Adsorption Isotherm and Pore Characteristics of Nano Alumina Derived from Sol-Gel Boehmite

This paper deals with a systematic investigation on the synthesis of aluminium oxide starting from mono hydroxy aluminium oxide (boehmite, (AlOOH)) which in turn is synthesized from aluminium nitrate. Boehmite on heating forms , transitional alumina and -alumina phases in the temperature range 400–600, 800–1050 and 1050–1100°C respectively. Calculation from XRD, using Scherer equation shows that -alumina has crystallite size in the range 5–10 nm, while and -alumina are in the range 10–20 nm and -alumina is about 33 nm. The textural features of aqueous sol-gel boehmite samples calcined at various temperatures were analyzed by specific surface area measurements and adsorption isotherm features. Maximum specific surface area of 266 m²/g is observed for the precursor calcined at 400°C and a minimum of 5 m²/g at 1100°C. Total pore volume is maximum for the precursor calcined at 600°C (0.2653 cm³/g). Average pore size ranges from 3 nm (400°C) to 11 nm (1100°C). The adsorption isotherms also show a change from Type IV to Type II with increase in temperature showing difference in surface properties. The information from t-plots, pore size distribution and cumulative pore volume data also indicates differences in porosity features of boehmite on calcination. The adsorption isotherm and pore size distribution analysis show maximum microporosity at 400°C, while maximum mesoporosity is observed at 600°C. At higher temperatures, porosity decreases, even though small fraction of pores in the mesopore range is still retained. At 1100°C, there is structural transformation from transitional to -alumina, with very low specific surface area 5 m²/g and pores in the size range of 11 nm. The various data presented in this study will be useful in the synthesis of alumina with tailor made properties.     

Elucidating the mechanism of carbocationic polymerization of α-methylstyrene

Summary The polymerization of -methylstyrene (MeSt) using the H₂O/SnCl₄ initiating system and ethyl chloride solvent has been investigated over the temperature range from –40° to –122°C in the presence and absence of the proton trap 2,6-di-tert-butylpyridine (DtBP). Arrhenius (In ¯M_n versus 1/T) plots obtained with poly (methylstyrene) (PMeSt) samples prepared in the absence of DtBP reveal the existence of two sharply defined temperature regimes (see Figure 1): at higher temperatures from –40 to –86°C, the slope of the Arrhenius plot yields H_{M n} = –4.90±0.25 kcal/mole whereas at lower temperatures, from –86° to –122°C,H_{M n} = –0.3 kcal/mole. With PMeSt samples prepared in the presence of DtBP the H_{M n} obtained for the higher temperature regime increases to –1.67 ± 0.20 kcal/mole whereas the H_{M n} reflecting the lower temperatures remains the same as that obtained in the absence of DtBP. These observations are readily explained by postulating a change in mechanism at –86°C: Evidently the ¯M_n of PMeSt is determined over the higher temperature regime by chain transfer to monomer which is frozen out at lower temperatures where termination becomes ¯M_n determinant. In the absence of DtBP chain transfer to monomer is operative which leads to the higher Arrhenius slope over the higher temperature regime; however, over the lower temperature regime where chain transfer is absent and termination is ¯M_n controlling, the Arrhenius slope remains unchanged. Evidence obtained from a Mayo plot (negligible intercept in the 1/¯M_n versus 1/[MeSt]_o plot, Figure 2) with samples prepared at –92°C, corroborate this postulate. Molecular weight dispersities (¯M_W/¯M_n) as a function of temperature have been determined in the presence and absence of DtBP (Figure 3). The proton trap affects ¯M_W/¯M_n only over the higher temperature regime which also suggests that chain transfer to monomer is frozen out at –86°C and that the polymerization becomes termination dominated at low temperatures.     

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.     

Creep and long-term strength of magnesia ceramics

Conclusions A study was made of the creep and long-term strength of magnesium oxide ceramics with crystal sizes of 12 and 25 at 1400–1550°C and 200–600 kg/cm². Ceramics with smaller crystal sizes are characterized, in identical conditions, by a higher creep rate, but lower long-term strength. The rate of the steady creep and the time prior to destruction are related for magnesia by the ratio _p=A()^–2. Magnesium oxide ceramics are characterized by the mechanism of destruction during creep.Translated from Ogneupory, No.6, pp.38–43, June, 1972.     

Sintering of magnesium hydroxide obtained from magnesium chloride solutions

Conclusions We studied two batches of magnesium hydroxide obtained by precipitating out (using dolomite milk) from magnesium chloride solution formed when processing potassium ores.The effect of the compaction pressure, the firing temperature, and prior heat treatment on the sintering process of magnesium hydroxide was investigated.The experimental samples of magnesium hydroxide exhibit high sinterability and facilitate the production of periclase powders (powder bodies) having a porosity of 6.8–9.9% at a firing temperature of 1600°C. The degree of sintering of magnesium hydroxide increases with increasing compaction pressure and firing temperature. Prior heat treatment of the material at 800–1000°C intensifies the sintering process with simultaneous reduction of shrinkage.The studies conducted on the specimens prepared from a fired briquette established that the periclase (magnesite) powder obtained from magnesium hydroxide is suitable for the production of magnesia refractories.A. V. Kushchenko and G. G. Eliseeva (UkrNIIO) participated in this investigation.Translated from Ogneupory, No. 2, pp. 7–10, February, 1988.     

Investigations on N-halogen cyclic ureide based magnesium primary cell system at various temperatures

This paper reports the results of experimental studies on a magnesium/magnesium perchlorate/N,N-dichlorodimethylhydantoin cell system at various temperatures ranging from 70 to –20° C. A constant current discharge method was employed to evaluate the battery parameters such as discharge capacity, energy density, coulombic efficiency and internal resistance. Cyclic voltammetric (CV) investigations of the N,N-dichlorodimethylhydantoin (DDH) were carried out in 0.1 M magnesium perchlorate medium in order to supplement the results. Scanning electron microscopy (SEM) was performed on post-polarized magnesium samples to follow the morphological changes in the anodic material with respect to temperatures. These investigations broadly reveal that the cell system can give rise to the open-circuit/closed-circuit voltage of 2.0 V and it is possible to obtain current densities of 20 mA cm^–2 during discharge.     

Dose and enantiospecific responses of white pine cone beetles,Conophthorus coniperda,to alpha-Pinene in an eastern white pine seed orchard

The white pine cone beetle, Conophthorus coniperda, exhibited dose and enantiospecific responses to -pinene in stands of mature eastern white pine, Pinus strobus, in a seed orchard near Murphy, North Carolina, USA. (–)--Pinene significantly increased catches of cone beetles to traps baited with (± )-trans-pityol. (+)--Pinene did not increase catches of beetles to pityol-baited traps and interrupted the response of beetles to traps baited with (±)-trans-pityol and (–)--pinene. Maximal attraction of cone beetles to pityol-baited traps was obtained with lures releasing (–)--pinene at a rate of 103 mg/day at 23^°C. Lures releasing (–)--pinene at rates lower or higher than 103 mg/day resulted in reduced catches to traps baited with (±)-trans-pityol. The sex ratio in all catches was heavily male biased. Attraction of the clerid predator, Thanasimus dubius, to traps baited with (±)-trans-pityol increased significantly with the presence of -pinene, irrespective of enantiomeric composition. Maximal attraction of T. dubius to pityol-baited traps occurred with devices releasing (–)--pinene at the highest rate tested, 579 mg/d at 23^°C, a sub optimal rate for cone beetles.     

Crystallization and Porosity Evolution of AlOOH–Fe(NO3)3 Gels

Monolithic planar boehmite gels of thickness 0.15 mm seeded by Fe(NO₃)₃ were examined by quantitative DTA, dilatometry, SEM, surface area and pore size distribution and on-line optical transmittance from RT to 1270°C. Dispersed Fe³⁺ ions resulted in 5wt% Fe₂O₃, seeds in monolithic boehmite gels which affected the crystallization of corundum phase and the associated evolution of porosity (the change of inter-colloidal to inter-grain porosity) during heat treatment. Transformation of to -(Al, Fe)₂O₃ took place within individual grains of 100–120 nm size and was similar to unseeded boehmite gels except the a significantly higher frequency of homogeneous nucleation of corundum phase occurred. Gradual elimination of inter-grain pores during sintering was associated with the increase of light transmittance. Ceramics became transparent after sintering at temperatures over 1270°C.     

The question of the stabilization and sintering of high purity zirconium dioxide

Conclusions The results of an investigation of the stabilization and sintering of test bodies based on zirconia of high purity (with 99.5% base oxide) and with the addition of 4–15 mol.% CaO or MgO established the following.In conditions of oxidizing fire at 1710°C with a 5-h soak, complete sintering occurs in the body containing 10 mol.% of stabilizing oxide (zero open porosity), there is adequate stabilization, the material assumes great strength and spalling resistance compared with the other bodies investigated.With an increase in the amount of stabilizing oxide to 12–15% although a sintered and fully stabilized product results there is a reduction in density which is especially strong with CaO additions. Furthermore, there is a sharp reduction in the strength and spalling resistance.The relatively low density of the fired bodies containing 10 mol.% stabilizing addition (for CaO 5.20, for MgO 5.28 g/cm³) is determined chiefly by the presence of pores both inside and on the boundaries of the crystals in the material.An increase in the firing temperature at 2200°C only slightly affects the density of the material. An increase in the density of the material containing 10 mol.% CaO is attained by:changing the form of the anion added with the stabilizer from CO₃ to F with this there is a sharp deterioration in spalling resistance of the material;by precalcining the original ZrO₂;by sintering the preliminary stabilized product as a result of which specimens are obtained with a bulk density of 5.54 g/cm³.     

Hot press compaction of magnesium oxide obtained by decomposing certain compounds

Conclusions A study was made of the densification during hot pressing at 1300–1600°C of magnesium oxide activated by decomposing the hydroxide and the basic magnesium hydrocarbonate. During decomposition of these compounds at 500–700°C with a soak of 15 min, magnesium oxide forms that is actively compacted almost to the theoretical density (98.5–99.5%) at relatively low temperatures (1500–1600°C) and pressures of 150 kg/cm².We investigated the influence of the time and temperature of heat processing of the hydroxide and the basic hydrocarbonate of magnesium on the fineness of the grains and the defects of the crystalline lattices of periclase thus formed, and also on the capacity for subsequent compaction during hot pressing.The reduction in the degree of compaction during hot pressing of the materials, heat processed at temperatures below 500°C, is due to the increase in the content in them of undecomposed residue, which hinders the diffusion sintering in subsequent stages of pressing.A reduction in the degree of compaction with rise in temperature of heat processing above 500°C or with an increase in the heat-processing time with the optimum temperature, is connected not with a reduction in the defects of the crystalline lattice of the periclase formed, but with the sizes and physical state of its particles.We also studied the effect of additions of magnesium oxide obtained by heat processing the hydroxide or the basic hydrocarbonate of magnesium on the compaction during hot pressing of industrial magnesia. The introduction of 10–20% of this additive ensures a reduction in the optimum pressing temperature of 100–300°C and an increase in the density of the specimens almost to theoretical.Translated from Ogneupory, No.2, pp.46–53, February, 1967.     

Electrical conductivities of aqueous ZnSO4−H2SO4 solutions

Conductivities of aqueous ZnSO₄–H₂SO₄ solutions are reported for a wide range of ZnSO₄ and H₂SO₄ concentrations (ZnSO₄ concentrations of 01.2 M and H₂SO₄ concentrations of 02 M) at 25°C, 40°C and 60°C. The results indicate that the solution conductivity at a given ZnSO₄ concentration is controlled by the H₂SO₄ (H⁺) concentration. The variation of the specific conductivity with ZnSO₄ concentration is complex, and depends on the H₂SO₄ concentration. At H₂SO₄ concentrations lower than about 0.25 M, the addition of ZnSO₄ increases the solution conductivity, likely because the added Zn²⁺ and SO ₄ ^2– ions increase the total number of conducting ions. However, at H₂SO₄ concentrations higher than about 0.25 M, the solution conductivity decreases upon the addition of ZnSO₄. This behaviour is attributed to decreases in the amount of free water (through solvation effects) upon the addition of ZnSO₄, which in turn lowers the Grotthus-type conduction of the H⁺ ions. At H₂SO₄ concentrations of about 0.25 M, the addition of ZnSO₄ does not appreciably affect the solution conductivity, possibly because the effects of increasing concentrations of Zn²⁺ and SO ₄ ^2– ions are balanced by decreases in Grotthus conduction.Nomenclature a ion size parameter (m) - a ^* Bjerrum distance of closest approach (m) - C stoichiometric concentration (mol m^–3 or mol L^–1) - I ionic strength (mol L^–1) - k constant in Kohlrausch's law - M molar concentration (mol L^–1) - T absolute temperature (K) - z _i electrochemical valence of speciesi (equiv. mol^–1) - z (z _–|z _–|)^1/2=2 for ZnSO₄ - z ₊ valence of cation in salt (=+2 for Zn²⁺) - z _– valence of anion in salt (=–2 for SO ₄ ^2– ) Greek letters fraction of ZnSO₄ dissociated - specific conductivity (^–1 m^–1) - ^expt measured specific conductivity (^–1 m^–1) - equivalent conductivity (^–1 m² equiv.^–1) - equivalent conductivity at infinite dilution (^–1 m² equiv.^–1) - ⁰ equivalent conductivity calculated using Equation 2 (^–1 m² equiv.^–1) - _cale measured equivalent conductivity (^–1 m² equiv.^–1) - _expt equivalent conductivity of ioni at infinite dilution (^–1 m² equiv.^–1) - reciprocal of radius of ionic cloud (m^–1) - viscosity of solvent (Pa s) - dielectric constant - ± mean molar activity coefficient - density (g cm^–3)     

Improving the thermal-shock resistance of siliceous refractories

Conclusion The thermal-shock resistance of siliceous specimens with additions, measured by the water-heat cycling method, as a rule, is increased with increase in the content of the additives from 5 to 30%, and is maximal for a content of 30% chromite-periclase. The crack resistance, determined by the air-heat cycling method is characterized by a maximum ¯R_T factor for each type of additive with a content of 15–20%. The maximum ¯R_T value is shown by specimens containing 20% chromite periclase.The maximum thermal-shock resistance of the specimens, without additives and with additions of silciding filling scrap, chrome-ore fractions <0.5 and 0.09 mm, clay, and chromitepericlase relate to each other respectively as follows: 11.82.52.83.55, and the crack resistance as 10.44.63.82.98.1. The number 2.9 obtained with specimens containing clay apparently is not maximal, since the clay content was limited to 15% for the reasons stated.Clay additives increase the thermal-shock resistance of the dinas [3], but markedly redice the temperature of initial softening under load and the mechanical strength. Therefore, thermal-shock resistant dinas with clay additions can be used only in special working conditions.The most promising of the compositions studied is one with an addition of chromite-periclase scrap. This helps to create a porous, microcracked structure which increases the thermal-shock resistance by five times (water-heat cycle tests) and the crack resistance by more than eight times (air-heat cycle method) compared with the ordinary dinas.The optimum content of chromite-periclase scrap, to obtain siliceous refractories with the maximum thermal shock resistance, is 15–20%. However, considering the reduced refractoriness under load noted with this (respectively to 1480 and 1370 deg C) we recommend that the additive be limited to 10%.Translated from Ogneupory, No. 4, pp. 9–11, April, 1991.     

Sintering high purity,magnesia with additions of hafnium dioxide

Conclusions The introduction of hafnium dioxide into high-purity active magnesia accelerates its sintering and with 0.1–0.3 mol % HfO₂ a ceramic with an apparent density of 99–100% of the theoretical (3.56–3.63 g/cm³) can be obtained after firing at 1300–1400°C. The optimum quantity of HfO₂ additive is close to 0.25 mol %.The calcination temperature of the mixture of magnesium hydroxide and additive obtained by precipitation from a solution of MgCl₂ and Hf(SO₄)₂ with ammonia, and the fabrication pressure do not greatly affect the final density of the ceramics.Sintering of spectrally pure MgO containing 0.25 mol. % HfO₂ begins at 950°C, then the apparent density grows rapidly with rise in firing temperature, approaching 3.40 g/cm² at 1100°C. Selective recrystallization at these low temperatures is slow and sintering is not accompanied by a substantial grain growth. At 1300°C and higher firing temperatures densification of the ceramics approaches the limit in several minutes.The mechanism of the transfer of substance during the sintering of these specimens is volume self-diffusion from the grain boundaries to the surfaces of the bridges formed between them. The energy of activation of this process [3.9 eV (6.2 × 10^–16 J) in the 1000–1300°C range] and the coefficient of self-diffusion for MgO calculated in accordance with this (6 × 10^–14 cm²/sec at 1100°C) correspond to existing data on the diffusion of magnesium into MgO.With the incorporation of 0.25 mol. % HfO₂ in less pure magnesia obtained from chemically pure MgCl₂, sintering is little different from the sintering of spectrally pure MgO, but the limiting apparent density of the ceramics in this case is somewhat lower-of the order of 99% of the theoretical.     

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.     

Commercial grades of alumina

Conclusions In an investigation of the properties of alumina GO, GK, and GU of Soviet origin and of the Hungarian alumina GV it was shown that the chemical composition is similar in all these materials but the alkali content is highest in the Hungarian alumina.The density of the alumina depends on the corundum content and increases in the order GOGU GVGK.Preliminary roasting of the alumina reduces the shrinkage of the specimens produced from these materials (with the exception of alumina GK) as a result of the formation of corundum during the roasting. The shrinkage of all specimens was similar when they were produced from alumina roasted at 1550°C.The porosity of the specimens depends primarily on the firing temperature. Firing the specimens at 1750°C resulted in complete sintering regardless of the temperature at which the original alumina was roasted. The cold-crushing strength was also at maximum in this case.Translated from Ogneupory, No. 3, pp. 46–52, March, 1976.     

Impedance of a porous electrode with an axial gradient of concentration

When the impedance is measured on a battery, an inductive impedance is often observed in a high frequency range. This inductance is frequently related to the cell geometry and electrical leads. However, certain authors claimed that this inductance is due to the concentration distribution of reacting species through the pores of battery electrodes. Their argument is based on a paper in which a fundamental error was committed. Hence, the impedance is re-calculated on the basis of the same principle. The model shows that though the diffusion process plays an outstanding role, the overall reaction rate is never completely limited by this process. The faradaic impedance due to the concentration distribution is capacitive. Therefore, the inductive impedance observed on battery systems cannot be, by any means, attributed to the concentration distribution inside the pores. Little frequency distribution is found and the impedance is close to a semi-circle. Therefore depressed impedance diagrams in porous electrodes without forced convection cannot be ascribed to either a Warburg nor a Warburg-de Levie behaviour.Nomenclature A D¦C¦ (mole cm s^–1) - B j+K¦C¦ (mole cm s^–1) - b Tafel coefficient (V^–1) - C(x) Concentration ofS in a pore at depthx (mole cm^–3) - C ₀ Concentration ofS in the solution bulk (mole cm^–3) - C C(x) change under a voltage perturbation (mole cm^–3) - ¦C¦ Amplitude of C (mole cm^–3) - D Diffusion coefficient (cm² s^–1) - E Electrode potential (V) - E Small perturbation inE namely a sine-wave signal (V) - ¦E¦ Amplitude of E(V) - F Faraday constant (96500 A s mol^–1) - F(x) Space separate variable forC - f Frequency in Hz (s^–1) - g(x) KC(x)¦E¦(mole cm s^–1) - I Apparent current density (A cm^–2) - I _st Steady-state current per unit surface of pore aperture (A cm^–2) - j Imaginary unit [(–1)^1/2] - K Pseudo-homogeneous rate constant (s^–1) - K Potential derivative ofK, dK/dE (s^–1 V^–1) - K ^* Heterogeneous reaction rate constant (cm s^–1) - L Pore depth (cm) - n Reaction order - P Reaction product - p Parameter forF(x), see Equation 13 - q Parameter forF(x), see Equation 13 - R _e Electrolyte resistance (ohm cm) - R _p Polarization resistance per unit surface of pore aperture (ohm cm²) - R _t Charge transfer resistance per unit surface of pore aperture (ohm cm²) - S Reacting species - S _a Total surface of pore apertures (cm²) - S ₀ Geometrical surface area - S _p Developed surface area of porous electrode per unit volume (cm² cm^–3) - s Concentration gradient (mole cm^–3 cm^–1) - t Time - U Ohmic drop - x Distance from pore aperture (cm) - Z Faradaic impedance per unit surface of pore aperture (ohm cm²) - Z _x Local impedance per unit pore length (ohm cm³) - z Charge transfer number - Porosity - Thickness of Nernst diffusion layer - Penetration depth of reacting species (cm) - Penetration depth of a.c. signal determined by the potential distribution (cm) - Electrolyte (solution) resistivity (ohm cm) - ₀ Flow of S at the pore aperture (mole cm² s^–1) - Angular freqeuncy of a.c. signal, 2f(s^–1) - Integration constant     

Kinetics of copper electrodeposition in citrate electrolytes

The kinetics of copper electrocrystallization in citrate electrolytes (0.5M CuSO₄, 0.01 to 2M sodium citrate) and citrate ammonia electrolytes (up to pH 10.5) were investigated. The addition of citrate strongly inhibits the copper reduction. For citrate concentrations ranging from 0.6 to 0.8 M, the impedance plots exhibit two separate capacitive features. The low frequency loop has a characteristic frequency which depends mainly on the electrode rotation speed. Its size increases with increasing current density or citrate concentration and decreases with increasing electrode rotation speed. A reaction path is proposed to account for the main features of the reduction kinetics (polarization curves, current dependence of the current efficiency and impedance plots) observed in the range 0.5 to 0.8 M citrate concentrations. This involves the reduction of cupric complex species into a compound that can be either included as a whole into the deposit or decomplexed to produce the metal deposit. The resulting excess free complexing ions at the interface would adsorb and inhibit the reduction of complexed species. With a charge transfer reaction occurring in two steps coupled by the soluble Cu(I) intermediate which is able to diffuse into the solution, this model can also account for the low current efficiencies observed in citrate ammonia electrolytes and their dependencies upon the current density and electrode rotation speed.Nomenclature b, b ₁, b ₁ ^* Tafel coefficients (V^–1) - bulk concentration of complexed species (mol cm^–3) - (si*) concentration of intermediate C^* atx=0 (mol cm^–3) - C concentration of (Cu Cit H)^2– atx=0 (mol cm^–3) - C C variation due to E - C concentration of complexing agent (Cit)^3- at the distancex (mol cm^–3) - C _o concentrationC atx=0 (mol cm^–3) - C _o C _o variation due to E - Cv _s bulk concentrationC (mol cm^–3) - (Cit H), (Cu), (Compl) molecular weights (g) - C _dl double layer capacitance (F cm^–2) - D diffusion coefficient of (Cit)^3- (cm²s^–1) - D ₁ diffusion coefficient of C^* (cm²s^–1) - E electrode potential (V) - f ₁ frequency in Equation 25 (s^–1) - F Faraday's constant (96 500 A smol^–1) - i, i ₁, i ₁ ^* current densities (A cm^–2) - i i variation due to E - Im(Z) imaginary part ofZ - j - k ₁, k ₁ ^* , K₁, K ₁ ^* , K₂, K rate constants (cms^–1) - K rate constant (s^–1) - K ₃ rate constant (cm³ A^–1s^–1) - R _t transfer resistance (cm²) - R _p polarization resistance (cm²) - Re(Z) real part ofZ - t time (s) - x distance from the electrode (cm) - Z _f faradaic impedance (cm²) - Z electrode impedance (cm²) Greek symbols maximal surface concentration of complexing species (molcm^–2) - thickness of Nernst diffusion layer (cm) - , ₁, ₂ current efficiencies - angular frequency (rads^–1) - electrode rotation speed (revmin^–1) - =K ^–1(s) - _d diffusion time constant (s) - electrode coverage by adsorbed complexing species - (in0) electrode coverage due toC _s - variation due to E     

Vibro-cast periclase refractories of a grainy structure and some of their properties

Conclusions We have studied the process of obtaining vibro-cast periclase refractories based on fine- and coarse-grained molding systems using a low-concentration solution of HCl as the binder.Using a combined experimental and calculation method based on the use of a cone, we have carried out studies to optimize the production parameters of the vibro-casting process.The materials obtained in the initial (unfired) state are characterized by fairly high strength (_comp=12–30 MPa) and can be used as refractory concretes.The process of sintering the materials was studied and it is shown that their properties are comparable with the normal periclase refractories of a grainy structure. After firing at 1550–1600°C the refractories show virtually no additional shrinkage and their true porosity is 19–23%, ultimate compressive strength 30–60 MPa, and ultimate bend strength 12–26 MPa.The results were proved industrially. Large details of the smelting unit of induction furnaces were manufactured and successfully tested.Translated from Ogneupory, No. 8, pp. 9–15, August, 1986.     

Thermophysical and elastomechanical properties of grainy corundum-zircon refractories and the effect of these properties on the thermal-shock resistance

Conclusions The thermophysical properties (l/l ₀ and ) and the elastomechanlcal properties (E, G, _bend) of corundum-zircon specimens in the 20–1200°C interval with various concentrations and grain sizes of zircon and different grain sizes and types of the corundum and zircon components in the grain and bondings have been investigated. It is established that in particular temperature regions there is a connection between the criteria R and R calculated from the values of the properties and the number of heat changes before the onset of primary cracking and total destruction of the specimens.Translated from Ogneupory, No. 1, pp. 56–60, January, 1980.     

Mass transport in a weakly stratified electrochemical cell

A study of natural convection in an electrochemical system with a Rayleigh number of the order 10¹⁰ is presented. Theoretical and experimental results for the unsteady behaviour of the concentration and velocity fields during electrolysis of an aqueous solution of a metal salt are given. The cell geometry is a vertical slot and the reaction kinetics is governed by a Butler-Volmer law. To reduce the effects of stratification, the flush mounted electrodes are located (symmetrically) in the middle parts of the vertical walls. It is demonstrated, both theoretically and experimentally, that a weak stratification develops after a short time, regardless of cell geometry, even in the central part of the cell. This stratification has a strong effect on the velocity field, which rapidly attains boundary layer character. Measured profiles of concentration and vertical velocity at and above the cathode are in good agreement with numerical predictions. For a constant cell voltage, numerical computations show that between the initial transient and the time when stronger stratification reaches the electrode area, the distribution of electric current is approximately steady.List of symbols a _i left hand side of equation system - b _i right hand side of equation system - c concentration (mol m^–3) - c dimensionless concentration - c _i concentration of species i' (mol m^–3) - c₀ initial cell concentration (300 mol m^–3) - c ₀ dimensionless initial cell concentration - c_wall concentration at electrode surface (mol m^–3) - dx increment solution vector in Newton's method - D _i diffusion coefficient of species i (m² s^–1) - D ₁ 0.38 × 10^–9 m² s^–1 - D ₂ 0.82 × 10^–9 m² s^–1 - D effective diffusion coefficient of the electrolyte (0.52 × 10^–9 m² s^–1) - _x unit vector in the vertical direction - _y unit vector in the horizontal direction - F Faraday's constant (96 487 A s mol^–1) - g acceleration of gravity (9.81 m s^–2) - i dummy referring to positive (i = 1) or negative (i = 2) ion - f current density (A m^–2) - f dimensionless current density - i₀ exchange current density (0.01 A m^–2) - J _ij Jacobian of system matrix - L length of electrode (0.03 m) - N _i transport flux density of ion i (mol m^–2 s^–1) - n unit normal vector - p pressure (Nm^–2) - p dimensionless pressure - R gas constant molar (8.31 J K^–1 mol^–1) - R _i residual of equation system - Ra Rayleigh number gL ³ c ₀/D (2.54 × 101¹⁰) - S _c Schmidt number /D (1730) - t time (s) - t dimensionless time - T temperature (293 K) - velocity vector (m s^–1) - dimensionless velocity vector - U characteristic velocity in the vertical direction - V _± potential of anode and cathode, respectively - x spatial coordinate in vertical direction (m) - x dimensionless spatial coordinate in vertical direction - x solution vector for c, and - y spatial coordinate in horizontal direction (m) - y dimensionless spatial coordinate in horizontal direction - z _i charge number of ion i Greek symbols symmetry factor of the electrode kinetics, 0.5 - volume expansion coefficient (1.24 × 10^–4 m³ mol^–1) - _s surface overpotential - constant in equation for the electric potential (–5.46) - _s diffusion layer thickness - scale of diffusion layer thickness - constant relating c/y to the Butler-Volmer law (0.00733) - kinematic viscosity (0.9 × 10^–6 m2 s^–1)