The removal of binder from ceramic or metal moldings by thermolysis involves the transport of degradation products through the parent organic phase and the vacated porous body. A numerical model has been developed to combine an equation which takes into account different gas-flow regimes with an equation for the transport of organic molecules in molten polymers. Computer modeling reveals the critical heating rate above which defects occur due to boiling of the polymer-monomer solution at the center of the molding. The situation in which a porous outer layer of the molding develops, offering resistance to flow of the evolved monomer gas, is then treated. This gives rise to a moving boundary with a variable concentration of diffusant which is dependent on the surface flux, gas transport coefficient, and thickness of the porous layer. The contributions of diffusion and viscous flow to gas transport are considered.

The growth process of a YBCO/LaAlO3/YBCO trilayered film made by pulsed laser deposition has been studied by high resolution transmission electron microscopy (HRTEM). The high resolution images of the cross-section samples have shown that a 7 nm layer of LaAlO3 has been grown epitaxially between c-axis oriented YBCO layers having the nominal thickness of 250 nm. A stacking fault in the LaAlO3 layer may introduce a stacking fault into the YBCO layer, which may form nucleation sites for α-axis oriented grains. A second phase had been formed at the interface between the LaAlO3 layer and the lower YBCO layer, which has been identified by image simulation and energy dispersive x-ray (EDX) analysis as a new tetragonal La–Al–Cu–O phase based on LaAlO3 in which some of Al atoms have been replaced by Cu. The approximate lattice parameters of the new phase are a = 0.38 nm and c = 0.76 nm. However, no second phase was found at the interface between the lower YBCO layer and the LaAlO3 substrate.

The first relative sensitivity factors (RSF) for detecting the major and dopant elements of optical materials by ultrahigh intensity post-ionization (UHIPI) mass spectrometry are determined. The post-ionization is performed using a single laser wavelength with intensities greater than 1014 W/cm2. Zn-implanted InP and In0.4Ga0.1As0.3P0.2 are used to investigate the photoionization of sputtered atoms and molecules. Under optimal conditions, the UHIPI RSF's for atomic singly charged In, Ga, and Zn are nearly equal; that is, the ratio of UHIPI signals is equal to the concentration ratio. In principle, no standards are needed for quantitative analysis. Arsenic and P, with higher ionization potentials, are not detected as efficiently as other elements. The detected mass balance is usually group III rich. An entire mass spectrum is necessary for complete characterization of all elements and adjustment of their RSF's because many sputtered molecules are detected containing the group V elements. Multiply charged species compose about 10% of the detected ions.

The thermodynamic stability of TlBa2Ca2Cu3O9–y (1223) and substitutionally related phases has been studied by performing extended high-temperature anneals in a two-zone furnace. This approach allows for independent control of each of the thermodynamic variables: the oxygen and thallous oxide partial pressures [P(O2) and P(Tl2O)], the sample temperature, and the sample composition. P(Tl2O) determines which of several superconducting phases form in the unsubstituted Tl–Ba–Ca–Cu–O system. TlBa2Ca2Cu3O9–y is stable only within a narrow window of P(Tl2O). Partially replacing Tl with Pb and Ba with Sr substantially increases the stability of the 1223 phase. The composition (TlxPb0.5) (Sr1.6Ba0.4)Ca2Cu3O9–y yields only the 1223 phase under two-zone conditions when P(Tl2O) exceeds a lower bound. The stability of Pb- and Sr-substituted 1223 relative to the 1212 phase is related to the substitutional stoichiometry, rather than P(Tl2O).

In associated papers [Y. S. Yuan, M. S. Wong, and S. S. Wang, J. Mater. Res. 11, 8–17 (1996); J. Mater. Res. (1996, in press)] it has been shown that weak thermo-mechanical properties of a bulk monolithic high-Tc superconductor (HTS) can be improved by introducing strong ceramic whiskers into the HTS ceramic materials. In this paper, we report a further study of incorporating Ag particulates, (Ag)p, in a bulk monolithic BPSCCO and in the MgO-whisker reinforced BPSCCO composite. Effects of the (Ag)p addition on processing, microstructure, and superconducting and mechanical properties of the bulk monolithic BPSCCO and the (MgO)w/BPSCCO composite are investigated. The results indicate that the highly ductile Ag particulates promote densification of the BPSCCO matrix phase in the composite during hot pressing. The microstructure of the HTS composite with the (Ag)p addition is similar to that in the HTS material without the (Ag)p. The (MgO)w/BPSCCO composite with 10% (by weight) Ag particulates has been shown to possess excellent superconducting properties. The (Ag)p addition to both the monolithic BPSCCO and the (MgO)w/BPSCCO is found to increase appreciably their fracture toughnesses, but has little effects on mechanical strengths of the materials. Quantitative relationships have been established among solid-state processing variables, HTS phase developments, microstructures, and superconducting and mechanical properties of the (Ag)p/BPSCCO and the (MgO)w/(Ag)p/BPSCCO HTS composites.

Polycrystalline (0001)-oriented thin films of ZnO (thickness 120 nm) were deposited by rf magnetron sputtering and post-deposition annealed at 500 °C in oxygen (1 atm). The films were subsequently implanted with copper at doses over the range 1016 to 1017 ions/cm2. X-ray diffraction (XRD) indicates the compressive intrinsic film stress is largely relieved by the preimplantation anneal, and does not change when implanted or when further annealed after implantation, suggesting that the dominant cause of intrinsic stress is the atomic packing density rather than the crystallographic defect density. Resistivity measurements indicate that annealing of pure ZnO films causes the perpendicular resistivity to increase from 1.3 × 105 Ω · cm to 5 × 1010 Ω · cm. Copper implantation results in a lower resistivity of the order of 107 Ω · cm, but subsequent annealing actually increases resistivity beyond that of annealed nonimplanted ZnO to 3 × 1012 Ω · cm. It is proposed that copper increases the resistivity of those annealed films by trapping free electrons with the Cu 3d hole state occurring in CuO (formed predominantly during annealing). In order to check this, the oxidation state of the implanted copper was studied before and after annealing by x-ray photoelectron spectroscopy (XPS) and extended x-ray absorption fine structure (EXAFS). Three oxidation states of copper (Cu0, Cu1+, Cu2+) are detected in the implanted films, and postimplantation annealing results in oxidation of copper to the Cu2+ state, confirming that the presence of CuO in ZnO is associated with increased resistivity.

Dynamic hot-stage microscopy and sessile drop experiments have identified three stages in the spreading of Ag–28 wt. % Cu liquid on the surface of high-purity Ni foil: (I) nonreactive flow, (II) secondary spreading, and (III) breakout flow. The first stage is deGennes-type spreading driven by capillary forces and resisted by viscous drag. A (Cu, Ni) reaction layer forms quickly at temperature along the liquid-solid interface. Stage I ends when the liquid braze attains a quasistatic contact angle on the reacted surface. Stage II spreading involves a complex advance of a thin liquid sheet outward from the triple line as a result of differences in wetting between Ni grain surfaces and grain boundaries. The advancing liquid meniscus is distorted as the liquid moves ahead along the better wetted grain boundary regions and is held back (pinned) on those surfaces that are poorly wet, resulting in a stick-slip motion of the triple line. The change in contact area with time is linear during this stage, and the rate of spreading is independent of temperature in the range of 780–870 °C. Although the diffusion of Cu into Ni grain boundaries likely drives the capillary flow, it is not the controlling process since an activation energy is not observed. The final stage of spreading, breakout flow, involves the flooding of the liquid braze over previously wetted surfaces due to a change in the balance of interfacial energies. Spreading ends during stage II or III either by isothermal solidification which stems from interdiffusion between the braze filler and the substrate or by curtailment of the liquid supply when it pulls back on the (Cu, Ni) reaction layer. Hold time, peak temperature, and heating rate all have an effect on both the terminal area of spread and the spreading kinetics of braze flow on polycrystalline Ni. The heating rate effect has not been emphasized in previously published literature for soldering and brazing and, if overlooked could easily impair one's ability to apply test results to other studies or practical situations. Roughness-enhanced spreading was not observed with the Ni foil surfaces used in this study. There was, however, a localized effect on the shape of the triple line that did not affect spreading kinetics or terminal area of spread in a systematic fashion.

This study investigates the influences of Y–Ba–Cu–O compounds solidification under different oxygen partial pressures by a zone melting method. In the Y–Ba–Cu–O system, P(O2) ranged from 0.02 to 1 atm, and the following reaction occurred during heating: YBa2Cu3O6−δ (Y123) → Y2BaCuO5 (Y211) + L. With an increase of oxygen partial pressure or decrease of pulling rate, the morphology of the solidified interface changed from mushy to equiaxed, cellular, and planar. The continuous Y123 grains were readily obtained under high oxygen partial pressure. Based on the constitutional supercooling theory and combining the result of the yttrium solubility limit for different oxygen partial pressures, the influences of oxygen partial pressure on Y123 morphological evolution were clarified.

We have used pulsed organometallic beam epitaxy (POMBE) to simultaneously deposit â- and ĉ-axis oriented Y1Ba2Cu3O7−x (YBCO) thin films at arbitrary locations on LaAlO3(100) substrates. Using photolithography and ion milling, several types of â-ĉ weak links have been fabricated at the boundary between the two films. The current-voltage (I-V) characteristics show a flux flow type behavior. The resistive transitions are broad and the critical current density is low, indicating weak coupling across these boundaries. With magnetic field applied parallel to the grain boundary plane, nonhysteretic I-V curves are obtained and the critical current goes to zero at an applied magnetic field of ∼7500 G.

This paper describes ZBLA fluoride glass thin films produced via an inexpensive, low-temperature reactive atmosphere sol-gel approach. Luminescence from erbium at 1.55 μm has been observed in x-ray-amorphous doped films deposited on calcium fluoride, polyimide, sapphire, and silicon substrates. Fluorescence studies of the erbium 4S3/2 → 4I13/2 transition, a characteristic emission for a reduced phonon energy host, were conducted for both sol-gel-derived films and conventionally prepared glass rods. The peak intensity observed from the sol-gel films was blue-shifted by 16 nm with a FWHM value approximately half that measured for the melt-quenched rods. Excitation studies indicate that, unlike conventionally prepared glasses, sol-gel materials suffer from nonradiative relaxation of the 4S3/2 excited state to the 4I9/2 level, where subsequent radiative emission to the 4I15/2 ground state occurs. The proposed source of the quenching mechanism are remnant species inherent to the sol-gel process. While this causes the luminescence behavior of rare-earth-doped sol-gel-derived fluoride materials to be similar to oxide hosts, these remnant species modify the branching ratios, resultantly leading to a novel 824 nm emission when excited at 488 nm.

Directional solidification has been shown to be a successful way of achieving high current densities in bulk YBCO. The lack of understanding of the growth kinetics, however, makes it difficult to fabricate longer samples and reduce the processing times. To study the growth kinetics, quenching experiments of undoped YBa2Cu3O7−x (Y-123) and Y-123 doped with Pt and Nd from above the peritectic temperature with different holding times, t, were conducted. The results of these experiments indicate that the average 211 particle size varies at t1/3. Growth rate experiments were also conducted on these samples to determine the maximum growth rate for plane front solidification, Rmax. This quantity was measured for undoped and doped Y-123, and it was found that the addition of Pt did not increase Rmax while the addition of Nd doubled the growth rate. Using the coarsening results together with the growth rate experiments, the diffusivity of Y in liquid and the 211-liquid interfacial energy for undoped and doped Y-123 were calculated.

Bimetallic copper-ruthenium catalysts supported on silica were prepared by the reduction of the metallic salts in aqueous solution at room temperature. The concentration of the two metal components was selected to span the entire range of composition. In spite of the known immiscibility for the copper-ruthenium equilibrium phase diagram, X-Ray Diffraction (XRD) measurements combined with X-ray Photoelectron Spectroscopy (XPS) data indicate that this method of preparation is able to produce nanocrystalline extended solid solutions and/or amorphous metastable phases. In the case of ruthenium-rich compositions, the hexagonal close-packed (hcp) ruthenium crystallites are covered by copper atoms which grow with the same hcp sequence of the ruthenium core. For intermediate compositions a nanocrystalline and/or amorphous phase is observed, while in the case of copper-rich samples a single-phase fcc extended solid solution is found. The surface composition of the samples appears systematically enriched with Cu, as obtained from XPS semiquantitative results. The phenomena of phase separation and growth induced by thermal annealing at 870 K are also presented and discussed.

Elemental powders of zirconium and aluminum in the atomic ratio of 70:30 were mechanically alloyed in an attritor under argon atmosphere using zirconia balls as milling media. Samples have been taken out for characterization after different durations of milling. The process of alloying and resultant amorphization had been studied using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) was carried out to study the morphological changes occurring during repeated cold welding and breaking of the particles. Samples for TEM study were prepared by dispersing the mechanically attrited particles in the nickel foil by electrochemical codeposition. TEM study of the initial stages of milling revealed that localized structural changes precede the bulk amorphization process during mechanical alloying (MA). The sequence of phase evolution has been identified as (i) the formation of nanocrystalline supersaturated solid solution of aluminum in α-zirconium, (ii) amorphization of localized regions at powder interfaces, (iii) ordering of aluminum-rich regions in the metastable Zr3Al (DO19) phase, and, finally, (iv) bulk amorphization of the powders.

Growth of In0.52Al0.48As epilayers on InP(100) substrates by molecular beam epitaxy at a wide range of substrate temperatures (470–550 °C) is carried out. Low temperature photoluminescence (PL) and double-axis x-ray diffraction (XRD) measurements showed a strong dependence of the PL and XRD linewidths and lattice mismatch on the substrate temperature. Minimum PL and XRD linewidths and lattice mismatch were found to occur at substrate temperatures of between ≈500 and 520 °C under the beam fluxes used in this study. The XRD intensity ratios (Iepi/Isub) were generally higher within the same substrate temperature range at which the lattice mismatch was the lowest. XRD rocking curves of samples grown at low temperatures showed the main epilayer peak to be composed of smaller discrete peaks, suggesting the presence of regions with different lattice constants in the material. PL spectra taken at increasing temperatures showed the quenching of the main emission peak followed by the evolution of a distinct lower energy peak which is possibly associated with deep lying centers.

The electrical activity of phosphorus in Si80Ge20 alloys prepared by two nonconventional doping processes has been investigated over the temperature range 25–1250 °C. Both a solid state (mechanical alloying) and gaseous phase doping processes were found to extend the electrical activity of phosphorus in Si80Ge20 alloys beyond the reported maximum equilibrium value (2.1 × 1020/cm3) to 2.5–2.9 × 1020/cm3 within the temperature range 900 1200 °C. It is likely that this extended electrical activity of phosphorous is associated with a high density of defects. The enhanced electrical activity of phosphorus enabled Si80Ge20 alloys to have 300 to 1000 °C integrated average electrical power factors in the range 30.1–35.7 μW/cm-°C2.

The inherently weak mechanical properties of bulk monolithic high-temperature superconductors (HTS) have been a concern. Properly selected reinforcements in fiber and whisker forms have been introduced to the HTS ceramics to improve their mechanical properties. In this paper, mechanical behavior of a MgO-whisker reinforced Pb-doped Bi-2223 (BPSCCO) HTS composite fabricated by a solid-state processing method is studied. The (MgO)w/BPSCCO HTS composite has been shown to exhibit excellent superconducting properties. Elastic properties, strengths, and notched fracture toughnesses of both the monolithic BPSCCO and the (MgO)w/BPSCCO HTS composite are investigated. Detailed mechanical properties are reported for the first time for the (MgO)w/BPSCCO HTS composite. Mechanisms of strengthening and toughening in the MgO-whisker-reinforced HTS composite are also discussed.

We examine the formation of Si1-xCx (x = 0.04–0.2) by means of CFy (y = 0,1,3) implantation in p-type Si, for application as a wide-bandgap emitter in a Si heterojunc-tion bipolar transistor. Upon implantation with 2.5 × 1016 CF+/cm2 at 45 keV, and subsequently with 2.5 × 1016 C+/cm2 at 30 keV, an amorphous top layer is formed. Annealing at temperatures up to 900 °C leads to a layer consisting of nanocrystalline material. High resolution transmission electron microscopy and secondary ion mass spectrometry show that a well-defined nanocrystalline/crystalline interface is created at an anneal temperature of 550 °C. At higher temperatures lattice defects start to develop. Preliminary attempts to dope the material via phosphorus or arsenic implantation indicate that temperatures of at least 900 °C are required to activate a fraction of the implanted dopants. This, however, adversely affects the adlayer/substrate interface.

The present paper describes the effect of Hf and Ti additions on the microstructures and mechanical properties of two-phase composites based on the Cr2Nb–Nb eutectic. The microstructures of directionally solidified in situ composites containing 50–70% by volume of the Laves phase Cr2Nb which was modified with Hf (7.5–9.2%) and Ti (16.5–26%) are described. Partitioning of Hf and Ti between the two phases is discussed using microprobe and EDS results. The tensile properties at 1100 and 1200 °C are described and compared with those of an analogous niobium silicide-based composite. The Cr2(Nb)–(Nb) composite tensile yield strengths at 1200 °C were increased over that of monolithic Cr2Nb to ∼130 MPa. However, at 1200 °C the yield strengths of the silicide-based composites were approximately twice those of the Cr2(Nb)–(Nb) composites.

Based on our recent experimental observations, a growth mechanism for the reactive diffusion between Si single crystal and NbC powder compact is proposed. In Si–NbC diffusion couples annealed at 1300 °C, a two-phase NbSi2 + SiC reaction layer formed with NbSi2 as the matrix and SiC as discontinuous particles. The NbSi2 grain sizes and SiC particle sizes are both in the μm range. We propose that the SiC particles nucleated at the void surfaces in the NbC powder compact. This proposed nucleation mechanism offers a potential way of controlling the SiC particle size by changing the void size and void density of the NbC powder compact. It is also pointed out that this microstructure requires Si to be the dominant diffusing species. Si must diffuse through the reaction layer, while C only has to undergo local rearrangement, and Nb need not diffuse at all.

A ferromagnetic and supersaturated fcc Fe60Cu40 solid solution was prepared by mechanical alloying (MA). The phase transformations of the as-milled Fe60Cu40 powder upon heating to 1400 °C and subsequently cooling to room temperature were characterized by differential thermal analysis (DTA) and thermal magnetic measurement. The fcc Fe60Cu40 solid solution decomposes into α–Fe(Cu) + γ–Fe(Cu) + Cu(Fe) upon heating from 300 to 460 °C, and on further heating, α–Fe(Cu) transforms to γ–Fe(Cu) at 640 → 760 °C; during cooling, the reverse transformation occurs from 800 → 640 °C (obtained from thermomagnetic measurement) or from 700 → 622 °C (obtained from DTA). The γ ⇆ α transformation in mechanically alloyed Fe60Cu40 nanocrystalline occurs in a wide temperature range; the transformation temperature is higher than that of the martensite transformation in as-cast Fe–Cu alloys, but is much lower than that of the allotropic transformation of pure Fe. These differences may be caused by the different fabrication process, the nonequilibrium microstructure of MA, as well as the inhomogeneous grain size in α–Fe(Cu). High resolution transmission electron microscope (HRTEM) observations carried out in the specimen after the DTA run show that N-W or K-S orientation relationships exist between α–Fe(Cu) and Cu(Fe), which also represent the orientation relationship between α–Fe(Cu) and γ–Fe(Cu) due to excellent coherency between γ–Fe(Cu) and Cu(Fe). The grain size of the α–Fe(Cu) is inhomogeneous and varies from 50–600 mm. Energy dispersive x-ray spectroscopy (EDXS) result shows that the Cu content in these α–Fe(Cu) grains reaches as high as 9.5 at. % even after DTA heating to 1400 °C, which is even higher than the maximum solubility of Cu in γ–Fe above 1094 °C. This may be caused by the small grain size of α–Fe(Cu).