Lawrence Baylor Robinson

Temperature dependence of ferromagnetic anisotropy energy in cubic crystals

Abstract The magnetic anisotropy constant K1 for cubic ferromagnetic crystals has been discussed based on the general expressions derived by Yang for hexagon crystals. By matching the experimental data, we obtained K 1 (T,H) K 1 (0,0) =−6.14 I 5 2 (T,H) + 3.36 I 9 2 (T,H) + 4.88m 2 (T,H) −1.10[ I 5 2 (T,H)] 2 for nickel, and K 1 (T,H)/K 1 (0,0)=I 9 2 for iron, where I 5 2 and I 9 2 are the hyperbolic bessel functions of order 2 and 4 respectively and m is the reduced magnetization. Both expressions have a theoretical basis.

Temperature dependence of the single-crystal elastic constants of co-rich co-fe alloys

The single‐crystal elastic moduli, C11, C12, and C44 of three fcc cobalt‐iron alloys (Co–6 at.% Fe, Co–8 at.% Fe, Co–10 at.% Fe) were measured in the range 0–315°C. In addition C11 for the Co–6 at.% Fe alloy, and C′=(1/2)(C11+C12+2C44) for the three alloys are measured over the temperature range 0–1250°C. Plots of the elastic moduli vs temperature exhibit a change in slope and deviation from linearity in the neighborhood of the Curie temperature. The temperature variation of the shear anisotropy in the fcc phase Afcc (≡2C44/C11−C12) differs among the three alloys. Afcc exhibits a highly positive temperature dependence in the Co–10 at.% Fe alloy and a slight negative dependence in the Co–6 at.% Fe and Co–8 at.% Fe alloys. Previous statements in the literature that the hcp⇄fcc transformation in cobalt is preceded by a highly negative temperature dependence of the shear anisotropy ratio A (≡C44/C66) in the hcp phase between 523°K and the transition at about 743°K is not borne out by the present results. Rath...

Effects of impurities on heat capacities at paramagnetic to ferromagnetic transitions

Abstract The influence of controlled impurities upon the detailed shape of the heat capacity of Gd near the Curie temperature is studied experimentally. The experimental results can be interpreted in terms of Watsons theoretical analyses which show that small scale, random inhomogeneities do not alter the shape of the heat capacity curve, whereas large scale inhomogeneities are found to reduce the sharpness of the critical singularities.

Thermal Conductivities of He, Ne, Ar, and of their Mixtures

The use of a line‐source transient‐heat‐transfer technique to determine the thermal conductivities of He, Ne, Ar, and of their mixtures is reported along with results. The method employs existing rigorous analytical theory for a finite line source possessing heat capacity. The experimental and analytical techniques minimize the effects of instrument size, radiation, free convection, and thermal separation of the mixtures. The linesource diameter was 0.00763 cm with a central 0.00203‐cm nickel temperature sensing coil to provide temperature data. Experimental times of 270 msec or less and total temperature rises of less than 10°C were used to minimize radiation, free convection, and thermal separation effects. The data show excellent agreement with existing data and an internal consistency and accuracy believed to demonstrate elimination of undesirable effects upon over‐all thermal‐conductivity determinations.

Electrical Breakdown Measurements of Cesium Vapor in a Rhenium Electrode Diode

The electrical breakdown of cesium vapor was measured in a guard‐ringed, variable parameter test vehicle, which had plane parallel rhenium electrodes. Experiments were conducted over a range of emitter temperatures from 1300° to 2000°K, with cesium pressures from 0.053 to 6.22 Torr and interrelectrode spacings from 0.001 to 0.07 cm. Because the breakdown occurred at relatively low voltages, the contact potential difference of the electrodes, as a function of temperature and cesium coverage, was taken into account. Paschen‐type curves exhibited minima in breakdown potential as a function of spacing at high pressures. At low pressures the necessary large spacings could not be achieved. The maximum breakdown voltage was 6 V at 0.053 Torr and the minimum breakdown voltage was 0.21 V at 6.22 Torr. At low cesium pressures the effect of the emitter temperature is pronounced. When the pressure is constant, the breakdown voltage shifts as a function of emitter temperature. At high pressures, the curves do not vary...

Diffusion of Silver into Nickel Single Crystals

Lattice diffusion of radioactove silver Ag110 into high‐purity nickel single crystals is measured over the temperature range 507–782°C. The lattice diffusion coefficient (DL)Ag/Ni, obtained by Gruzins residual activity method, is given by (DL)Ag/Ni=0.025 exp(−47 200/RT) cm2/sec. The activation energy QAg/Ni=47 200 cal/mole for silver diffusion in nickel is much smaller than QNi/Ni for self‐diffusion in nickel (66 800 cal/mole) but is about equal to QAg/Ag for self‐diffusion in silver (46 000 cal/mole.

A calculation of magnetic transition temperatures of rare‐earth alloys

A theoretical expression for magnetic transition temperatures of binary rare‐earth alloys is proposed based on the similarity between the expressions of spin‐disorder resistivity and transition temperatures of pure rare‐earth metals. The proposed expression for the magnetic transition temperature for binary rare‐earth alloys is kTc (or Tp)=(−3πZ2)/EFΩ2)ΣRn≠0Φ(2KFR) × [x(1−x)Vab2 + xGa2(ga−1)2ja(ja+1)+(1−x)Gb2 (gb−1)2jb(jb+1)], where Φ(2KFR)=(KFR×cosKFR ‐ sinKFR)/(KFR)4 and the other symbols have their usual meaning. The above expression is obtained from considering an equation for spin disorder resistivity given by Dekker. The experimental data of magnetic transition temperatures of Gd–Dy system have been used to test the validity of the proposed equation. The agreement between theory and experiment is reasonably good.

Some theoretical aspects of spin‐disorder resistivity of rare‐earth alloys

A theoretical expression for the spin‐disorder resistivity of binary rare‐earth alloys at temperatures above the magnetic transition point has been derived by Dekker. Some experimental results have been obtained by Bozorth and Suits and also by Yang and Robinson. A comparison of theory and experiment has been made in this note. According to Dekkers theory, the spin‐disorder resistivity ρd for a binary alloy containing two magnetic components can be expressed approximately as ρd = c1x(1 − x)+(c2 − c3)x + c3, where c1, c2, and c3 are some constants and x is the percentage composition of one of the components. The experimental spin‐disorder resistivity of the Gd–Dy system conforms to this theoretical expression. The constants c1, c2, and c3 have a theoretical basis.

An Analysis of Direct Energy Converters by the Methods of Irreversible Thermodynamics

Methods of thermodynamics of irreversible processes are presented within the framework of the Burgers‐Pipkin solution of the Boltzmann transport equation. The thermionic energy converter is treated as a thermocouple and analyzed by the nonequilibrium thermodynamic methods which are developed.Following a general treatment, the cesium diode is treated as a specific example. Detailed calculations are made of the thermal conductivity and the electrical conductivity, as well as the ratio. In making the calculations, contributions from the electrons, positive ions, and neutral particles, are considered as well as interactions among the various types of particles. Binary interactions between the neutral particles, collective (Coulomb) interactions among charged particles, and (quantum‐mechanical) collisions between electrons and neutral particles are taken into account. Each type of interaction has its appropriate effect on the thermal and electrical conductivities.A brief discussion of the results and implicati...

Effect of Compton Scatterings by Air Molecules on the Sharpness of a Beam of Gamma Rays

A special problem involving the passage of a beam of gamma rays through air is examined. The effect of Compton scatterings on the sharpness of the gamma‐ray beam is discussed. Other mechanisms whereby the beam is softened are not included because the interest is only in the component of the beam which suffers little energy loss.The problem of interest is similar, in some respects, to multiple scattering of electrons with negligible energy loss. The mathematical expression for the collision process is different, however. A method used by Fermi in regard to multiple scattering of electrons is also used here. The Klein‐Nishina cross section for Compton scatterings is introduced into the present problem in order to obtain scattering probabilities.Some numerical results are obtained which can be applied to cobalt‐60 gamma rays. The results are presented in graphical form.