Obtaining corundum-zircon suspensions,their rheological and bonding properties

Conclusions It is shown that the combined grinding of corundum (80%) and zircon (20%) will produce suspensions with high concentrations and binding properties. A study was made of the effect of pH and slip concentration on their rheological properties, density, and the strength of the castings. With a pH of 2.5–3.5 and of 0.50–0.54 we obtained castings with an open porosity of up to 28–30% and _bend of up to 8–9 MPa.Substantial sintering of the material is accomplished even at reduced (1450°C) temperatures. The factors for _bend (on specimens measuring 7×7×70 mm) and _comp (on specimens measuring 10×10×10 mm) with an open porosity of 16–18% are up to 180 and 740 MPa respectively. At 1100 and 1200°C the value of _comp equals 300–340 and 250–300 MPa, respectively.Translated from Ogneupory, No. 6, pp. 22–25, June, 1984.     

Anisotropy in the strength of cellulosic fibres

Conclusions It is proposed to use the equation of a lemniscate, or the Cassini curve, to determine the destructive stress at break for cellulose films in any direction. Thereupon, to determine , it is necessary to know the destructive stress along two basic directions (₀ and ₉₀).To obtain cellulosic films which are isotropic in plane, a necessary and sufficient condition is ₀=₉₀.Translated from Khimicheskie Volokna, No. 6, pp. 34–35, November–December, 1987.     

Phenomenological model of the scale effect of the strength of polymer fibers

A new derivation of the scale effect equation for polymer fibers is proposed. The equation is: AL ^–, where is the average strength of the fiber; L is the length of the fiber; A and are coefficients. The equation was obtained from the general premises of scale effect theory without indication of the concrete form of the probability function F(S).Ukrainian Scientific-Research Institute of Polymer Fibers, Kiev. Translated from Khimicheskie Volokna, No. 4, pp. 34–36, July–August, 1992.     

Studies on electrical conductivity of gamma irradiated polyaniline

Summary de electrical conductivity at surface of -irradiated polyaniline (PAn) has been studied. EPR spectroscopic results indicate that the variation of spin concentration is consistent with the increase of _de. Electrical conductivity (_de) versus temperature (T) characteristics of unirradiated and irradiated PAn were performed, which demonstrate that the unirradiated curve can fit to ln_deT₁, while the irradiated curve fit to ln_deT_1/4.     

Microwave absorption measurements of the electrical conductivity of small particles

A microwave absorption technique based on cavity perturbation theory is shown to be applicable for electrical conductivity measurements of both a small, single-crystal particle and finely divided powder samples when values fall in either the low (<0.1 ^–1 cm^–1) or the intermediate (0.1 100 ^–1 cm^–1) conductivity region. The results here pertain to semiconductors in the latter region. If the skin depth of the material becomes significantly smaller than the sample dimension parallel to theE-field, an appreciable error can be introduced into the calculated conductivity values; however, this discrepancy is eliminated by correcting for the field attenuation associated with the penetration depth of the microwaves. A modification of this approach utilizing the skin depth allows a first-order correction to be applied to powder samples which results in the accurate measurement of absolute values, and results with doped Si powders are compared to values obtained from one small single particle using this microwave technique as well as reported DC values determined with single crystals. The use of this microwave absorption technique with small particles having high surface/volume ratios, such as catalyst supports and oxide catalysts, under controlled environments can provide fundamental information about adsorption and catalytic processes on such semiconductor surfaces. An application to a ZnO powder demonstrates this capability.     

The impedance of the alkaline zinc-manganese dioxide cell. II. An interpretation of the data

The impedance of small alkaline zinc-manganese dioxide cells has been interpreted in terms of a controlling charge-transfer and diffusion process at the zinc electrode throughout the early stages of discharge. After about 20% of the available charge has been removed, it becomes necessary to include the manganese dioxide electrode circuit components. This network has the circuit elements for charge transfer and a proceeding chemical reaction. The Warburg component for the manganese dioxide electrode need not be considered since the effective area considerably exceeds that of the zinc. The relative areas are confirmed by the magnitudes of the circuit element components. The decomposition of the impedance data has been successfully accomplished as far as 80% discharge; after this point cells show considerable differences from cell to cell, especially in the low-frequency range, which makes a confident interpretation difficult. It is considered that this is due to the loss of the physical definition of the system.Nomenclature C _m,C _z double-layer capacitances of MnO₂ and Zn electrodes, respectively - C _X,R _X parallel branch accounting for current density varying with fractional electrode coverage - R resistance of electrolyte - V open-circuit voltage of cell - Z, Z, Z impedance of cell,resistive component ofZ and reactive component ofZ, respectively - _m, _z transfer resistance of MnO₂ and Zn electrodes, respectively - , _R, _C in Warburg equation:Z _W = ^–1/2(1–i) orZ _W = _R^–1/2– i_Cco^–1/2     

Microcalorimetric, reaction kinetics and DFT studies of Pt–Zn/X-zeolite for isobutane dehydrogenation

Microcalorimetric measurements of the adsorption of H₂ and C₂H₄ were carried out at 300 K on a Pt–Zn/X-zeolite catalyst (Pt:Zn atomic ratio equal to 1:1). The initial heats of H₂ and C₂H₄ adsorption were equal to 75 and 122 kJ/mol, respectively, and these values are weaker than the values of 90 and 155 kJ/mol typically observed for supported Pt catalysts. Reaction kinetics measurements for isobutane dehydrogenation over the Pt–Zn/X-zeolite catalyst were carried out at temperatures from 673 to 773 K, at isobutane pressures from 0.01 to 0.04 atm, and at hydrogen pressures from 0.1 to 0.7 atm. The catalyst shows high activity and selectivity for dehydrogenation of isobutane to isobutylene. The reaction kinetics can be described with a Horiuti–Polanyi reaction scheme. DFT calculations were carried out for the adsorption of ethylene on slabs of Pt(111), Pt₃Zn(111) and PtZn(011). Results from these calculations indicate that addition of Zn to Pt weakens the binding energies of -bonded ethylene, di--bonded ethylene, and ethylidyne species on atop, bridged, and three-fold Pt sites, respectively. These effects are most significant for the bonding of ethylidyne species, and they are least significant for -bonded ethylene species. Results from DFT calculations for the adsorption of formaldehyde show that addition of Zn to Pt weakens the di--bonding at Pt–Pt sites; however, this weakening effect of Zn on formaldehyde adsorption is less significant than the effect on ethylene adsorption. Moreover, the preferred location for adsorption of formaldehyde on PtZn(011) is a Pt–Zn site, whereas the preferred location for adsorption of ethylene is a Pt–Pt site. Thus, formaldehyde is adsorbed more strongly by 53 kJ/mol on PtZn(011) compared to the di--adsorption of ethylene, whereas formaldehyde and ethylene adsorb in the di--forms with comparable energies on Pt(111). This preferred adsorption of formaldehyde compared to ethylene on PtZn(011) may be at least partially responsible for the enhanced selectivity of Pt–Zn-based catalysts for hydrogenation of C=O groups compared to C=C bonds in ,-unsaturated aldehydes.     

Electrical Properties and the Structure of Halogen-Containing Lead Borate Glasses: I. The PbF2–2PbO · B2O3 System

The concentration and temperature dependences of the dc electrical conductivity of glasses in the PbF₂–2PbO · B₂O₃ system is investigated. It is found that, at 20–25 mol % PbF₂, the dependences log = f([F]) and E = f([F]) exhibit kinks. The concentration dependence of the true transport number of fluorine ions _F – is studied using the Tubandt method. It is shown that the introduction of 20–25 mol % PbF₂ leads to a crossover from the protonic conductivity to the unipolar fluorine ionic conductivity ( ). The results obtained are discussed in terms of the Myuller theory of the microinhomogeneous glass structure associated with the selective interaction of the components during the synthesis.     

Role of structure and capacity for energy absorption in increasing the strength and crack resistance of refractory ceramics

Conclusions We identified the influence of the parameters of a heterogeneous structure and also of particles possessing the capacity to absorb energy of deformation on the strength _rup and the crack resistance K_lc of refractory ceramics. The proposed relationships facilitate quantitative evaluation of the influence of these factors and indicate ways of increasing the levels of _rup and K_lc. Comparison of the calculated and experimental values of _rup and K_lc obtained from tests of various types of ceramics indicate the suitability of the equations for assessing the characteristics of strength and crack resistance.Translated from Ogneupory, No. 7, pp. 8–13, July, 1987.     

On the Charge Carrier Concentration in Sodium–Calcium Aluminophosphate Glasses

The dc and ac conductivities of the xNa₂O–(35 – x)CaO–7.5Al₂O₃–57.5P₂O₅glasses (mol %) with x= 0–35 are studied in the frequency range from 20 Hz to 1 MHz at different temperatures. It is found that the dc conductivity strongly depends on xonly for glasses with x 10. For glasses with x 5, the dc conductivity is virtually composition independent. The dependence of the ac conductivity plotted on the log(()/_dc) vs. log(/_dc) coordinates is analyzed. The ac conductivity represented in these coordinates depends on xonly for glasses with x 5, in which the dc conductivity does not depend on x. For glasses with xin the range from 10 to 35, all the isotherms of the ac conductivity closely coincide with each other. This result is discussed in the framework of two hypotheses: (1) the conductivity () is determined by the dynamic polarization (of the Maxwell–Wagner type at low frequencies) of the material due to spatial fluctuations of the density of paths providing the migration of sodium ions and (2) the concentration of charge carriers is independent of x.     

Crack resistance of corundum refractories over a wide range of temperatures

Conclusions The ultimate bend strength _bnd and the critical stress intensity factor K_lc of a group of corundum refractories were determined over a wide range of temperatures. It was shown that the temperature dependences of _bnd and K_lc have a similar nature. In the refractories having the usual porous structure, the rate of decrease of K_lc with increasing temperature is less than that of _bnd. Owing to this, the coefficient of crack resistance K_t of the refractories increases at high temperatures.The refractories having a porous-fissured structure, exhibit an almost identical reduction of _bnd and K_lc with increasing temperature and the magnitude of K_t increases very insignificantly.Translated from Ogneupory, No. 3, pp. 8–11, March, 1987.     

Prospects for strengthening fibres and films

Conclusions A model of the orientation strengthening of polymers which is based on the concept of the existence of a network of nodes which rearrange during the stretching process has been used to calculate the limiting values of degree of stretch, , and the strength, , of fibres of polyethylene, polycaproamide, polyvinyl alcohol, polypropylene, polyacrylonitrile, and poly-p-phenyleneterephthalamide.According to calculation, increasing the molecular weight of the polymer and decreasing the concentration of the solution being spun should lead to an increase in and ; for Terlon with a molecular weight of about 250,000, =100 and =10 GPa.Translated from Khimicheskie Volokna, No. 5, pp. 33–34, September–October, 1983.     

Electrical conductance of some cellulose derivatives

Summary The interrelation between the electrical conductance, and some chemically modified cellulose molecule produced through partial substitution of the hydroxyl groups by different groups namely, acetate, methoxyl, carboxymethyl and sodium carboxymethyl has been studied in the temperature ranges 283–333 k. The results show that of the examined samples change in the descending order: sodium carboxymethyl cellulose >carboxymethyl cellulose >methyl cellulose>cotton cellulose>cellulose acetate. The variable response of to the chemical modification and the physical changes of cellulose accompanying such modification has been discussed in terms of the molecular structure, chain flexibility and degree of polymerization of the examined samples.     

Thermal Conductivity of a Glass: I. Measurement by the Glass–Metal Contact

The thermal (phonon) conductivity of glass has been measured by contacting the sample with a metal at a different uniform initial temperature. The subsequent temperature response in the metal is measured by a tiny thermocouple just underneath the (contact) surface. The coefficient of heat penetration c _p follows directly from the fitted asymptotic temperature jump or drop for long times. Division by the separately measured heat capacity c _p yields the thermal conductivity . The conductivity measurement reproducibility was = 3%. The standard deviation between validation measurements and round robin test results on Pyrex glass was = 5.8%, somewhat more than the accuracy = 5.2% of the round robin test results. The measurement method is insensible for slight imperfections of the thermal contact and infrared radiation diffusion (photon conductivity) in a hot glass. The method has been used with minor modifications for solid and molten samples at temperatures of 50–850°C and conductivities of 0.1–25 W/(m K). The thermal (phonon) conductivity of the investigated soda-lime silicate glasses increases slightly (27–30%) with temperature from ambient up to around the glass transition.     

Surface characterization of zirconium titanate (ZrTiO4) powder by measurements of electrical photoconductance and photoassisted oxygen isotope exchange

The photosensitivity of a ZrTiO₄ sample (33 m²/g) prepared by a sol-gel method has been assessed in the presence of O₂ by both photoconductance and oxygen isotope exchange (OIE) measurements at room temperature at wavelengths > 290 nm. For oxygen pressures < ca. 13.3 Pa, the steady-state photoconductance of ZrTiO₄ was unaffected by , which indicated that the direct recombination of the photoproduced charges played the dominant role. At higher pressures, varied as the reciprocal of , which was consistent with the fact that the electronic equilibrium was then governed by O₂ + e^– O ₂ ^– . OIE over ZrTiO₄ occurred predominantly via the overall mechanism which involves the exchange of two surface oxygen atoms for each exchange act. It was very slow as compared with OIE over photocatalytically active anatase samples which, in addition, occurs via another mechanism. These results allow one to predict that this ZrTiO₄ sample is a poorly active photocatalyst for oxidations involving gaseous oxygen, and further illustrate the interest of and OIE measurements to evaluate the photosensitivity of semiconductor oxide samples.     

Influence of static and cyclic compression on the electrical conductivity of FEF black-loaded rubbers

Summary The influence of static and cyclic compression on the temperature dependence of the electrical conductivity (T) of 100 phr FEF black-loaded rubber vulcanizates (SBR, NR, IIR, BR and NBR) has been studied. It was found that increases with static pressure for all rubber vulcanizates. The temperature coefficient of conductivity proved to be negative; its maximum value ranges between –0.1 and –0.01 deg^–1 depending upon the applied stress and type of rubber vulcanizate. The pressure coefficient of conductivity at room temperature varies with the static pressure from 0.78 to 0.04 (kg/cm²)^–1. Menwhile, a was found to decrease with the number and stress amplitude of the cyclic compressions, while increases with the number of compression cycles.     

A vibrational study of the activation sequence of C–H and C–C bonds of isobutene and 1-butene on Mo(110) and (4 × 4)-C/Mo(110) surfaces

The thermal decomposition pathways of isobutene and 1-butene on both Mo(110) and 4 × 4-C/Mo(110) surfaces have been studied using high-resolution electron energy loss spectroscopy (HREELS) in order to highlight the substantially different activities of these two surfaces towards the cleavage of C–H and C–C bonds. On clean Mo(110), the CH₂ group of isobutene decomposes upon heating to 150 K, producing either a /-bonded isobutenylidene [(CH₃)₂CCH] species or a 1,1-di-/-bonded isobutenyl [(CH₃)₂CC] species. Upon further heating, extensive C–H bond scission occurs to form hydrocarbon fragments which do not contain CH₃ or CH₂ groups, but appear to have largely intact carbon skeletons. By contrast, isobutene is molecularly adsorbed on the carbide-modified surface at 150 K. Further heating produces isobutylidyne [(CH₃)₂HCC] by 300 K, which subsequently decomposes via C–C bond scission to generate surface methyl groups. The different activation sequence of the C–H and C–C bonds of isobutene on clean and carbide-modified Mo(110) surfaces is also qualitatively confirmed by comparative studies of 1-butene on the two surfaces.     

Connection between velocity of sound,sound damping coefficient,and strength of ultra-high modulus fibres

Conclusions The strength of high-modulus yarns is connected by a direct linear dependence with the rate of sound propagation and by an inverse linear dependence with the damping coefficient of ultrasonic waves.Thermal aging leads to a monotonic decrease in yarn strength and rate of sonic wave propagation, and to a linear increase in the damping coefficient, a change in molecular orientation always causing a more marked change in the damping coefficient than in the velocity of sound.The linear dependence between c () and () is broken up in the case of heat-drawn yarns; this is connected with formation of defects in the material.Translated from Khimicheskie Volokna, No. 5, pp. 26–27, September–October, 1989.     

Electrical conductivity of praseodymia doped ceria

The electrical conductivity () and oxide ion transference number (t ₀) of praseodymia doped ceria systems were measured. The former increased rapidly with the praseodymia content, while the latter decreased. At 600° C, for instance, CeO₂ and Ce_0.6Pr_0.4O₂ under 0.21 atm of oxygen were 2.0×10^–5 and 3.6×10^–2 S cm^–1; andt ₀ in them were 0.59 and 0.11, respectively. This mixed conductor having high electrical conductivity might be useful as a fuel cell electrode if it could be combined with a suitable solid electrolyte.