摘要:

With the help of the lattice momentum, phase‐matched optical generation of coherent phonons with frequencies higher than 50 GHz appears feasible. We have considered two cases: (i) direct conversion of millimeter or submillimeter photons in a piezoelectric superlattice, (ii) optical mixing of two laser beams in a nonpiezoelectric superlattice.

ISSN:0021-8979    

DOI:10.1063/1.1662386

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A technique is presented for measuring the thermal diffusivity of solids. This technique differs from others in that boundary conditions are obtained by use of a well‐stirred constant‐temperature bath. By varying the bath temperature and the initial temperature of the specimen, thermal diffusivity values may be determined as a function of temperature. Accuracies within ±5% may be obtained and the major sources of error in the technique may be isolated and controlled. The method lends itself to remote‐control testing of hazardous materials such as high explosives and solid propellants. In the present studies thermal diffusivities ranging from 0.005 to 0.010 ft2/h were measured between −65 and 165°F on a typical high explosive material.

ISSN:0021-8979    

DOI:10.1063/1.1662387

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A method based upon the use of two algebraic transformations is applied to the equation of heat conduction in anisotropic media having nonlinear generation. The feasibility of the method is manifested when the principal conductivities together with the volumetric heat capacity vary with temperature in a power fashion. Also, the conductivities are taken parallel to the principal axes. Except for the unsteady term, the transformed equation reduces to its counterpart in isotropic media with constant properties. Applicable boundary and initial conditions of general type are analyzed.

ISSN:0021-8979    

DOI:10.1063/1.1662388

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

p‐type semiconducting diamond was grown by vapor deposition from a 0.83% diborane in methane gas mixture at 1050°C and 0.2 Torr on 0 to 1‐&mgr; nominal size natural type‐I diamond powder. Total mass increases of about 9% were achieved which correspond to average linear growth rates of less than 10−3&mgr;/day. Evidence showing the growth was boron‐doped diamond included chemical etching, x‐ray and electron diffraction, density measurements, Seebeck and resistivity measurements, chemical analysis, optical measurements, induced electron emission spectroscopy, and scanning electron microscopy. The crystalline quality of the new diamond has not been established; it may be highly defective. A distinct change in color of the diamond seed crystals from an off‐white or gray for virgin crystals to light blue after growth was observed. The results are further confirmation that diamond may be grown at low pressures where it is thermodynamically metastable with respect to graphite. It is also further evidence that boron is the element responsible for bluep‐type diamonds.

ISSN:0021-8979    

DOI:10.1063/1.1662389

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Pulse experiments are described in which a pure copper sample was bombarded for 20 sec with 1‐MeV electrons at temperatures between 293 and 430°K, then the continuing increase in accumulation of point defects at dislocations was monitored at temperature by internal friction and elastic modulus data. No discontinuity in the rate of arrival is found at the moment that the irradiation is discontinued, indicating that the arriving point defect must be characterized in its motion by an energy of 0.6 eV. The postirradiation kinetics are analyzed to show that the energy of migration of this defect along dislocations is ∼ 0.2 eV less than the energy for motion in the lattice, away from the dislocations. The above kinetic analyses are carried out with the aid of the recently formulated ``defect dragging'' theory of Simpson and Sosin and constitute further verification of this theory.

ISSN:0021-8979    

DOI:10.1063/1.1662390

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The steady‐state flows of gas atoms and vacancies leaving a pore that is either on the surface, or at various positions inside of a spherical grain have been calculated by approximation methods for the conditions that exist during the final stage of sintering. Gas and vacancies were assumed to leave pores by volume diffusion only. It was found that the position of a pore in a grain should have little effect on its shrinkage rate—a result that appears to be in general accord with previous experimental observations on various materials. The rate of shrinkage of pores on grain boundaries in the one material investigated appears to be many times greater than that predicted by the calculations, implying that shrinkage of such pores was not controlled by volume diffusion. Pore‐to‐pore diffusion and removal of porosity by grain‐boundary migration are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1662391

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Elastic Debye temperatures &thgr; were calculated by averaging elastic stiffness coefficients. For cubic symmetry, eight averaging methods were evaluated with respect to a computationally exact &thgr;. Reuss's &thgr;, corresponding to uniform stress, gave better agreement than Voigt's &thgr;, corresponding to uniform strain. Hill's geometrical &thgr; gave the best agreement.

ISSN:0021-8979    

DOI:10.1063/1.1662392

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The heat transfer problem associated with pulse thermal diffusivity measurements is analyzed for the cases of (i) triangular pulses whose widths are comparable with the transit time of temperature fronts across a sample (finite pulse width effects), (ii) heat losses from sample faces, and (iii) the simultaneous occurrence of heat losses and finite pulse‐width effects. Methods for the analysis of experimental results which are affected by these conditions are discussed. Tabular and graphical data which facilitate the analysis are given.

ISSN:0021-8979    

DOI:10.1063/1.1662393

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The electrical resistivity of Al&sngbnd;Ni alloys in the composition range 2–28 wt% Ni was investigated. The eddy‐current decay method was used and the measurements were made at three temperatures: 297, 77, and 4.2 K. The results are explained in term of a series and parallel resistor model which takes into account the existence of different regions in the alloy (i.e., the continuous eutectic mixture, and the dispersed proeutectic constituents). This model permitted the resistivity curve to be fitted at the three temperatures and resistivity value for Al3Ni to be computed. The sharp maximum of resistivity at the eutectic composition, appearing at low temperature, has been attributed to the scattering of electrons on precipitate boundaries and to peculiar morphological features.

ISSN:0021-8979    

DOI:10.1063/1.1662394

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

High‐velocity compression tests were conducted on pure polycrystalline aluminum. Data obtained at compressive strain rates ranging from 0.4×104to 12×104sec−1could be divided into two regions of interest. For strain rates from 0.7×104to 1.4×104sec−1a linear relationship was found to exist between the applied stress and the strain rate, indicating that the moving dislocations are viscously damped in this region. The values of the damping constantBobtained in this region ranged from 1.9×10−4to 3.0×104dyn sec/cm2and showed good agreement withBdetermined by other investigators. Beyond these strain rates, the damping reduced at first and then increased, with very high stresses being obtained at the highest strain rates reached. The drop in the damping has been attributed to the reduction in the volume around the cores of the moving dislocations that can exchange energy with the lattice phonons. The rise in the damping at the higher strain rates may be due to relativistic effects as the moving dislocations approach the velocity of sound. Using the expression for the strain field around a fast‐moving dislocation in the phonon viscosity theory and the general expression for dislocation damping due to phonon scattering, an expression for the damping constant was obtained embodying these effects. This was found to show good agreement with the data.

ISSN:0021-8979    

DOI:10.1063/1.1662395

出版商:AIP    

年代:1973

数据来源: AIP