摘要:

The metalorganic chemical vapor deposition (MOCVD) of epitaxial III‐V semiconductor alloys on III‐V substrates is reviewed in detail. The emphasis is placed on both practical and theoretical knowledge of the equipment and deposition process. The chemistry of the source alkyls and the dynamics of the transport process are discussed. The growth of the GaAs and AlxGa1−xAs systems are treated as prototypical examples (and the most studied) of the III‐V materials. Latter sections review InP, Ga1−xInxAs, and related alloys. Finally, the antimonide and the other systems are reviewed. Electronic and optical devices fabricated from MOCVD‐grown materials are used as examples of the capabilities of the growth technique.

ISSN:0021-8979    

DOI:10.1063/1.336296

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A digitally controlled thermally stimulated depolarization current facility is described and measurements on poly(ethylene oxide) (PEO) and PEO complexed with potassium thiocyanate (KSCN) are reported. The results complement and extend previous electrical relaxation studies on these materials. In particular, in PEO the presence of a relaxation previously labeled &agr;cis confirmed. The relaxation is assigned to defects in the crystalline phase. In addition, the &ggr; relaxation region is shown to consist of at least two relaxations. Further, a high‐temperature family of peaks is obtained which are associated with space charge. For KSCN‐complexed PEO three low‐temperature relaxations are observed in the &ggr; relaxation region. The relationship between these relaxations and previously reported dielectric relaxation results is established and possible origins of the relaxations are discussed. Finally, differential scanning calorimetry studies were carried out on all of the materials.

ISSN:0021-8979    

DOI:10.1063/1.335878

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Thermal insulation characterization of polymers and other materials is an important requirement for present and future aerospace missions, as well as for the home construction industry. Some dispersed composites and layered samples have been successfully characterized by the thermal‐diffusivity flash technique, while the interpretation of these and other experiments for other systems remains problematical. One refinement for layered‐sample data reduction is investigated here. An exponentially decaying spatial penetration of radiant energy into the sample is accounted for. Under the conditions chosen for numerical evaluation, an 18% reduction in the back‐face rise timet1/2is predicted for a case in which only 5% as much radiant energy is deposited on the midplane as on the front surface. It is suggested how the use of the entireV(t) response function might lead to experimental methods of estimating the penetration depth of the radiant energy for particular samples.

ISSN:0021-8979    

DOI:10.1063/1.335879

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Polyethylene single‐crystal mats 20 &mgr;m thick were heated to the annealing temperatures (120–132 °C) in less than 1 s. The Cornell High Energy Synchrotron Source (CHESS) and a TV detector system were used to obtain wide‐ and small‐angle x‐ray scattering patterns from these mats during annealing with a time resolution of 0.3 s. Both whole polymer and a molecular weight fraction showed rapid melting and recrystallization, with a minimum crystallinity at 2–4 s after reaching the annealing temperature. Recrystallization was essentially complete in 60–90 s and most curves could be fitted to the Johnson–Mehl–Avrami equation with exponentn=1. Two‐thirds of the material could be molten before there was any loss of the original crystal orientation. The increase in long period was discontinuous at high annealing temperature. The original small‐angle long‐spacing peak quickly lost intensity as crystallinity fell and a new peak appeared at larger long spacing. This new long spacing increased with time for the whole polymer and was constant for the fractionated material. At lower annealing temperatures the amount melting was small and the small‐angle intensity and long period increased continuously.

ISSN:0021-8979    

DOI:10.1063/1.335880

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A simple, three‐constant pressure‐volume‐temperature PVT equation of state, previously applied to polymer liquids, is applied to polymer solids, both crystalline and amorphous. In terms of reduced variablesP¯=P/B0,V¯=V/V0, andT¯=T/T0, the equationP¯ V¯5=T¯3/2−ln V¯ fits experimental volume data taken from the literature for 19 polymers to within the accuracy of the measurements, about 0.1%. Using the different sets of reducing parameters for the liquid and glassy states of an amorphous polymer, a second order phase transition is predicted. At zero pressure, this transition temperatureT2lies about 20 K below the glass transition temperatureTgbut has a greater pressure dependence. It was also found that the combination of reducing parametersRT0/B0V0is approximately constant for amorphous and crystalline polymers.

ISSN:0021-8979    

DOI:10.1063/1.335881

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The propagation of a craze as quasifracture under repeated cyclic stressing in polymeric systems has been under intensive investigation recently. Based upon a time‐dependent crazing theory, the governing differential equation describing the propagation of a single craze as quasifracture in an infinite viscoelastic plate has been solved for sinusoidal stresses. Numerical methods have been employed to obtain the normalized craze length as a function of time. The computed results indicate that the length of a quasifracture may decelerate and decrease indicating that its velocity can reverse. This behavior may be consistent with the observed and much discussed craze healing and the enclosure model in fatigue and fracture of solids.

ISSN:0021-8979    

DOI:10.1063/1.335882

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

We have studied the structure and morphology of the useful high‐temperature/high‐strength polymer polyaryletherketone (PEEK) by transmission electron microscopy and three‐dimensional electron diffraction after finding suitable solvents (&agr;‐chloronaphthalene and benzophenone) that allowed casting of the required ultrathin polymer films. When crystallized from the melt, PEEK grows in the form of spherulites consisting of narrow lamellae and having thebaxis of the unit cell radial. Additionally, at high temperatures in these ultrathin films, the spherulites attain an extraordinary cylindrical symmetry as a result of growth of their lamellae on edge, with theccrystallographic direction parallel to the film plane and theadirection corresponding to the cylinder axis. Reasons for this mode of growth are attributed to the highly anisometric molecular cross section normal to the chain direction, which favors crystal nucleation on the substrate with thebcplane. At lower temperatures during crystallization from the melt, a more random lamellar disposition is seen in these thin‐film spherulites, although lamellae on edge still predominate. Crystallization by heating from the quenched glassy phase yields random lamellar aggregates and small spherulites. The glassy phase in ultrathin PEEK shows no consistent morphological features down to a level of resolution of 1.0 nm. Scanning electron microscopic examination of the free surfaces of bulk samples crystallized under controlled conditions both from the melt and from the glass, show that our findings from ultrathin films (with the exception of the quasicylindrical spherulitic substructure) also apply to these thicker specimens.

ISSN:0021-8979    

DOI:10.1063/1.335856

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Copolymers of vinylidene fluoride‐trifluoroethylene P(VDF‐TrFE) exhibit large piezoelectric and pyroelectric effects. In addition to the most common application of the pyroelectric effect (radiant detection) it is possible to convert heat directly into electrical energy by pyroelectric conversion. This study reports the first measurements of high (constant) field pyroelectric effects of P(VDF‐TrFE) which are relevant to pyroelectric conversion. Electric displacement changes which result from temperature changes of the copolymer were measured for thermal cycling from room temperature to 90 °C at fields up to 1 MV/cm. The displacement changes (0.1–0.3 &mgr; C/cm2) were dependent upon the molar ratio of the constituent monomers. The resistivity of the copolymer was also measured in the temperature range 20 to 90 °C and was found to vary from 1012to 1016&OHgr; cm depending on monomer ratio, temperature, field, and time. The resistivity and displacement change data are discussed in terms of predicted pyroelectric energy conversion performance.

ISSN:0021-8979    

DOI:10.1063/1.335857

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The surface compositions of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends have been investigated using x‐ray photoelectron spectroscopy (XPS). The XPS results demonstrate that there are significant differences between the composition of the surfaces (e.g., outmost ∼6 nm) and the bulk of both miscible and immiscible blends. The surfaces of PS/PVME films dip‐coated from either toluene or trichloroethylene solutions are found to be enriched in PVME. An additional enrichment in the PVME content of the surface is also observed when miscible PS/PVME blends are phase separated by heating them above the lower critical solution temperature. The average composition profile of a miscible 50/50(w) blend has been obtained by angular‐dependent XPS measurements. This experiment suggests that the enrichment is the result of a concentration gradient rather than a monolayer coating. These results are discussed in terms of the surface free energies of PS and PVME and the degree of mixing in the surface layer.

ISSN:0021-8979    

DOI:10.1063/1.335858

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Coexistence curves for binary polymer/solvent solutions are asymmetric when volume fraction is used as the concentration variable. Coexistence curves for polystyrene/methylcyclohexane solutions can be symmetrized by a simple transformation of variables; in the new dimensionless variable &psgr;, all of the data satisfy a universal scaling law: ‖&psgr;−&psgr;c‖=&psgr;0(&egr;N0.313)&bgr;; &egr;≡(Tc−T)/Tc, whereNis the degree of polymerization and &psgr;cand &psgr;0are constants independent ofN. The experimental value of the critical exponent &bgr; (0.327±0.002) agrees well with recent renormalization group calculations for the Ising model. The exponent onN(0.102±0.002) may be universal and should be compared with its mean‐field value of 1/4; a calculation of its nonclassical value remains a challenging theoretical problem.

ISSN:0021-8979    

DOI:10.1063/1.335859

出版商:AIP    

年代:1985

数据来源: AIP