摘要:

We investigate the effect of polydispersity in fiber cross section on the effective properties of a fiber‐reinforced composite. For a model microstructure in which the fibrous inclusions are fully penetrable cylinders with varying diameters, we compute the microstructural parameters &zgr;’1and &eegr;1which determine Milton’s upper and lower bounds on transport coefficients and elastic moduli of the composite. From our results we infer that polydispersity in fiber size has very little influence on these properties.

ISSN:0021-8979    

DOI:10.1063/1.337786

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Using ultrasonic methods at ambient temperatures, for niobium carbide we determined the monocrystalline elastic stiffnesses:C11,C12, andC44in Voigt’s contracted notation. From these, we calculated the quasi‐isotropic (polycrystalline) elastic constants and the elastic Debye characteristic temperature. Results derived from a Blackman diagram suggest that ionic forces contribute significantly to the elastic constants and to interatomic bonding. This conclusion applies not only to NbC but also to other MX carbides with an NaCl‐type crystal structure.

ISSN:0021-8979    

DOI:10.1063/1.337249

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Explicit results are presented for Walpole bounds on the effective elastic properties of overall isotropic multiphase composites of randomly oriented crystals of materials with either cubic or hexagonal symmetry. The Walpole bounds are considerably narrower than the widely used Voigt and Reuss bounds, but slightly further apart than the Hashin–Shtrikman bounds. The difference between the Walpole and Hashin–Shtrikman bounds arises because of the way the comparison material that optimizes the bounds is chosen. The usual Hashin–Shtrikman (and Voigt‐Reuss) approach to estimating the overall elastic moduli of multiphase composites from the single‐crystal elastic stiffnesses of the component phases calculates polycrystalline averages for each phase separately, then uses the arithmetic means of the polycrystalline bounds as ‘‘the’’ elastic properties of the (isotropic) components of the multiphase aggregate. This technique is not appropriate for all isotropic multiphase composites, depending on how the composite is formed. The Walpole averaging technique performs the averaging for all the components simultaneously. We demonstrate that the widely used Voigt–Reuss–Hill average can differ from the arithmetic mean of the Walpole or Hashin–Shtrikman bounds up to 10 or 20% in composites with large differences in the elastic properties of the components.

ISSN:0021-8979    

DOI:10.1063/1.337250

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The fluid encapsulation technique allows for simultaneousinsitupressure and temperature calibration, at present up to 6 GPa and 800 °C, using on‐line data processing. Each experimental run is done quasi‐isobarically with continuous variation of temperature. Temperature is determined by the thermal emf’s ofS‐ orK‐type thermocouples placed in contact with accepted standard calibration materials which undergo pressure and temperature dependent phase transitions. In the course of this study, the phase diagrams of Bi, Hg, Sn, Tl, Pb, and Fe have been reinvestigated and self‐consistency between transitions of all of these materials was used to determine thep,T(emf) coordinations. Thermal emf’s, for theS‐type thermocouple, are converted to temperature using a 20‐degree polynomial that fits the IPTS‐68 data to within 0.4 K and is continuously differentiable in the temperature range from 0 to 1705 °C.

ISSN:0021-8979    

DOI:10.1063/1.337251

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The diffusivities of B, P, and As implanted in TiSi2are analyzed between 500 and 900 °C by secondary ion mass spectroscopy. It is shown that P and As have high (and almost equal) diffusivities compared with B which appears immobile. This difference is presumed to be related to the very high stability of TiB2(as compared with TiSi2) and the probable precipitation of B in the form of a titanium boride. The lattice diffusion coefficients for As and P are deduced from the diffusion profiles; they range from 10−17to 10−14cm2/s between 550 and 800 ° C. The activation energies are found to be, respectively, 1.8 and 2.0 eV; values close to the activation energy for the self‐diffusion of Si in TiSi2, 1.8 eV. The diffusion profiles also show a high grain boundary diffusivity and an accumulation of dopant at the TiSi2–Si interface.

ISSN:0021-8979    

DOI:10.1063/1.337252

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

We report an investigation of the materials properties of GaAs on Si epitaxial layers. By using properly oriented substrates, we have found that a substantial reduction in the density of threading dislocations can be achieved. In the presence of steps, dislocations with their Burgers vectors in the (100) substrate plane are preferentially generated, which are more effective in accommodating lattice mismatch and do not thread into the epitaxial layer. We have also found that the density of threading dislocations can be reduced significantly by the use of GaAs/InGaAs pseudomorphic superlattices. Using these techniques, dislocation densities of as low as 103–104cm−2have been achieved in 2‐&mgr;m‐thick GaAs on Si epitaxial layers. In growth on nominal (100) orientations, where it is known that single atomic steps dominate, we have found no evidence of antiphase domains by transmission electron microscopy or chemical etching. This result suggests that it may not be energetically favorable for antiphase domains to form in these samples. Alternatively antiphase domains may propagate along the {111} directions and annihilate one another. For GaAs/(Al,Ga)As double heterojunction (DH) laser structures on Si substrates, the dislocation control techniques have made possible electroluminescence intensity (spontaneous emission) within a factor of 2 of state‐of‐the‐art DH lasers on GaAs (which for reference lased at a current threshold density of 600 A/cm2). These results compare to within experimental determination. Electroluminescence intensities were also found to increase with increasing initial growth temperature. Stripe geometry lasers with room‐temperature pulsed threshold currents as low as 170 mA for 10×240 &mgr;m2stripe have been obtained using these techniques.

ISSN:0021-8979    

DOI:10.1063/1.337253

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The incorporation of impurities in GaAs epitaxial layers grown from trimethyl gallium (TMG) and AsH3has been studied in detail by varying a large number of growth parameters. These include the V/III ratio, temperature, the axial position in the reactor, gas sources, substrate and susceptor material, carrier gas, substrate misorientation, and the crystallographic orientation of the substrate. As main characterization techniques photoluminescence and Hall–van der Pauw measurements have been used. Donor and acceptor concentrations in the layers have been found to vary not only with temperature and V/III ratio, but also with the axial position in the reactor, giving rise top/ntransitions and maxima in the carrier mobility. The V/III ratio is shown to be effectively constant for larger axial distances in the cell. Highly doped substrates have been found to give rise to outdiffusion of defect complexes into the layers. The main acceptor impurities found in this work are zinc, silicon, and carbon. They are shown to originate from the TMG gas source, the hot quartz parts in the cell, and the TMG growth component, respectively. Incorporation of these elements appears to be orientation, and in the case of carbon also misorientation, dependent. The results for {001} and misoriented {001} crystals are discussed on the basis of two models for impurity incorporation: CAsacceptors are concluded to be incorporated by a trapping process at growth steps, whereas ZnGa, SiAs, and residual donors most probably incorporate via equilibrium processes. The conclusions from a study of the influence of the misorientation are especially important for this interpretation.

ISSN:0021-8979    

DOI:10.1063/1.337254

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A detailed analysis of the photocapacitance signal at the near‐band and extrinsic energetic ranges in Schottky barriers obtained on horizontal Bridgman GaAs wafers, which were implanted with boron at different doses and annealed at several temperatures, has been carried out by using the optical isothermal transient spectroscopy, OITS. The optical cross sections have been determined as well as the quenching efficiency of the EL2 level which has been found to be independent of the annealing temperature. Moreover, the quenching relaxation presents two significant features: (i) a strong increase of the quenching efficiency from 1.35 eV on and (ii) a diminution of the quenching transient amplitude in relation with that shown by the fundamental EL2 level. In order to explain this behavior, different cases are discussed assuming the presence of several energy levels, the existence of an optical recuperation, or the association of the EL2 trap with two levels located, respectively, atEv+0.45 eV andEc−0.75 eV. The theoretical simulation, taking into account these two last cases, is in agreement with the experimental photocapacitance data at low temperature, as well as at room temperature where the EL2 filling phototransient shows an anomalous behavior. Moreover, unlike the previous data reported for the EL2 electron optical cross section, the values found using our experimental technique are in agreement with the behavior deduced from the theoretical calculation. The utilization of the OITS method has also allowed the determination of another level, whose faster optical contribution is often added to that of the EL2 level when the DLOS or standard photocapacitance is used.

ISSN:0021-8979    

DOI:10.1063/1.337255

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Deep levels in single crystals of CdSe intentionally doped with copper have been investigated by photoconductivity and space‐charge region capacitance techniques. The only center which can be conclusively associated with the copper impurity was found to have an activation energy of ∼1 eV with respect to the valence band. Estimates of 10−13and 10−18cm2were made for the hole and electron capture cross sections for this center. The capture cross‐section ratio of ∼105indicates that this center is behaving as a photoconductivity sensitizing center. However, another center with an activation energy of ∼0.65 eV with respect to the valence band, and which is commonly observed in undoped material, would appear to be the dominant sensitizing center for all but the highest levels of copper doping. Two further centers, with activation energies of ∼0.5 and ∼0.9 eV, relative to the conduction band, were also found. In addition, there is evidence for the existence of at least two distinct centers with the same activation energy of ∼0.2 eV with respect to the valence band, one of which appears to be copper related.

ISSN:0021-8979    

DOI:10.1063/1.337256

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A nondestructive microwave photoconductance technique has been employed to investigate the uniformity of electrical transport properties in semi‐insulating, doped, or implanted GaAs. Although the measurement time is increased, the technique is also applicable to Si. A review of the advantages and limitations is discussed and some example applications to a variety of GaAs and Si structures are presented.

ISSN:0021-8979    

DOI:10.1063/1.337257

出版商:AIP    

年代:1986

数据来源: AIP