Takashi Mukaibo

Crystal structure of graphite bromine lamellar compounds

Abstract The isotherm of bromine on pyrolytic graphite was studied at 20°C by stepwise bromination, and several phases of graphite bromide were identified, which have compositions of C 8 Br, C 12 Br, C 16 Br and C 20 Br. X-ray studies show that these possess the socalled 2nd, 3rd, 4th and 5th stage structures respectively. At bromine concentrations between these definite compositions, X-ray diffraction patterns show mixtures of the higher stage and the lower stage structures. The density of the intercalated bromine molecules in a layer is believed to stay unchanged with the change of the composition. During the course of the reaction with the saturated vapor of bromine, an intermediate metastable lamellar compound structure was found in which the bromine layers are intercalated into every second layer spacing but for which the bromine concentration is believed to be much lower than that of C 8 Br.

A new thermal imaging system utilizing a Xe arc lamp and an ellipsoidal mirror for crystallization of refractory oxides

Abstract A newly developed thermal imaging system utilizing a Xe arc lamp and a total ellipsoidal mirror for floating-zone crystal growth at temperatures of 1600–2800°C is described. Methods to achieve a desirable temperature destribution are discussed. Single crystals of CaO, Y 2 O 3 , MgAl 2 O 4 , mullite, rutile and CaO- or Y 2 O 3 -stabilized ZrO 2 have been grown.

Theory of polarographic maximum current—I. Conditions for the onset of hydrodynamic instability in a liquid metal electrode system

Abstract The maximum current of the first kind, an extraordinary current peak appearing at the rising portion of a current—voltage curve with a liquid metal electrode—liquid electrolyte system, has been analysed in terms of momentum equations for liquid metal and liquid electrolyte, the diffusion equation for active ionic species, and some electrochemical conditions. Consequently, a kinematic theory has been deduced to explain that the maximum current is caused by the intense motion of the liquids near the interface, resulting from longitudinal variations of interfacial tension. It is proposed that the maximum current is a manifestation of electrochemical and hydrodynamical instability, which originates from incessant, small and random disturbance about the surface of the electrode. A linearized mathematical model has been introduced to explain the initial processes of amplifying or decaying small disturbances. The characteristic equation obtained from the mathematical model gives the conditions under which the maximum current occurs; the instability sets in when the slope of the electrocapillary curve is positive for the cathodic deposition process, and negative for the anodic dissolution process.

Laser-flash calorimetry I. Calibration and test on alumina heat capacity

Abstract A new method and apparatus for laser-flash heat-capacity calorimetry over the temperature range 80 to 1100 K are described. Experimental results on an alumina standard sample agreed with the National Bureau of Standards within ±0.5 per cent from 100 to 800 K and within ± 1 per cent at temperatures from 80 to 100 K and from 800 to 1100 K. The estimated inaccuracies are within ± 0.8 per cent at 300 K and ± 2 per cent at 1100 K.

Uranium mononitride: Heat capacity and thermal conductivity from 298 to 1000 °K

Abstract The heat capacity and the thermal diffusivity of uranium mononitride were measured by a laser flash method at temperatures ranging from 298 to 1000 °K. The samples were arc-melted UN having nearly zero porosity and sintered UN having porosity of 10.1%. The heat capacity of UN was represented by C p = 12.08 + 2.548 × 10 − 3 T − {−1.252 × 10 5 T −2 cal/mol · deg K (298–1000 °K)}. From the heat capacity data, entropy, enthalpy and the Gibbs energy function of UN were calculated. The thermal conductivities of arc-melted UN, calculated from the heat capacity and the thermal diffusivity data, at 350 and 1000 °K were 0.031 and 0.045 cal/ cm · sec · deg K, respectively. The results agreed reasonably well with those of Moore et al. obtained at lower temperatures.

Vacancy diffusion in magnetite

Abstract The chemical diffusion coefficient in a single crystal of magnetite was measured by observing the relaxation of deviations from stoichiometry responding to a stepwise change in oxygen partial pressure between 1300 and 1450°C. The chemical diffusion coefficient was proportional to (∂ In δ ∂ In p o2 ) −1 . The vacancy diffusion coefficient was calculated with the help of nonstoichiometric data and was found to be independent of the vacancy composition. The value of Dv was D v = (0.14 ± 0.08) exp (− (32,500 ± 1800) RT) cm 2 s −1 .

Theory of polarographic maximum current—II. Growth or decay rate of the electrochemical and hydrodynamic instability

Abstract The maximum current of the first kind essential to an electrode process in a liquid metal—liquid electrolyte system is attributed to the intense motion of the liquid near the interface, resulting from longitudinal variations of interfacial tension. In the preceding paper it was stated that the maximum current is a manifestation of electrochemical and hydrodynamical instability, originated by incessant and random imposition of perturbation about the surface of the electrode; and that the conditions when the maximum current occurs can be obtained theoretically. Therefore, in this paper the growth or decay rates of the imposed perturbations are calculated within the limit of linear approximation for a typical electrode system. Then we compare the rates with that of the maximum waves observed in water—mercury and fused lead—alkalihalides systems. This comparison suggests that the maximum current is weakened by (1) small polarization, (2) polarization near the point of zero charge of the electrocapillary curve, (3) low concentration of the active species in the electrolyte, and (4) sufficiently high electric conductivity. These suggestions are in good accordance with the observations.

Structure and properties of graphite-chromyl chloride lamellar compound

Abstract Crystal structure, electric conductivity and thermal stability of graphite-CrO 2 Cl 2 lamellar compound were investigated. X-ray diffraction studies of C 27·5 CrO 2 Cl 2 show that it possesses a 3rd stage lamellar structure with I c of 14·87 A. This lamellar compound is unusually stable and its electric conductivity, as well as its composition, show only a slight change after complete evacuation of reacting CrO 2 Cl 2 gas. When heattreated in a vacuum, its electric conductivity shows a considerable decrease, whereas its composition changes very slowly up to 300°C.

A new compound — UNF

Abstract A new compound UNF has been prepared by the reaction of UF4 with silicon and nitrogen. The crystal structure of UNF has been determined to be tetragonal with dimensions of a = 5·612 ± 0·001 A , b = 5·712 ± 0·001 A . This compound is converted to U2N3 in nitrogen atmosphere, and also converted to UN in argon atmosphere at elevated temperatures. This compound is not stable in air even at room temperature due to its oxidation.

Anodic behaviour of platinum in the LiClKCl eutectic melt

Abstract The mechanism of anodic passivation of platinum in a dehydrated LiClKCl eutectic melt was studied by means of potential-sweep, galvanostatic, potentiostatic and potential-decay measurements. The main results were as follows: (i) When the potential-sweep method was employed, there was a sharp peak current on a current/potential curve, proportional to the square root of the sweep rate. (ii) The time for passivation under galvanostatic conditions was inversely proportional to the square of the applied current. (iii) When an electrode was polarized potentiostatically, a gradual decrease in current was followed by an abrupt fall. The current at the beginning of the abrupt fall was linear with the applied potential. (iv) At the instant of cutting-off of current, the electrode potential fell to a constant value, regardless of the applied potential. The results are explained theoretically on the following assumptions. (a) When the electrode potential reaches the passivation potential, the electrode becomes covered with a non-conducting film. (b) The electrode process is controlled by the diffusion of platinum ions to the solution before passivation occurs.