systems

Summary Starting with the concept of temperature dependent apparent activation energy of flow as a specific temperature function of the considered polymer system a general appraoch is presented, which includes both the Arrhenius and the WLF behaviour. It is demonstrated how local flow can be characterized directly from mechanical measurements via temperature dependent apparent activation energy of flow, using two parameters only, the apparent activation energy of flow extrapolated to infinite temperature, and the Vogel temperature as obtained by extrapolation to infinite activation energy.Examples are given in order to confirm the usefullness of these parameters for molecular interpretation of changes in the local flow mechanism as well as for clarification of the dependences between the polymer structure and the local flow process. As an unique effect it has been found that the energetic interaction of oligomeric polystyrene with poly(vinylmethylether) is much more pronounced than that with high molecular weight polystyrene. This findings may exhibit technical relevance.Herrn Prof. Dr. Dr. H. C. Mult. G. V. Schulz zu seinem 80. Geburtstag mit allen guten Wünschen gewidmet     

HEAT TRANSFER IN THE THERMAL ENTRANCE REGION OF AN INTERNALLY HEATED FLOW

The passage of an electrical current through a liquid causes the evolution of heat and when the electrical conductivity depends on the temperature, the rate of evolution varies from point to point in the fluid. Within the thermal entrance region of a duct flow, competition between the rate of evolution and the rate of conduction to bounding surfaces, coupled with differences in residence time between points in planes perpendicular to the flow direction causes local maxima in the temperature field in such planes. The evolution of such temperature fields is studied here using Galerkin's method to approximate solutions of the appropriate differential equation. Temperature fields of this sort influence the electrophoretic migration of small particles in the process known as continuous flow electrophoresis.     

HEAT TRANSFER IN THE THERMAL ENTRANCE REGION OF AN INTERNALLY HEATED FLOW

The passage of an electrical current through a liquid causes the evolution of heat and when the electrical conductivity depends on the temperature, the rate of evolution varies from point to point in the fluid. Within the thermal entrance region of a duct flow, competition between the rate of evolution and the rate of conduction to bounding surfaces, coupled with differences in residence time between points in planes perpendicular to the flow direction causes local maxima in the temperature field in such planes. The evolution of such temperature fields is studied here using Galerkin's method to approximate solutions of the appropriate differential equation. Temperature fields of this sort influence the electrophoretic migration of small particles in the process known as continuous flow electrophoresis.     

Water Air Bubble Grid Turbulence

The interaction of a swarm of bubbles with the isotropic grid turbulence generated in a vertical water channel has been studied experimentally. Measurements of local void fraction, mean velocity and all three turbulence intensity components are presented at a distance from the grid where isotropic turbulence prevails in single‐phase flow. At low gas flow rate the interaction is rather mild with minor redistribution of the void but significant increase of the turbulence intensity. High gas flow rates result in significant modifications of the flow structure. Void distribution presents maxima at locations between the center of the channel and the wall reminiscing of the Segre‐Sielberberg effect. Similar maxima are observed in mean velocity measurements at values much higher than the corresponding single‐phase flow. Turbulence intensity components increase is following different trends in longitudinal and transverse direction and the isotropy of the flow is destroyed.     

Residual stresses in injection-molded amorphous polymers

Nonisothermal flow of a polymer melt into a cold cavity and its rapid cooling give rise to the buildup of flow and thermal stresses in the molded article. In the present investigation the resultant residual stresses (RS) induced by these two sources were studied in two stages. First, the flow-induced stresses were relaxed by proper heat treatment followed by quenching, resulting in only thermal stresses. The experimentally determined RS profiles in polysulfone and amorphous polyamide showed a parabolic shape and were correlated with the initial and final quenching temperatures, the glass transition temperature, and Biot Number. In the second stage, the combined effect of thermal- and flow-induced stresses was studied using injection-molded specimens prepared under a wide speptrum of molding conditions including melt and mold temperatures and injection rate and pressure. Results here indicated that the basic thermal-induced parabolic RS profiles are altered by the flow-induced stresses resulting in complicated profiles including local maxima and unbalanced RS. Finally, the tensile mechanical properties obatained for plaques molded under the various injection-molding conditions were studied and correlated in part with the previously determined RS profiles. Results have shown that a property gradient exists as a function of distance from both the gate and surface of the molded plaque.     

子波分析识别环流反应器泡状结构实验

用热膜测速技术以高于对应最小湍流时间尺度的分辨率精细测量了环流反应器内气液二相流速度的时间序列信号,对气液二相流速度信号中自动识别泡状流结构进行了研究。用子波分析方法,将热膜测速仪输出的气液二相流混合速度信号在时域空间和频域空间同时进行时频双局部化多尺度分解,研究了气液二相流中液相流和泡状流结构的分尺度时域特征和分尺度能谱等统计特征,利用子波能谱分析的能量最大准则,提出了一种用子波变换自动识别气液二相流中泡状流结构的新方法。     

Couette grain flow experiments: The effects of the coefficient of restitution, global solid fraction, and materials

Granular flows are complex flows of solid granular material which are being studied in several industries. However, it has been a challenge to understand them because of their non-linear and multiphase behavior. The present experimental work investigates granular flows undergoing shear, by specifically studying the interaction between rough surfaces and granular flows when the global solid fraction and the material comprising the rough shearing surface are varied. A two-dimensional annular shear cell, with a stationary outer ring and inner driving wheel, and digital particle tracking velocimetry (DPTV) technique were used to obtain local granular flow properties such as velocity, local solid fraction, granular temperature, and slip. A customized particle drop test apparatus was built to experimentally determine the coefficient of restitution (COR) between the granular and surface materials using high-speed photography. Results showed that wheel surface materials that produce higher COR values exhibit higher velocity and granular temperature values near the wheel, and lower slip velocities. The local solid fraction appears inversely related to the COR values. The global solid fraction seemed to correspond with velocity and granular temperature, while displaying an inverse relationship to slip. Results also showed an initial decrease in the kinetic energy of the flow as the global solid fraction increased, due to the formation of a distinct contact region. This was followed by a rise in kinetic energy as the global solid fraction continued to increase, based on the increase of particles present in the kinetic region of the flow.     

Characterization and continuous extrusion of isotropic and anisotropic poly(p-phenylene terephthalamide)/sulfuric acid solutions to form ribbons and blown film

An experimental characterization and processing study of redissolved Kevlar® and poly(p-phenylene terephthalamide)/sulfuric acid solutions is reported. Polarized light microscopy studies show the development of an anisotropic phase. When two phases coexist, negatively birefringent spherulites are observed. At higher concentrations, a single phase of coalescing spherulites is seen and following melting and subsequent cooling, nematic (threadlike) structures are observed. Viscosity and normal stresses were measured at various concentrations and temperatures. A yield stress is exhibited at room temperature. Both room temperature and 60°C viscosity vs concentration curves display maxima. The solution in concentration ranges from 2 to 12 percent have been extruded as ribbons and as annular blown tubular film. Processing variables and problems are discussed. Wide angle X-ray scattering patterns of films show orientation. Tensile properties have been measured on films.     

Transformation Range Behavior of Lithium Galliosilicate Glasses

The transformation range viscosity and thermal expansion behavior of lithium galliosilicate glasses were studied. The glass transformation and dilatometric softening temperatures exhibit maxima at gallium to lithium ratios of somewhat greater than one. Similar behavior was observed for the transformation range viscosity. The effect of phase separation on the dilatometric softening temperature is discussed in detail. These results are compared with those obtained for lithium alumino-silicate glasses in other studies. The results are discussed in terms of the structural role of aluminum and gallium in the glass network.     

OH concentration fluctuations in turbulent natural gas jet flames

OH radical concentrations in a turbulent non-premixed natural gas flame were measured using laser-induced fluorescence. Instantaneous concentration profiles along a line were obtained using a diode array camera. Investigation of the molecular transitions during laser excitation shows that concentrations, where calibrated with a one-dimensional laminar premixed flame, are biased with a factor of about two. This bias is similar for different flames, so that results of different flames can be compared with an accuracy of about 20%. Three different flames were studied, with fuel jet Reynolds numbers of 9.7 × 10³, 6.8 × 10³ and 4.9 × 10³. Average concentrations and probability density functions show that concentrations close to the nozzle in the flame with highest turbulence are low, which may indicate local extinction. Integral length scales and Taylor micro scales, derived from spatial correlation, exhibit minima at radical locations where OH fluctuations exhibit maxima.     

Isothermal physical aging of a fully cured epoxy—amine thermosetting system

The rate and effects of isothermal physical aging of a fully cured epoxy—amine/glass fiber composite specimen were studied for a wide range of isothermal aging temperatures (−180 to 200°C) using a freely oscillating torsion pendulum technique: torsional braid analysis (TBA). As assigned from the maxima in the mechanical loss vs. temperature, the glass transition temperature, T_g, was 182°C (0.9 Hz), and the principal glassy-state secondary transition temperature, T_β, was ≈ −30°C (1.9 Hz). Plots of the increase in the isothermal modulus and of the decrease in the isothermal mechanical loss were linear vs. log aging time; their slopes provided aging rates. It was found that the isothermal aging rate varies with isothermal aging temperature (T_a) and that there are two maxima in the aging rate vs. T_a. A correlation presumably exists between the two maxima in the aging rate and the two transitions. This is not surprising since mechanical loss maxima (i.e., transitions) and aging rate maxima both correspond to specific, localized, and restricted submolecular motions. Effects after isothermal physical aging were investigated vs. temperature in terms of change of modulus of the specimen. The effect of isothermal aging existed primarily in a narrow temperature region localized about T_a. The majority of the isothermal aging effect can be eliminated by heating to temperatures above T_a, but below T_g. Theoretical and practical implications of this observation are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 55–67, 1997     

ESTIMATION AND MEASUREMENT OF THE DISTRIBUTION OF TENSILE MODULI IN INJECTION MOLDED THERMOPLASTICS

A variety of techniques have been employed to study anisotropic behavior, as a result of processing, in injection molded parts. The techniques include density, sonic modulus, birefringence, wide-angle X-ray diffraction, infrared dichroism and mechanical testing.

Generally, maximum density is observed in the core of the molding and near the gate, while minimum density is observed near the surface. Maximum orientation occurs at or near the surface, as indicated by both birefringence and sonic measurements. X-ray diffraction suggests a complex pattern of orientation for the three cyslallographic axes. The crystallographic a-axis tends to be oriented in the flow direction as shown by both the X-ray and infrared techniques. In general, raising the molding temperature has only a marginal influence on crystallinity and orientation. However, the two resins included in the study exhibited substantial differences in the distribution of these properties. Both elongation at break and at yield exhibit maxima near, but not at, the surface. On the other hand, tensile stress at yield tends to increase monotonically from the surface towards the centre.

The results of crystallinity and orientation are combined to predict the distribution of tensile moduli using available theories. Good agreement is obtained between the calculated and measured values.     

Rheostructural studies on a synthetic mesophase pitch during transient shear flow

The steady, shear viscosities of a synthetic mesophase pitch (Mitsubishi AR-HP) obtained from rate-sweep experiments at 0.1-10 s⁻¹ exhibited shear-thinning (region I) and plateau responses (region II), but displayed a hysteresis during the decreasing rate sweep. Transient tests revealed that the shear stress (and consequently the shear viscosity) displayed a local maximum and a minimum before approaching a steady state. Following steady flow at 1 s⁻¹, a reversal of flow direction or a very short interruption in flow did not lead to the maxima or minima in the transient shear stress, but the maxima and the minima reappeared in the transient stress after a rest time of ∼1000 s. An experimental protocol was developed to preserve the rheological samples, and their microstructure was characterized in three orthogonal planes for the initial and final states. The initial microstructure was found to have a weak, but preferred, orientation of mesophase layers in the radial direction of the rheometer cone-plate (due to the initial squeezing flow). The initial microstructure changed to a flow-aligned fibrous structure after shearing in the viscometric flow.     

Polyvinylchloride melt rheology II—the influence of molecular weight on flow activation energy

The flow behaviors of a series of polyvinylchloride (PVC) resins covering a broad range of molecular weight have been examined at several temperatures. It has been shown that the influence of temperature on viscosity depends on the temperature range. That is, the flow activation energy is not constant but can be approximated by two values, one applicable to low temperatures, the other to high temperatures. The flow activation energy based on viscosities at constant shear rate decreases as the molecular weight increases. In contrast, the flow activation energy from viscosities at constant shear stress increases with molecular weight. The fact that the activation energy is dual valued does not seem to be associated with the polymer type. Both emulsion and suspension resins exhibit this behavior. Addition of certain modifiers appears to alter the activation energy at lower temperatures. These observations indicate that the shift in the activation energy in the low temperature range is due to a change in the flow mechanism.     

常温及热态通气条件下多层桨三相搅拌反应器中局部气含率分布

Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration (a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT+2WHU) recommended in previous work has been used in this work. The operating temperatures were 24°C and 81°C, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are investigated and discussed. Results show that the increasing of agitator speed or gas flow rate leads to an increase in local void fraction at the majority of measurement points in both cold and hot systems. However, the uniformity of gas dispersion does not always increase as the rais-ing of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase stirred tanks.     

Parametric sensitivity of chain polymerization reactors exhibiting the trommsdorff effect

The recently developed mathematical technique for studying the parametric sensitivity of reactors is extended to include chain polymerization systems exhibiting the gel and glass effects, as well as physical property variations. The sensitivities of the two temperature maxima with respect to various parameters are computed. It is found that, for a sample system, poly(methyl methacrylate), all the sensitivities of the gel effect induced temperature peak attain their maxima at the same conditions—this leads to a generalized temperature sensitivity constraint applicable to reactor design or operation. This sensitivity boundary is associated with high conversions and high molecular weights. The analysis shows that the dimensionless propagation activation energy, ?_p, and the dimensionless initiation activation energy, ?_d, are the two most important parameters governing the system performance. Sensitivities of the gel effect-induced number average chain length peak with respect to various parameters are also obtained. Again, all of these chain length sensitivities show maxima at the same condition, leading to the concept of a generalized chain length sensitivity criterion of constraint. Most importantly, the temperature and chain length sensitivity boundaries are virtually identical.     

The electrochemistry of silver in koh at elevated temperatures—III. Potentiostatic study

The electrochemistry of polycrystalline silver in 1 mol kg^?1 KOH at temperatures between 295 K and 478 K has been studied using potentiostatic polarization. At potentials corresponding to silver dissolution and to the early stages of Ag₂O formation, the transient current data at all the temperatures studied exhibit regions that are linearly dependent on the inverse square root of time. This dependence indicates mass transport to be a rate limiting step in these processes. Log (steady-state current) vs potential data show that at all temperatures silver undergoes an active-to-passive transition. However, the potential of the transition shifts to less positive values and the passive current increases with increasing temperature. In the region of formation of surface oxide phases, the transients exhibit maxima due to nucleation and growth phenomena. The effect of temperature on the transients is similar to the effect of overpotential; in both cases, the time taken to reach the maximum current decreases.     

ON THE UNIMODALITY OF THE EXACT LIKELIHOOD FUNCTION FOR NORMAL AR(2) SERIES

Abstract. For stationary second-order autoregressive normal processes, the conjecture of uniqueness of the solution of the exact likelihood equations is examined. A sufficient condition for uniqueness is given; this condition is satisfied with very high probability if the number of observations is not extremely small. Moreover, it is shown that not more than two maxima may exist. Examples of data which actually produce a likelihood function with two local maxima are given.