Showing papers in "MRS Proceedings in 1986"

Accurate Interatomic Potentials for Ni, Al and Ni3Al

Abstract: To obtain meaningful results from atomistic simulations of materials, the interatomic potentials must be capable of reproducing the thermodynamic properties of the system of interest. Pairwise potentials have known deficiencies that make them unsuitable for quantitative investigations of defective regions such as crack tips and free surfaces. Daw and Baskes [Phys. Rev. B 29, 6443 (1984)] have shown that including a local “volume” term for each atom gives the necessary many-body character without the severe computational dependence of explicit n-body potential terms. Using a similar approach, we have fit an interatomic potential to the Ni3Al alloy system. This potential can treat diatomic Ni2, diatomic Al2, fcc Ni, fcc Al and L12 Ni3Al on an equal footing. Details of the fitting procedure are presented, along with the calculation of some properties not included in the fit.

Analysis of Phases in Cement Paste using Backscattered Electron Images, Methanol Adsorption and Thermogravimetric Analysis

Abstract: In backscattered electron (bse) images of polished cement sections, anhydrous material, calcium hydroxide, other hydration products (mainly C-S-H) and porosity can be distinguished on the basis of their grey level in the image. Using an image analyzer connected directly to the SEM, it is possible to resolve these four components and so measure their relative proportions and distributions. The effects of magnification and the number of fields measured on the accuracy of bse image analysis are examined. The volume fractions of anhydrous material, porosity and calcium hydroxide derived from bse image analysis are compared with those obtained by other techniques and good correlation was found for the measurement of anhydrous material and porosity.

Chemistry And Applications Of Inorganic-Organic Polymers (Organically Modified Silicates)

Abstract: The combination of inorganic polymeric networks with organic components leads to inorganic-organic polymers. A convenient method for the introduction of organic radials into an inorganic backbone is the use of organosubstituted silico esters in a polycondensation process. This leads to≡Si-O-Si≡ network containing materials, so-called organically modified silicates (ORMOSILs). For the synthesis of the inorganic backbone, in opposition to the high temperature preparation of non-metallic inorganic materials like ceramics, “soft chemistry” methods have to be applied in order to preserve organic groupings to be incorporated. Therefore, the sol-gel process is a suitable technique [1–5]. A review over basic synthesis principles and chemical methods, their effect on special material properties and the application potential will be given.

Ceramics from Hydridopolysilazane

Abstract: The Advanced Ceramics Based on Polymer Processing Program is sponsored by the Defense Advanced Research Projects Agency (Materials Science Division). The objectives of the program include development of a family of Si-C, Si-N, and Si-C-N ceramic fibers which may be used for reinforcement in ceramic, metal and plastic matrices, and development of economical process technology for fabricating ceramic matrix composites with high fracture toughness, high temperature performance and no inherent limitations to forming complex shapes. Attainment of these objectives would circumvent two limitations of existing ceramic material technology - lack of design reliability due largely to catastrophic failure related to brittleness, and inability to fabricate complex shapes [1].

Volume Relationships for C-S-H Formation Based on Hydration Stoichiometries

Abstract: Volume changes can be calculated from the hydration stoichiometry of C3S if the composition of C-S-H is taken as C1.7SH4.0 with a density of 1.85 g/cm3. The results are in general agreement with the volume changes determined by Powers. However, the calculated evaporable water content is higher and the space-limiting water-cement ratio is calculated to be 0.42. The calculations can be applied also to the pozzolanic reaction and predict a marked increase in solid volume. In that case the composition of C-S-H is modified to C1.5SH3.8.

Polymeric Precursor Synthesis of Ceramic Materials

Abstract: For the past twenty years we have successfully synthesized a wide range of ceramic materials by using organic precursors. The organic precursors are formulated into a glass before pyrolysis. Synthesized compounds include, for example, titanates, zirconates, silicates, chromites, niobates, tantalates, ferrites and molybdates. This preparation technique is based upon having individual cations complexed in separate weak organic acid solutions. The individual solutions are gravimetrically analyzed for the respective cation concentration to a precision of 10–100 ppm. In this way it is possible to precisely control all of the cation concentrations, and to mix the ions on an atomic scale in the liquid state. There is no precipitation in the mixed solution as it is evaporated to the rigid polymeric state-in the form of a uniformly colored transparent glass. The glass retains homogeneity on the atomic scale, and may be calcined at a relatively low temperature of only a few hundred degrees Celsius to the homogeneous single phase of precise cationic stoichiometry and particle size of a few hundred A. The advantage of having such well characterized, thermodynamically defined compounds is illustrated by the resulting understanding of the solid state chemistry of multicomponent compounds that has occurred.

Studies of Organometallic Precursors to Aluminum Nitride

Abstract: The reaction of trialkylaluminum compounds with ammonia has been examined as a potential route to high purity AlN powder and to AlN thin films. This reaction proceeds in stages in which the initially formed Lewis acid/base adduct undergoes thermal decomposition to a series of intermediate alkylaluminum-amide and -imide species with increasing Al-N bonding, i.e., $$\matrix{ {{{\rm{R}}_3}{\rm{Al}} + {\rm{N}}{{\rm{H}}_3} \to {{\rm{R}}_3}{\rm{Al}}:{\rm{N}}{{\rm{H}}_3} \to \; \to \; \to {\rm{AlN}} + 3{\rm{RH}}} \cr {({\rm{where}}\;\;{\rm{R}} = {\rm{C}}{{\rm{H}}_3},\;\;{{\rm{C}}_2}{{\rm{H}}_5},\;\;{{\rm{C}}_4}{{\rm{H}}_9},\;\;{\rm{etc}}.)} \cr } $$ The structure and properties of several of these species have been studied using various physical and chemical methods, leading to a better understanding of the chemistry of this novel AlN precursor system. The structure of the intermediate organoaluminum amide, (CH3)2AlNH2, has been determined by single crystal X-ray diffraction methods and found to contain molecular trimer units with a six-membered Al-N ring structure similar to those which make up the wurzite structure of AlN. This compound is readily volatile and has been used to deposit AIN thin films on Si surfaces by a low-pressure CVD process. This approach has also been used to prepare AlN as a high surface area, high purity powder.

The Effect of Hydrolysis Conditions on the Characteristics of PbTiO3 Gels and Thin Films

Abstract: Sol-gel processing represents a promising method of fabrication for thin films of electronic ceramics which are useful in a number of packaging and device applications. In this study, the influence of acid and base catalysts on the structure of PbTiO3 gels and films (0.1-1.0μm) was investigated, for the purpose of inducing and identifying gel structures which were the most suitable as precursors for thin dielectric layers. Continuous, crack-free films, with dielectric strengths in excess of 106 V/cm were developed. Basic solutions gelled rapidly, phase separated, and were probably more crosslinked than acidic gels. Acidic gels seemed more capable of polymeric rearrangement during drying, yielding denser amorphous structures with microcrystalline regions. High-field dielectric constants (1 MV/m ac) in the range K=30–40, and K=160–170, were determined for amorphous and crystalline films, respectively.

XeCl Excimer Laser Annealing Used to Fabricate Poly-Si Tfts

Abstract: Mo-gate n-channel poly-Si TFTs have been fabricated for the first time at a low processing temperature of 26°C. 500 to 1000A thick a-Si:H was successfully crystallized by pulsed XeCl excimer laser (308nm) annealing without heating the glass substrate. The channel mobility of the TFT was 180 cm2/V.sec when the a-Si:H was annealed at energy density of 200 mJ/cm2.

MBE Growth of GaAs on Si: Problems and Progress

Abstract: Several fundamental problems are reviewed that must be solved if GaAs on Si growth is to be achieved with device-quality already close to the GaAs/Si interface, rather than relying on thick buffer layers: (a) antiphase disorder, (b) interface charge and cross-doping, and (c) misfit dislocations. An extensive discussion is given of the mechanism by which antiphase disorder is suppressed on (100)-oriented substrates

Influence of Type of Cement and Curing on Carbonation Progress and Pore Structure of Hydrated Cement Pastes

Abstract: Different series of cement paste specimens were prepared with ordinary Portland cement, with portland, blast furnace slag cements having slag contents of 30, 50 and 75% by mass, with commercial fly ash cement and with portland cement containing fly ash additions of 10, 20, 30 and 50% by mass. Moist curing of the specimens varied between 3 and 28 days before the pore size distribution and characteristics of the phase composition were analyzed. Subsequent to curing, the specimens were subjected to drying in air of 65% RH with a controlled CO2 content of 0, 0.03 and 2% CO2 by volume. Depth of carbonation, pore size distribution of the carbonated paste, and the phase composition were investigated after 28 days and 6 months of drying, respectively. The results show that carbonation alters the prevailing pore structure of the hydrated paste. Important parameters are the type of cement used and the duration of curing.

High Temperature Ordered Intermetallic Alloys

Abstract: This paper is intended to be a general introduction to this conference and is therefore not a review of the state of our current knowledge. Instead, it will address questions like the following: Why are intermetallic compounds interesting? What alloys are being studied, and which are being ignored? Since most research work is now being performed on L12 alloys, with by far the greatest emphasis on Ni3Al, the balanceof the paper will concentrate on strengthening mechanisms and mechanisms of ductility control in Ni3Al, pointing out the interesting questions and controversies which arose during this conference. The conclusions to be drawn from this paper are that ordered intermetallic alloys are very valuable materials for high temperature use, but engineers probably must become more sophisticated in the use of materials with limited ductilities at low temperatures before intermetallics will gain widespread usage. Furthermore, additional research needs to be performed on more complex intermetallic compounds than L12 since L12 compounds, as a group, do not have particularly high melting temperatures. However,since alloys with complex structures, e.g. Laves phases, are well known for their brittleness at low temperatures, it is all the more important that the properties of such alloys be studied and methods found to improve their ductilities.

Stresses in Adhesively Bonded Bi-Material Assemblies Used in Electronic Packaging

Abstract: The magnitude and the distribution of stresses in elongated adhesively bonded bi-material assemblies subjected to uniform heating or cooling are determined and discussed. The suggested approach enables one to evaluate the stresses in the assembly components themselves, as well as the shearing and normal (peeling) stresses in the interface, with consideration of the attachment compliance. The case of an epoxy bonded assembly is used to illustrate the developed theory.

Silicon as a Mechanical Material

Abstract: Now that single crystal silicon has established itself as the preeminent electronic material, increasing applications are being implemented in which silicon's MECHANICAL properties are more important. This paper reviews some of the history behind this trend, explains how and why silicon can be an economical and effective mechanical material, and describes several of the devices which have been demonstrated in this technology.

Surface Reactions in Discharge and Cvd Deposition of Silane

Abstract: Glow discharge deposition of hydrogenated amorphous silicon films involves; (A) the electron collisions which produce the reactive species, (B) the gas reactions these species undergo while diffusing or drifting to the surfaces, and (C) the surface reactions involved in film growth and gas processing. I will first describe our knowledge of the electron and gas reactions in these discharges, then of the surface reactions, and finally I will offer some conjectures regarding the influence of these different surface reactions and bombardments upon film properties.

Liquid-Phase Epitaxy of Hg1−xCdxTe from Hg Solution: A Route to Infrared Detector Structures

Abstract: Over the past few years, liquid-phase epitaxy (LPE) has become an established growth technique for the synthesis of HgCdTe. This paper reviews one of the most successful LPE technologies developed for HgCdTe, specifically, “infinite-melt” vertical LPE (VLPE) from Hg-rich solutions. Despite the very high Hg vapor pressure (> 10 atm) and the extremely low solubility of Cd in the Hg solution (< 10−3 mol%), this approach was believed to offer the best long-term prospect for growth of HgCdTe suitable for various device structures. Since the initial demonstration of LPE growth of HgCdTe layers from Hg solution in experiments conducted at SBRC in 1978, the VLPE technology has advanced to the point where epitaxial HgCdTe can now be grown for photoconductive (PC) and photovoltaic (PV) as well as monolithic metal-insulator-semiconductor (MIS) and high-frequency laser-detector devices with state-of-the-art performance in the entire 2–12 μm spectral region. A historical perspective and the current status of VLPE technology are reported. Particular emphasis is placed on the important role of the ther-modynamic parameters (phase diagram) and on control of stoichiometry (defect chemistry) and impurity doping (distribution coefficient) for growth of HgCdTe layers from Hg solution. Critical material characteristics, such as transport properties, minority-carrier lifetime, morphology and crystal structure, are also discussed. Finally, a comparison with the LPE technol-ogy using Te solutions, which has been the mainstay of the remainder of the IR community, is presented.

Tm Analysis of Interfaces Beiween Nb and Al2O3 Precipitates

Abstract: A Nb—Al alloy has been internally oxidized to produce A12O3precipitates. The interface between plate—like AI2O3 precipitates and the Nb matrix has been investigated by diffraction, CTEM, and HREM techniques. Periodic arrays of misfit dislocations were observed which acted as diffraction gratings for the electrons. The location of misfit dislocation cores could be determined from HREM images within an accuracy of a few lattice spacings. It can be concluded that the misfit dislocations lead to a good matching of atomic rows across the interface. The dislocation arrangement at this partially coherent interface is in accordance with theoretical expectations from dislocation theory.

Microstructure and Microchemistry of Slag Cement Pastes

Abstract: Pastes of a portland cement (60%) blended with a granulated blastfurnace slag (40%) were examined, principally by SEM with EDX analysis. Reaction rims around slag particles and relicts of fully reacted slag particles had compositions compatible with mixtures, in varying proportions, of C-S-H having Si/Ca -0.62 and a phase of the hydrotalcite family having Al/Mg -0.38. Calculations taking into account relevant densities and water contents indicated that replacement of the slag by its in situ hydration products entails little or no change in the numbers of Mg and 0 atoms per unit volume, but that substantial proportions of the Ca, Si and Al are released and an equivalent amount of H gained. In other respects, the microstructures qualitatively resembled those of pure portland cement pastes of similar ages, but less CH was formed and the C-S-H not formed in situ from the slag had a Si/Ca ratio of 0.56, higher than that of 0.50 to 0.53 found in the absence of slag. None of the individual phases in the slag cement pastes showed significant compositional variation with time in the 28 day to 14 month period studied. The relative amounts of Ca, Si and Al expelled from the slag are such that, in order to form C-S-H and AFm phase, more Ca is required. It is obtained partly at the expense of CH formation, and partly through increase in the Si/Ca ratio of the C-S-H formed from the clinker phases. Mass balance, volume composition and bound water content were calculated for the 14 month old paste and compared with corresponding results for the pure portland cement.

Electrical Characterization of ZrN

Abstract: We have investigated reactively sputtered ZrN, the most thermally stable of the refractory metal nitrides, for its applicability as a submicron gate electrode. One would require for such gate electrode in addition to the lowest possible resistivity, a metal work function that will tailor the threshold voltage of metal/oxide/semiconductor (MOS) field effect transistor in such a way as to minimize the channel implant. Having these requirements in mind, we have measured the temperature dependence of resistivity, its dependence on the annealing temperature and work function of ZrN films. Our best films have resistivity, ρ of 20 µΩ-cm, comparable to TiSi2. We find that ρ of as-deposited films being nearly stoichiometric, decreases about 20% after 1000°C anneal. Such anneal does not significantly alter the average grain size, which remains about 300 A even at this temperature. The temperature dependence of ρ at low temperatures indicates phonon scattering mechanism and we suggest that small amounts of oxygen are responsible for residual resistance. The films undergo superconducting transition at 7.5–8 K and this transition provides an additional check on the compositonal homogeneity of the films. The work function, determined from the capacitance-voltage (C-V) characteristics of ZrN capacitors with various thicknesses of SiO2, has value φm = 4.6 Volts, at the midgap between n+ and p+ polysilicon and thus is applicable to 0.5 µm N-MOS and submicron C-MOS.

TEM Studies of Cement Hydration

Abstract: The application of transmission electron microscopy to the study of the early stages of hydration C3S and the microstructure of very mature C3S paste is reported. In the former case the effect of water-cement ratio appears to be very important. In the latter, a detailed evaluation of the microstructures of inner C-S-H, outer C-S-H and CH is possible and the relationship of these to models of cement hydration is discussed.

Atomistic simulations of surface relaxations in Ni, Al, and their ordered alloys

Abstract: We have performed a series of simulations to examine the atomistic nature of surface relaxations in pure metals and ordered alloys. The surface relaxations (..delta..d/sub n,n+1/) are shown to be oscillatory and to decay rapidly into the bulk. The period and form of the oscillation may be determined by simple geometrical arguments. The oscillation wavelength is always of order an atomic diameter. In pure metals, the surface layer of atoms always displaces inward. However, in the ordered alloys the larger atom may displace outward. On planes composed of more than one atom types, rippling occurs.

Natural Analogues: Their Application to the Prediction of the Long-Term Behavior of Nuclear Waste Forms

Abstract: One of the unique and scientifically most difficult aspects of nuclear waste isolation is the extrapolation ofshot-term laboratory data (hours to years) to the long time periods (103-105 years) required by regulatory agencies for performance assessment. The direct verification of these extrapolations is not possible, but methods must be developed to demonstrate compliance with government regulations and to satisfy the lay public that there is a demonstrable and reasonable basis for accepting the long-term extrapolations. Natural analogues of both the repository environment (e.g. radionuclide migration at Oklo) and nuclear waste form behavior (e.g. alteration of basaltic glasses and radiation damage in minerals) have been used to demonstrate the long-term behavior of large scale geologic systems and, on a smaller scale, waste form durability. This paper reviews the use of natural analogues to predict the long-term behavior of nuclear waste form glasses. Particular emphasis is placed on the inherent limitations of any conclusions that are based on “proof” by analogy. An example -- corrosion of borosilicate glass -- is discussed in detail with specific attention to the proper and successful use of natural analogues (basaltic glass) in understanding the long-term corrosion behavior of borosilicate glass.

Mechanical Property and Microstructure of TZP and TZP/Al2O3 Composites

Abstract: There are various types of microstructure in the TZP materials and their composite materials. The fine-grained microstructure is well-known as the basic microstructural type of TZP materials. Y-TZP and Ce-TZP belong to this group. The large-grained microstructure can exist in the TZP consisting of the tetragonal phase with low metastability, that is, Y-TZP doped with TiO2 and Ce-TZP containing a high CeO2 content. The composite material between Y-TZP and β-lanthanum alumina possessed a unique microstructure including elongated grains of β-Al2O3 type structure. This study provides a summary of the mechanical properties of these TZP and β-Al2O3 type structure composite materials, and points out how the mechanical behavior depends on the microstructural features.

Transformation Plasticity and Transformation Toughening in Mg-PSZ and Ce-TZP

Abstract: The constitutive behavior of transformation plasticity in single phase fine grain Ce-TZP and polyphase coarse grain Mg-PSZ was studied. A material independent pressure sensitivity and a microstructure dependent strain rate sensitivity and temperature sensitivity were found to be associated with the transformation yield stress. Autocatalysis of various extent operates in all cases, forming broad macroscopic shear bands in TZP and fine crystallographic slip bands which terminate at grain boundaries in PSZ. Transformation plasticity is apparently athermal near the M b temperature in TZP, but not in PSZ except in brittle fracture. A strong crystallographic texture of the transformed phase develops during deformation. These results are analyzed in terms of a shear-dilatant yield criterion and a rate equation which account for stress assistance in martensitic transformation. An elastic-plastic fracture mechanical model is developed to estimate both the shear and dilatation contributions to the transformation plastic work in the crack tip plastic zone. On the basis of approximately equal shear and dilatation contributions to transformation plasticity, the model predicts a transformation zone height which is four times that of the previous model, and a toughness increment which is two times that of the previous model. These predictions are found in good agreement with the reported toughness-zone size relationship. The effect of temperature and chemical stability on toughness is also rationalized.

A Review of Recent Developments in Iron Aluminides

Abstract: This paper presents a review of recent research conducted at the Air Force Wright Aeronautical Laboratories (AFWAL)/Materials Laboratory to develop iron-aluminides as elevated temperature structural materials. The research consisted of investigations on the microstructure, tensile behavior, deformation, and fracture mechanisms of the DO3 and B2 ironaluminides. Binary Fe-Al alloys with a wide range of Al contents, solidsolution ternary alloys and precipitation- and dispersion-strengthened two-phase alloys have been investigated. It is shown that iron-aluminides have a potential to be structural materials at least up to 650°C.

Raman Spectra of Rings in Silicate Materials

Abstract: Raman spectroscopic studies on gel-derived silicates have confirmed that narrow bands near 607 cm-1 and 492 cm-1, first observed in the Raman spectrum of fused silica, are associated with three- and four-fold siloxane rings. Using these results, we have identified three- and four-fold siloxane rings in other high-surface-area silica materials, including leached glasses and Cab-OSil. This Raman spectroscopic evidence not only shows that small siloxane rings are a common characteristic of a number of silica materials but also suggests that they form preferentially at silica surfaces. This paper reviews the Raman spectroscopic evidence that led to the identification of the vibrational frequencies of the small siloxane rings and presents the results of Raman experiments on high-surface-area silica materials in which the concentration of small siloxane rings is enhanced compared to fused silica.

High-Speed Photoconductive Detectors Fabricated in Heteroepitaxial GaAs Layers

Abstract: Response times of ∼60 and 25 ps (FWHM), respectively, have been measured for photoconductive detectors fabricated in GaAs layers grown by molecular beam epitaxy on silicon substrates and silicon-on-sapphire substrates. Photoconductive detectors, which can be readily combined with GaAs logic devices such as MESFETs to provide high-speed optical to electrical conversion, could be used in optical interconnects that are integrated with Si circuits on monolithic GaAs/Si wafers. Transconductance values of 120 mS/mm have been obtained for MESFET’s fabricated in GaAs layers grown on silicon-on-sapphire substrates.

Diffusion Mecflanisms of Srrontium and Cesium in Compacrtd Sodium Bentonite

Abstract: In this research the diffusion of strontium and cesium through bento- nite slabs in stationary state has been measured. The measurement gives directly the transport through the sample and the corresponding effective diffusivity. The apparent diffusivity, diffusivity of the sorbed ions as well as the sorption factors can also be evaluated on the basis of the same diffusion measurement. The sorption on the coxmpacted bentonite has also been measured in a special sorption test where a radioactive tracer has been allowed to diffuse into compacted bentonite until equilibrium was reached. The measured effective diffusivities were clearly higher than they would be in the case of pure pore diffusion. The effective diffusivities also increase with K and the apparent diffusivities are almost independent on KD. This supports the theory that also the sorbed ions diffuse in their own phase. The sorption factors of the compacted bentonite correspond to the values of the batch experiments.

Choice of French Clays as Engineered Barrier Cokponents for Waste Disposal

Abstract: Results are presented of physical measurements on candidate buffer materials for use in nuclear fuel waste disposal. The materials being considered as constituent elements of engineered barriers are essentially calcium smectite clays, in other terms swelling clays, coming from fourteen french deposits. The criteria for good candidates are mainly: smectite content in the clay materials, carbonate and organic material content and bulk density of the material, compacted under a pressure of 100 MPa.