Thermal conductivity of gaseous fluorocarbon refrigerants R 12, R 13, R 22, and R 23, under pressure

The thermal conductivity of four gaseous fluorocarbon refrigerants has been measured by a vertical coaxial cylinder apparatus on a relative basis. The fluorocarbon refrigerants used and the ranges of temperature and pressure covered are as follows: R 12 (Dichlorodifluoromethane CCl₂F₂): 298.15–393.15 K, 0.1–4.28 MPa R 13 (Chlorotrifluoromethane CClF₃): 283.15–373.15 K, 0.1–6.96 MPa R 22 (Chlorodifluoromethane CHClF₂): 298.15–393.15 K, 0.1–5.76 MPa R 23 (Trifluoromethane CHF₃): 283.15–373.15 K, 0.1–6.96 MPaThe apparatus was calibrated using Ar, N₂, and CO₂ as the standard gases. The uncertainty of the experimental data is estimated to be within 2%, except in the critical region. The behavior of the thermal conductivity for these fluorocarbons is quite similar; thermal conductivity increases with increasing pressure. The temperature coefficient of thermal conductivity at constant pressure, (/T) _p , is positive at low pressures and becomes negative at high pressures. Therefore, the thermal conductivity isotherms of each refrigerant intersect each other in a specific range of pressure. A steep enhancement of thermal conductivity is observed near the critical point. The experimental results are statistically analyzed and the thermal conductivities are expressed as functions of temperature and pressure and of temperature and density.     

Electrocal,thermal, thermoelectric and related properties of magnesium silicide semiconductor prepared from rice husk

Polycrystalline, 10m size magnesium silicide was prepared by alloying 99.9% purity polycrystalline silicon obtained from rice husk ash and high-purity magnesium powder. The material in sintered pellet form was characterized for its structural, electrical, thermal, thermoelectric and other properties. A typical sintered pellet exhibited a room-temperature (30°C) thermoelectric power of 565 V K^–1 and an electrical resistivity of 35 cm. On the other hand, the material was found to be thermally quite stable up to 650°C with a room-temperature thermal conductivity of 6.3×10^–3cals^–1cm^–1K^–1 (2.6 J s^–1 m^–1 K^–1). These properties of the material indicate that the material can find potential applications as a thermoelectric generator and in other semiconductor devices. Furthermore, an indigenous technology for large-scale production of silanes (SiH₄) can be developed using this Mg₂Si which could be prepared in large quantities by a simple and low-cost process.     

Non-Contact Measurements of Surface Tension and Viscosity of Niobium, Zirconium, and Titanium Using an Electrostatic Levitation Furnace

The surface tension and viscosity of liquid niobium, zirconium, and titanium have been determined by the oscillation drop technique using a vacuum electrostatic levitation furnace. These properties are reported over wide temperature ranges, covering both superheated and undercooled liquid. For niobium, the surface tension can be expressed as (T)=1.937×10³–0.199(T–T _m) (mN·m^–1) with T _m=2742 K and the viscosity as (T)=4.50–5.62×10^–3(T–T _m) (mPa·s), over the 2320 to 2915 K temperature range. Similarly, over the 1800 to 2400 K temperature range, the surface tension of zirconium is represented as (T)=1.500×10³–0.111(T–T _m) (mN·m^–1) and the viscosity as (T)=4.74–4.97 ×10^–3(T–T _m) (mPa·s) where T _m=2128 K. For titanium (T _m=1943 K), these properties can be expressed, respectively, as (T)=1.557×10³–0.156(T–T _m) (mN·m^–1) and (T)=4.42–6.67×10^–3(T–T _m) (mPa·s) over the temperature range of 1750 to 2050 K.     

Authorship patterns in life sciences,preclinical basic and clinical research papers

The present paper examines the multiple authorship in research papers in biomedical sciences from the more basic aspects to clinically oriented research. Seventeen journals were chosen for analysis — nine from the general and life sciences categories and eight from medical sciences group with clinical orientation. All these were high impact journals as per the Science Citation Index and come in the top ten journals in their respective desciplines. The average authors/paper was significantly higher (P<0.001) in medical journals –4.299 (range 3.21–5.35) as compared to general biomedical journals –3.298 (range 3.21–5.35). Data from highly cited papers (1961–78) also indicate that papers in clinical sciences have higher average authors (2.71) as compared to preclinical basic research (2.25: P<0.26) and more basic research areas like biochemistry and molecular biology (2.208; P<0.02). The team size in research in clinical subjects is therefore appreciably larger as compared to basic biomedical sciences. Also the general and biomedical sciences articles were relatively longer (average 7.75 pages; range 2.69–10.07) as compared to medical papers with a clinical orientation (avarage 4.24 pages; range 1.80–12.92; P<0.001).     

Temperature dependence of the piezoelectric,mechanical, and dielectric characteristics of rubber-elastic piezoelectric composites

New rubber-elastic piezoelectric composite materials are investigated by a resonance technique in the temperature range from –60C to +80C. A slight change in the bulk piezoelectric parameters is noted in the interval 5–70C g _v = 60–75 mVm/N). A theoretical physical model is proposed for calculating the piezoelectric characteristics. A piezoelectric cable and large-area piezoelectric sheets have been developed using the new materials.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 54–57, June, 1995.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Sintering Behavior of Hot-Pressed Ca–αSialon Ceramics

On the basis of phase relationships in the Ca–Si–Al–O–N system, a Ca––sialon ceramic was synthesized using the hot-pressing technique. The reaction sequences and densifications of the Ca––sialon vs. firing temperatures have been characterized in detail. The present experiments reveal a reaction sequence as follows: at 1250°C the reactant mixture started to soften, at 1300°C a gehlenite phase was produced, at 1500°C the gehlenite phase was resolved into a liquid phase and a Ca––sialon started to form, and at 1600°C the formation of Ca––sialon was complete. The product was stable and almost entirely single phase Ca––sialon. Accompanying to the above sequences, densification also proceeded via a liquid-phase sintering, particle rearrangement, solution–reprecipitation, and grain growth process. In the final microstructure elongated grains of Ca––sialon were obtained, improving the fracture toughness of this Ca––sialon ceramic.     

Effect of calcination conditions of self-sintering mesocarbon microbeads on the characteristics of resulting graphite

Mesocarbon microbeads are now-a-days used as a prominent self-sintering precursor for the production of high density monolithic graphite. The quality of this graphite is highly dependent on the characteristics of these microbeads, such as the quinoline and toluene insoluble contents, -resins content and volatile matter content, which in turn, can be controlled to desired values by suitable treatments of their extraction and calcination. In the present paper, the authors give an account of the study conducted to see the effect of calcination conditions of mesocarbon microbeads on the characteristics of the resulting graphite. A calcination at a temperature in the range of 280–320 °C for 30 min. under an ambient pressure of nitrogen, or at a temperature of 245–310 °C for 10 min. under a reduced pressure (5 cm Hg) of nitrogen, results in mesocarbon microbeads having a quinoline insoluble content of 83.6–89.8%, toluene insoluble content of 94.4–99.7%, -resins content of 6.8–11.9% and a volatile matter content of 10.2–13.5%. Such microbeads have been found to lead to a monolithic graphite possessing a bulk density 1.91–2.02 g cm^–3, bending strength of 62–70 MPa, Shore hardness of 58–69, electrical resistivity of 2.1–2.6 m cm and a degree of anisotropy of 1.02–1.05.     

High level illumination effect on MS'S solar cell characteristics with a new material Ga2Se3, as an intermediate layer

Compound semiconductor heterostructure (Al–Ga2Se3–nSi) uses a new material for photovoltaic applications. This Metal-Semiconductor-Semiconductor (MS'S) structure with Ga2Se3 as an intermediate layer has been proposed as a better alternative to conventional Metal-Insulator-Semiconductor (MIS) solar cells for normal illumination. It is shown here that in the whole range – starting from lower intensity, i.e. at a concentration ratio, Cr1, to very high illumination level, Cr2000, the proposed new structure gives better results than corresponding MIS solar cells. © 1998 Chapman & Hall     

Crystalline transformation of Ti-B-O,Al-Ti-B-O,Si-Ti-B-O and Al-Si-Ti-B-O by thermal treatments

The thermal transformations of Ti-B-O, Al-Ti-B-O, Si-Ti-B-O and Al-Si-Ti-B-O have been investigated using the methods of thermal analysis and X-ray powder diffraction. The materials are a crystalline series of TiO₂ with partial replacement of titanium by the elements, aluminium, boron and silicon. The anatase form of the materials was transformed to the rutile form at 520680 °C for Ti-B-O, 880950 °C for Al-Ti-B-O, 10801280 °C for Si-Ti-B-O and 11801330 °C for Al-Si-Ti-B-O. The rate constant for the anatase-rutile transformation of Ti-B-O was 6.908×10^–3 min^–1 under isothermal conditions at 680 °C. Analysis of the kinetic data obtained by differential thermal analysis (DTA) gave the activation energy for transformation of anatase into rutile as 663.7 Kcal mol^–1 for Al-Ti-B-O. The lattice parameters for the compounds studied at various temperatures were calculated by least-squares fitting of the X-ray powder diffraction data.     

Density Determination of Liquid Copper, Nickel, and Their Alloys

A method for the determination of the density of electromagnetically levitated metallic liquids has been developed. This method employs an enlarged beam of parallel laser light to produce a shadow image of the sample. The shadow is recorded by a digital CCD-camera, and the images are analyzed using an edge detection algorithm. The circumference is fitted by Legendre polynomials that can be used for calculations of the volume of the sample. The method has been tested successfully on various alloys of copper-nickel (Ni _x Cu _y ), as well as on the pure elements, Cu and Ni. Densities were measured for each sample at different temperatures below and above the melting point, and a linear behavior was observed. At the melting point the densities for copper and nickel were 7.9 and 7.93gcm^–3, respectively. For T=1270°C liquid copper has a density of 7.75gcm^–3 which strongly increases up to roughly 8.1gcm^–3 if a small amount (10–40 at.%) of nickel is added to the system. For nickel concentrations larger than 50at.% the density remains nearly constant.     

Creep of (Mg,Fe)O single crystals

The creep behaviour of (Mg, Fe)O single crystals compressed along 1 0 0 has been investigated over the temperature range 1300 to 1500° C, at stresses between 20 and 70 MPa, for oxygen partial pressures between 10^–4 and 10² Pa, and with iron concentrations between 70 and 11 900 p.p.m. Under these conditions, the dependence of the steady-state strain rate on stress, temperature, oxygen partial pressure, and iron concentration can be summarized by the flow law, exp (–445 kJ mol^–1/RT. These results suggest that the steadystate strain rate is controlled by dislocation climb with a jog velocity which is limited by lattice diffusion of oxygen by a vacancy pair mechanism. The activation energy for creep, 445 kJ mol^–1 is larger than that reported for self-diffusion of oxygen, 330 kJ mol^–1, because the formation energy for jogs is relatively large, 115 kJ mol^–1.     

Kinetics of direct nitridation of pelletized silicon grains in a fluidized bed: experiment, mechanism and modelling

A fluidized-bed nitridation of pelletized silicon grains having a wide size distribution was carried out in the temperature range 1200–1300°C under conditions free of external heat and mass transfer effects. N2(30%–90%)–H2(5%–50%)–Ar (balance) mixtures were used as the nitriding gas at atmospheric pressure. Both the yield of -Si3N4 and the final overall conversion of silicon are affected by temperature and nitrogen gas concentration in a nitriding atmosphere, but hydrogen gas has a minor effect on either of these. After accounting for some of the structural changes that occur during nitridation, a simple model was derived. The model has shown that the pseudo-asymptotic exponential conversion trend in the second nitridation stage could be explained by various reaction mechanisms, adjusted for properties of the size distribution of silicon grains and the experimentally observed spalling of the product scale from the silicon surface. In the investigated range of experimental conditions, nitridation could be considered as having an apparent activation energy of Eapp340 kJ mol-1. © 1998 Chapman & Hall     

EXAFS, magnetic, spectral, and thermal studies on a new polymeric copper mixed-valence compound with the 2-thioisoorotato bridging ligand

From the reaction, in aqueous medium, of 2-thioisoorotic acid and copper carbonate, an orange compound can be isolated. The formula unit contains only one copper and one 2-tioisoorotato anion. This compound has been studied by means of extended X-ray atomic force spectroscopy (EXAFS), magnetic measurements, spectral (infrared, electronic and electron paramagnetic resonance (EPR)) and thermal methods (thermogravimetry, differential scanning calorimetry, evolved gas infrared analysis). EXAFS measurements point out the existence of a first shell around the copper atoms formed by a sulphur atom (Cu–S 0.2209 nm) and two oxygen atoms (Cu–O 0.1947 nm). This coincides with the coordination mode proposed for the ligand from infrared data. The structure may be described as a chain-like structure in which the metal ions are bridged by 2-thioisoorotato anions through both oxygen atoms of the carboxylate group and the sulphur atom, (S2)]–Cu–[(O5,O5)–2-thioisoorotato–(S2)]–Cu–[(O5,O5). Also, the weak involvement of an endocyclic nitrogen atom in the four-membered chelate formation with the sulphur atom cannot be rejected except at distances Cu–N beyond 0.23 nm. Whereas EXAFS measurements detect the only presence of Cu(I), magnetic measurements indicate there is a mixture of valence states of copper atoms (10% CuII+90% CuI). On the other hand, the compounds exhibit an axial EPR spectrum which is typical from a magnetically diluted Cu(II) compound with a stronger ligand field in the equatorial plane. Finally, in order to determine the homogeneity of this compound, we have analysed this sample by electron microscopy. It is composed of spheres with radii ranging between 1.3 and 1.7 m and cylinders whose length ranges between 6 and 10 m and width between 1.2 and 1.6 m. Energy dispersive spectroscopic analysis was performed on several points and areas of this polymer. In all analyses, the ratio Cu : S was the expected 1 : 1 without any appreciable difference between spheres and cylinders. These results seem to indicate a similar composition in both morphologies, pointing to the presence of two polymorphic phases. This could be the reason for the unsuccessful attempts to obtain single crystals of this polymer to date. © 1998 Chapman & Hall     

Deviation from Wiedemann–Franz Law for the Thermal Conductivity of Liquid Tin and Lead at Elevated Temperature

The thermal conductivities of tin and lead in solid and liquid states have been determined using a nonstationary hot wire method. Measurements on tin and lead were carried out over temperature ranges of 293 to 1473 K and 293 to 1373 K, respectively. The thermal conductivity of solid tin is 63.9±1.3 Wm^–1K^–1 at 293 K and decreases with an increase in temperature, with a value of 56.6±0.9 Wm^–1K^–1 at 473 K. For solid lead, the thermal conductivity is 36.1±0.6 Wm^–1K^–1 at 293 K, decreases with an increase in temperature, and has a value of 29.1±1.1 Wm^–1K^–1 at 573 K. The temperature dependences for solid tin and lead are in good agreement with those estimated from the Wiedemann–Franz law using electrical conductivity values. The thermal conductivities of liquid tin displayed a value of 25.7±1.0 Wm^–1K^–1 at 573 K, and then increased, showing a maximum value of about 30.1 Wm^–1K^–1 at 673 K. Subsequently, the thermal conductivities gradually decreased with increasing temperature and the thermal conductivity was 10.1±1.0 Wm^–1K^–1 at 1473 K. In the case of liquid lead, the same tendency, as was the case of tin, was observed. The thermal conductivities of liquid lead displayed a value of 15.4±1.2 Wm^–1K^–1 at 673 K, with a maximum value of about 15.6 Wm^–1K^–1 at 773 K and a minimum value of about 11.4±0.6 Wm^–1K^–1 at 1373 K. The temperature dependence of thermal conductivity values in both liquids is discussed from the viewpoint of the Wiedemann–Franz law. The thermal conductivities for Group 14 elements at each temperature were compared.     

Phonon Modes in CMR Manganites at Elevated Temperatures

Phonon modes of the colossal magnetoresistance manganites La_0.7Ca_0.3MnO₃ and La_0.8Li_0.2MnO₃ have been investigated by far-infrared (100–700 cm^–1) reflectivity spectroscopy at elevated temperatures (300–800 K). The three principal optical phonons all move systematically to lower frequency as temperature increases, in accordance with a Grüneisen relationship. The La(Ca,Li)–(MnO₃) vibration (170 cm^–1) varies with cation mass as expected. The Mn–O stretching mode (580 cm^–1) has a higher frequency in the Li-doped compound than in the Ca-doped, whereas the O–Mn–O bending mode )340 cm^–1) occurs at the same frequency in each, demonstrating that the cation strongly influences the Mn–O– length but not the bond angle.     

Dielectric properties of (Ba,Sr)O-(Sm,La)2O3-TiO2 ceramics at microwave frequencies

The dielectric properties of (Ba, Sr)O-(Sm, La)₂O₃-TiO₂ material at microwave frequencies were investigated. By varying the amount of strontium from 0–25 mol% in the 0.15(Ba_1–x Sr _x )O-0.15Sm₂O₃-0.7TiO₂ composition, it was possible to adjust the frequency temperature coefficient, _f, from –13 p.p.m. °C^–1 to + 30 p.p.m. °C^–1. When 7 mol% Sr was substituted for barium, _f=0 p.p.m. °C^–1 was obtained. TiO₂ with rutile phase (_f400 p.p.m. °C^–1) acted as a dominant element in _f variation of the 0.15(Ba_1–x Sr _x )O-0.15(Sm_1–y La _y )₂O₃-0.7TiO₂ (0x0.25, 0y0.6) system. Additionally, increasing the quantity of lanthanum substitution for samarium had a greater positive effect on _f than strontium substitution for barium. When 60 mol% La was substituted for samarium with 7mol% Sr substitution barium, _f of the system reached 95 p.p.m. °C^–1. The effect on microwave dielectric characteristics of the 0.15(Ba_0.93Sr_0.07)O-0.15Sm₂O₃-0.7TiO₂ (BSST) ceramics by varying the calcination and/or sintering conditions or doping additives, were studied. The added SnO₂ acted as a firing agent to lower the sintering temperature, and the dielectricQ(Q _d) value was improved by properly adding CdO. With 1 wt% CdO addition, the highestQ _d value of the BSST resonator, after calcination at 1100 °C/2 h and sintering at 1370 °C/4 h, reached 4180 at 4 GHz with a small _f of –4 p.p.m. °C^–1 and an _r of 80.7 was obtained.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

X-ray,electrical conductivity,and infrared studies of oxide spinels containing cobalt,titanium and manganese

X-ray, electrical conductivity and infrared studies of the system Co_2–x Ti_1–x Mn_2x O₄ were carried out with a view to investigate the cation distribution. Compounds 0 x 0.6 showed cubic symmetry, compounds 0.8 x 1 are tetragonal. X-ray intensity calculation indicated the presence Of Co²⁺ and Mn³⁺ at both octahedral and tetrahedral sites. A plot of activation energy against composition shows a break where crystal symmetry changes. The electrical conductivity-temperature behaviour obeys the Raschhinrichsen law. The mobility of holes calculated from infrared and conductivity data is of the order Of 10^–9 cm² V^–1 sec^–1. X-ray intensity calculations and conductivity measurements suggest the ionic configuration of the system to be Co _1–x ²⁺ Mn _x ³⁺ [Co _2–2x ²⁺ Mn _x ³⁺ Ti _x ⁴⁺ ]O ₄ ^2– .     

Effects of tensile stresses on corrosion parameters for model crack grooves

Summary Measurements have been made on the electrochemical parameters in a model corrosion gap as affected by tensile stresses varying from zero up to the yield point of the material. It is shown on various constructional materials (D16T aluminum alloy, steel 45, Kh18N12T steel, and PT-3V titanium alloy) that the passivation time for a freshly renewed surface is reduced by a factor 1.5–3.5 as increases, while the galvanic current increases by a factor 1.2–3.0 and the electrode potentials shift by 30–100 mV in accordance with the pH. Numerical values are given for the parameters as functions of and pH.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 4, pp. 22–25, July–August, 1990.