Coaxial variable-length resonator for determining the electromagnetic parameters of materials at normal and higher temperatures

Conclusions It is possible by means of the above resonator, according to our analysis and in the absence of air gaps between the sample and the line, to evaluate the real components and of permeability and permittivity respectively in the range of 2 to 100 with an error between ±3% and ±10% in the temperature range from room temperature to +400°C, and the imaginary components and (for tan and tan in the range of 0.001 to 2) with an error of 7 to 20% over the same temperature range.     

Rheology of stimulated whole saliva in a typical pre-orthodontic sample population

The dynamic viscosity () of stimulated whole saliva in a typical pre-orthodontic sample population was characterized as a function of temperature (T). Samples were collected from 30 adolescents or young adults, after screening for factors that are known to have an effect on salivary viscosity. Using a cone and plate viscometer, 1.5 ml of stimulated whole saliva was evaluated at a constant shear rate of 450 s^-1 from T=20°C to T=40°C. Data from the -T plots showed a negative dependence of the form, =a–bT, over a range of from 1.08 to 2.45 centipoise (cps) at 34°C. Most of the samples fell into a narrow envelope, where the mean of the saliva samples ranged from 2.42±0.61 cps at 20°C to 1.57±0.32 cps at 37°C. With regard to sample stability, viscosity-time plots indicated that a small but predictable decrease in occurred during the 3 h period. The -T plots generated from fresh and frozen saliva samples demonstrated an appreciable change in as a result of refrigeration. With regard to sample reproducibility, viscometric data obtained from a typical pre-orthodontic patient over a 1-week period fluctuated within a fairly broad envelope of values.Presented, in part, at the 21st Annual Meeting and Exhibition of the A.A.D.R., Boston, Massachusetts, March, 1992.     

Study on surface cracking of alumina scratched by single-point diamonds

Scratching experiments have been carried out on hot-pressed alumina by using single-point diamonds of conical shape (conical angle 85, 108, 128 and 65° and nose radius 1.6, 1.1, 1.9 and 45.0m, respectively). With an increase of the scratching depth, the material exhibits the following behaviour: macroscopic plastic deformation scale-like cracking cracking or chipping. Cracking or chipping is absent only if the groove depths are less than certain values which vary with the diamond shape. The crack penetration depth is approximately in proportion to the depth of cut. For a depth of cut less than 2m, the penetration depth of crack induced by the diamond of nose radius 45.0m is almost 8 times those produced by the other diamonds. For a depth of cut more than 2m, the former is more than twice the latter. The coefficients of pile-up increase as the depths of cut decrease.     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

The role of starting powder size on the compressive response of stand-alone plasma-sprayed alumina coatings

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 m, 19 m and 32 m alumina; the resulting effective lamella diameters were 10 m, 28 m, and 55 m, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 m powder exhibited greater moduli than tubes sprayed from either 19 or 32 m powders. This difference was attributed to the greater percentage of unmelted -Al₂O₃ in the coating.     

Effect of fiber content on the thermoelectric behavior of cement

The effect of discontinuous stainless steel fibers (diameter 60 m) as an admixture in cement paste on the thermoelectric behavior (the Seebeck effect) was systematically studied as a function of fiber volume fraction from 0 to 0.50 vol%. Without fibers, cement paste has an absolute thermoelectric power of +3 V/°C. A fiber content of up to 0.20 vol% makes the absolute thermoelectric power more negative (down to –63 V/°C), whereas a fiber content of 0.20–0.50 vol% makes the absolute thermoelectric power more positive (up to +31 V/°C)—even more positive than the positive value for the steel fiber by itself (+8 V/°C). The value is zero at a steel fiber content of 0.27 vol%. The effects are probably due to carrier scattering rather than conduction.     

Effects of grain size on pyroelectric and dielectric properties of Pb0.9La0.1TiO3 ceramics

Pyroelectric and dielectric properties of Pb0.9La0.1TiO3 ceramics with grain sizes in the range of 1.3–6.0 m were investigated. The temperature stability of the pyroelectric coefficient at room temperature was improved as the grain size became smaller. The pyroelectric coefficient of Pb0.9La0.1TiO3 at room temperature reached the lowest value near the grain size of 2.7 m. Above 2.7 m, the relationship between the pyroelectric coefficient and the grain size could be explained by the increase of 90° domain switching. However, below 2.7 m, the relationship was not clear. The variation of the relative dielectric constant of Pb0.9La0.1TiO3 with the grain size showed a similar relationship with that of the pyroelectric coefficient. The variation of the pyroelectric figure of merit on voltage responsivity as a function of the grain size was similar to that of the pyroelectric coefficient because the pyroelectric coefficient varied more strongly than the relative dielectric constant. © 1998 Chapman & Hall     

The dielectric properties of glass-ceramic-on-metal substrates for microelectronics packaging

The dielectric properties of some glass-ceramic-on-metal substrates have been determined over the frequency range 500 Hz to 5 MHz using a.c. bridge techniques. The substrates consisted of cordierite-based glass-ceramics screen printed on molybdenum. For glass layers of thickness greater than 100 m both the permittivity, and the dielectric loss, , are frequency independent over this frequency range at room temperature giving the value of =6.5 and tan =8×10^–3; the room-temperature data are consistent with the universal law of dielectric response. The variation of permittivity with temperature has also been examined and, below 120 °C, the temperature coefficient [(–1) (+2)]^–1 (/T)_p, was found to be 1.3×10^–5 K^–1. The results are compared with those previously reported for Al₂O₃ and AIN substrates.     

Grain refinement of a TiAl alloy by heat treatment through near gamma transformation

Casting of TiAl alloys is receiving more and more research because of its relatively low cost. One problem with this technology is that the coarse microstructure developed during solidification is harmful to material properties. Thus it is essential to seek an approach to refining the cast microstructure and this approach may also be applicable to cast components. In this study, a novel heat treatment route is proposed to obtain a fine fully lamellar structure from a cast TiAl alloy with a grain size of 1000 m. This route consists of three steps, namely pretreatment to have a feathery structure, annealing in the + region to have a fine near gamma structure and solution treatment to develop a fully lamellar structure with a grain size of 30 m.     

Dissolution and adhesion behaviour of radio-frequency magnetron-sputtered Ca–P coatings

Radio-frequency magnetron sputtering deposition was used to produce calcium phosphate sputter coatings with three different thicknesses (0.1, 1 and 4 m) on titanium discs. Half of the as-sputtered coatings were subjected to an additional heat treatment for 2 h at 500°C. X-ray diffraction demonstrated that annealing at 500°C changed the amorphous 1 and 4 m sputtered coatings into an amorphous–crystalline structure, while the amorphous 0.1 m changed in a crystalline apatite structure. Further, scanning electron microscopy (SEM) inspection demonstrated that annealing of the 1 and 4 m coatings resulted in the appearance of some cracks. The dissolution behaviour of these Ca–P coatings was determined in a simulated body fluid. It was found that after incubation for 4 weeks the dissolution was determined by the crystallinity of the deposited coating. SEM and Fourier transform infrared evaluation showed that all the heat-treated sputter coating appeared to be stable under the test conditions and a Ca–P precipitate was always deposited on the coating surface. On the other hand, the amorphous 0.1 and 1 m coatings dissolved completely within 4 weeks, while the amorphous 4 m coating showed only signs of surface dissolution. Scratch testing demonstrated that there is a linear correlation between the critical load, L _c, and the coating thickness. A heat treatment for the CaP-4 coating resulted in an additional decrease in the critical load. On the basis of these findings, we conclude that already a 0.1 m heat-treated Ca-P sputter coating is of sufficient thickness to show in-vitro adequate bioactive and adhesive properties.     

Strength of the pyroceram So115M under lengthy static loading

The article presents the results of long-time strength tests of the casting pyroceram SO115M at room temperature by the method of three-point bending. We obtained the power dependence of the time to failure _f on the applied load : _f ^–11.7.Translated from Problemy Prochnosti, No. 1, pp. 102–104, January, 1996.     

Electrical and magnetic properties of Ni0.8Zn0.2Fe2O4/silica composite prepared by sol-gel method

Ultrafine Ni_0.8Zn_0.2Fe₂O₄ particles dispersed in silica (SiO₂) matrix are produced by sol-gel method. The powders were subjected to X-ray diffraction to confirm the formation of crystalline phases. The physical properties such as bulk density, true density, % of porosity and % of linear shrinkage were studied. The magnetic permeability as the function of frequency from 1 kHz to 13 MHz and temperatures from room temperature to 300°C were studied for samples sintered at temperature 1250°C. The AC electrical resistivity as the function of frequency and DC electrical resistivity as the function of temperature were studied. The AC-resistivity of the order of 10⁵ cm and DC-resistivity 10⁸ cm were obtained at room temperature. Microstructural features of sintered samples show the presence of ferrite grains of 1–2 m size.     

Precipitation characteristics of μ-phase in wrought nickel-base alloys and its effect on their properties

Thermal exposures consisting of 1–16000 h at 540, 650, 760, and 870°C were used to study the susceptibility of selected nickel-base alloys to precipitation of -phase and its effect on mechanical strength and corrosion resistance. Analytical electron microscopy and X-ray diffraction were used to characterize the -phase. A -phase of the type Mo₆Ni₇ in nickel-base alloys was found to be stabilized by critical concentrations of iron in an excess of about 3 wt%. Generally, the -phase had a characteristic defect structure consisting of twins and stacking faults, and it exhibited a preferential tendency for precipitation at existing molybdenum-rich carbide particles within the alloy matrix and at grain boundaries. Precipitation of -phase was found to produce a moderate loss of room-temperature tensile ductility; however, it resulted in a considerable degradation of impact toughness and corrosion resistance. In contrast, it had no significant effect on elevated temperature tensile properties. A correlation was found to exist between the Ni/Fe + Co ratio as well as the Mo + W content of the alloy and susceptibility to precipitation of -phase.     

Absorption Spectrum of ZnO Precipitates in ZnSe

The absorption spectrum of ZnO precipitates in ZnSe saturated with oxygen is studied in the spectral range 500–2000 cm^–1 and is shown to correlate with the transmission spectrum of single-crystal ZnO. Saturation of ZnSe with oxygen in the course of growth leads to ZnO precipitation during cooling. The precipitates give rise to narrow absorption bands in the range 5.8–7.1 m, which correlate with the components of the multiphonon absorption spectrum of ZnO, formed by combinations of LO and TO phonons. The 2LO mode defines the long-wavelength transmission edge of ZnO. In addition, the spectrum shows strong absorptions due to the LO + TO(10.2 m) and 2TO (11.3 and 12.7 m) modes. The possible role of SeO₂ is discussed. It is suggested that ZnO with a high carrier concentration may act as a gray filter in the spectral range 2–9 m.     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 m and a zirconia grain size of 0.2 m. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the alumina phase stage shifted to lower temperature. During pressureless sintering the to alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature.     

Effects of carbon content on microstructures and magnetic properties of annealed or solution treated Fe-Cr-Ni-C alloys

The correlations between microstructures and magnetic properties were studied in four Fe-17.5 mass%Cr-2.0 mass%Ni-Xmass%C (X = 0.3–0.6) alloys. Each alloy consisted of ferromagnetic and M₂₃C₆ carbide phases by step-annealing at 1053 K and 848 K. The number of the M₂₃C₆ carbide particles increased with the carbon content, and this microstructural variation caused a deterioration of the soft magnetic properties. On the other hand, the alloy with 0.3 mass% carbon content consisted of ferromagnetic and paramagnetic retained structures after solution treatment at 1473 K. The amount of the stable structure increased with the carbon content, while the amount of the structure decreased. This microstructural variation caused a decrease in the relative permeability, _r, and the stabilization of the paramagnetic property to low temperatures below room temperature. The temperature stability of the _r values was closely related to the martensite start temperature, M _s. From an equation for the estimation of M _s from the chemical compositions of the phase, the M _s's of the alloys with 0.3, 0.4, 0.5 and 0.6 mass%C were estimated to be 326, 289, 241 and 216 K respectively. These values were consistent with the M _s's expected from the microstructures and temperature dependences of the _r values.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

Measurement of temperature in a non-steady-state gas flow

A method is described for measuring the temperature of a non-steady-state gas flow with a thermocouple which is an inertial component of the first order.Notation T*_f non-steady-state gas flow temperature - T_t thermosensor temperature - thermal inertia factor of thermosensor - time - C total heat capacity of thermosensor sensitive element - S total heat-exchange surface between sensitive element and flow - heat-liberation coefficient - temperature distribution nonuniformity coefficient in sensitive element - Re, Nu, Pr, Bi, Pd hydromechanical and thermophysical similarity numbers - P^* total flow pressure - P static flow pressure - T^* total flow temperature - d_t sensitive element diameter - w gas flow velocity - flow density - flow viscosity - _f flow thermal conductivity - k gas adiabatic constant - R universal gas constant - M Mach number - T thermodynamic flow temperature - _o, _o and values at T=288°K - A, m, n, p, r coefficients - _c heat-liberation coefficient due to colvection - _r heat-liberation coefficient due to radiation - _b emissivity of sensitive element material - Stefan-Boltzmann constant - T_e temperature of walls of environment - _c, _r, _tc thermosensor thermal inertia factors due to convective, radiant, and conductive heat exchange - L length of sensitive element within flow - a thermal diffusivity of sensitive element material - _t thermal conductivity of sensitive element material Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 59–64, July, 1984.