摘要:

The electrical resistivity of a Hf filament containing 4.9 at. % Zr was measured in the temperature interval from 1155° to 2250°K. The resistivity increases from 37 &mgr;&OHgr;‐cm at room‐temperature to 187 &mgr;&OHgr;‐cm at 2025°K where the alloy undergoes a transition from an hcp to a bcc structure with a corresponding decrease in resistivity to 173 &mgr;&OHgr;‐cm. From data in the literature the transition temperature of pure Hf is estimated to be 2030°±30°K. Graphs of total emissivity versus temperature indicate that changes in the surface characteristics of the sample occurred during the measurements. After surface conditions became constant, the total hemispherical emissivity increased linearly with temperature from about 0.26 at 1200°K to 0.31 at 2000°K.

ISSN:0021-8979    

DOI:10.1063/1.1713855

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Magnetoeleastic pulses with variable echo time were observed in a rod of yttrium iron garnet. Data were taken at room temperature in the frequency interval 0.5 to 3.3 Gc/sec with input power in the milliwatt range. At a fixed carrier frequency, pulses were observed for external magnetic fields ranging over several hundred oersted; characteristically the echo time increased with decreasing field. The carrier frequency‐magnetic field‐echo time behavior can be explained in terms of a &thgr;=0 magnetoelastic disturbance traveling along the rod axis. It is launched at the position where &ohgr;c=&ggr;Hi. The disturbance propagates along the rod axis to the nearer end‐face where it is reflected. On arrival back at the launch site, a fraction of the energy is detected and the remainder reflected for another round trip. The process is repeated several times to give rise to a pulse sequence. The experimental results can be understood in terms of a magnetoelastic wave packet with calculated travel timeT=∫um−1dz≈∫us−1dz+∫ue−1dz, where the group velocities,u, refer to magnetoelastic, spin, and elastic waves, respectively. Spin‐wave loss has been included and gives improved agreement between theory and experiment.

ISSN:0021-8979    

DOI:10.1063/1.1713856

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Spin relaxation has been measured at 3 kOe and at lattice temperatures from 1.24° to 250°K for theE′ centers generated in one type of vitreous silica by 2‐MeV gamma rays. An unusual temperature dependence was found in which the spin‐lattice relaxation time varies roughly asT−2,5over most of the range. All of the data are accurately fitted by analytical functions derived from a coupling of the individual defect centers to the vitreous host involving excess local strain due to mechanical resonances of about 4.1×1011and 6.0×1012cps at eachE′ center. The strength of the coupling and the characteristic frequencies of mechanical resonance are similar to those in crystalline SiO2.

ISSN:0021-8979    

DOI:10.1063/1.1713857

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Superconducting wire made from heavily cold‐worked ductile niobium‐titanium wire shows much less degradation in solenoids than does the corresponding widely used niobium‐zirconium alloy. The origin of this behavior is discussed in terms of flux jumping and the differences in metallurgical properties of the two alloy systems. The superior niobium‐titanium alloy has been used in the inner sections of a 100 000‐G solenoid which has been driven normal repeatedly without damage. Details of design, construction, and operation of this solenoid are discussed with particular reference to critical rate of excitation under flux creep conditions. The normalization properties of the solenoid are analyzed in terms of the thermal propagation model, and the voltage rise and temperature maximum are estimated from this model.

ISSN:0021-8979    

DOI:10.1063/1.1713858

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The cause of the current degradation of superconducting solenoids, a subject of continuing interest, is elucidated further in this work. Two types of measurement are reported here: (1) Flux pinning energies and straight wire critical currents of 0.25‐mm (10‐mil) Nb&sngbnd;Zr wire are measured as a function of magnetic field. (2) Solenoid critical currents are reported as a function of temperature for the same material. These measurements indicate that high pinning energy causes flux jumping which causes solenoid degradation. They further indicate that pinning energy can be weakened by increasing the temperature; therefore, flux jumping and coil degradation can be suppressed by increasing the temperature

ISSN:0021-8979    

DOI:10.1063/1.1713859

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The coercive forceHcas a function of the angle between the easy axis of magnetization and the external field has been studied in orthoferrite single crystals using the vibrating sample technique. The experimental results are in good agreement with the Kondorsky 180° wall motion model which predicts the relationHc(0)/Hc(&thgr;)=cos&thgr;.

ISSN:0021-8979    

DOI:10.1063/1.1713860

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

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 implications of the analysis is given, along with a reiteration of the assumptions and the limitations of the calculation.

ISSN:0021-8979    

DOI:10.1063/1.1713861

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The elastic coefficients for an arbitrary rectangular coordinate system are calculated as a function of direction cosines in the crystal. Young's modulus, shear modulus, and Poisson's ratio are defined in general and values tabulated for some of the more important directions in the crystal. Graphs of these moduli are also plotted as a function of crystal direction for orientations in the (100) and (110) planes as well as planes determined by the [110] direction and any perpendicular direction.

ISSN:0021-8979    

DOI:10.1063/1.1713863

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The pressure as a function of lattice parameter and temperature has been calculated for NaCl over a pressure range of 0 to 500 kbar for temperatures between 0° and 1500°C. The calculation used the Mie‐Gru¨neisen equation of state with Born‐Mayer type repulsion terms between first and second nearest neighbors. The Gru¨neisen constant was expanded about its value at room temperature and atmospheric pressure; the first coefficient in the expansion being evaluated by forcing the calculated thermal expansion at atmospheric pressure to fit the experimental results of Enck. The two empirical parameters in the repulsion terms were evaluated using the experimental lattice parameter and isothermal compressibility at atmospheric pressure. The calculated pressure vs volume agrees with Bridgman's room‐temperature measurements in NaCl below 100 kbar to within 3% and with high‐pressure high‐temperature shock data to better than 2%. It is proposed to use the numerical results to calibrate the pressure in high‐pressure high‐temperature apparatus.

ISSN:0021-8979    

DOI:10.1063/1.1713864

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Neutron diffraction measurements have shown that the sublattice magnetizations in lithium ferrite are significantly higher, in the temperature range from 0.4Tcto 0.9Tc, whereTcis the Curie temperature, than predicted by molecular field calculations. An approximate analysis of the effect of adding a biquadratic term to the exchange interaction suggests that the principal effect is to distort the temperature scale in such a way as to makeT/Tc= (T′/Tc) (1+&bgr;⟨SA·SB⟩), whereT′ is the temperature calculated from a molecular field model and &bgr; is of the order of the ratio of the biquadratic to bilinear exchange coefficients. If theA‐A B‐Binteractions are neglected, but a biquadratic term is included in theA‐Binteraction with &bgr;=0.04, the calculated curves are in good agreement with experiment.

ISSN:0021-8979    

DOI:10.1063/1.1713865

出版商:AIP    

年代:1965

数据来源: AIP