Showing papers in "MRS Proceedings in 1991"
TL;DR: In this article, a simple geometrical model describing the changes in the surface topography during ion beam etching is proposed. But the model is not applicable to a wide range of materials and to XTEM preparation.
Abstract: The thinning technique is based on a simple geometrical model, describing the changes in the surface topography during ion beam etching. A high ion beam density makes jt possible that a thinning with an incidence angle of 0.5–7° ( measured from the sample surface) can take place within a reasonable time. Our method is applicable to a wide range of materials and to XTEM preparation.
109 citations
IBM1
TL;DR: In this article, the Tripod Polisher is used to produce a tapered, wedge-shaped specimen, which has the area of interest at the thinnest edge of the taper.
Abstract: Cross sections of material specimens for TEM analysis must be produced in the shortest time possible, contain few, if any, artifacts and have a large area available for analysis. The analyst must also be able to prepare these cross sections from specified areas of complex, heterogeneous structures on a routine, reproducible basis to meet the growing needs of the semiconductor industry for TEM analysis. The specimen preparation spatial resolution required for preparing precision cross sections is substantially less than one micron. Cross sections meeting these requirements can be prepared by mounting a specimen to the Tripod Polisher and mechanically polishing on one side of the specimen, using a sequence of progressively finer grit diamond lapping films, until the area of interest is reached. This polished surface is then very briefly polished on a cloth wheel with colloidal silica to attain the final polish on that side. The specimen is then flipped over on the Tripod Polisher and polished from the other side, using same sequence of diamond lapping films to reach the predefined area of interest. The Tripod Polisher is set at a slight angle, to produce a tapered, wedge-shaped specimen, which has the area of interest at the thinnest edge of the taper. The specimen is polished with the diamond lapping films and the colloidal silica until it is 1000 Angstroms or less in thickness. The specimen is removed from the polisher and mounted on a 2 × 1mm slotted grid with M-Bond 610 epoxy. After the epoxy is cured the specimen can be taken directly to the microscope for analysis. The need for ion milling has been eliminated or reduced to a few minutes in most of our work because of the thinness of the final specimen. The total specimen preparation time is between 2.5 and 4 hours, depending on the specimen and the size of the specified area. The area available for analysis ranges from 0.5mm up to the full size of the mounting grid opening. The wedge shape of the specimen provides the mechanical stability needed for a long thin specimen.
80 citations
TL;DR: This report concerns the tissue-engineered growth of new cartilage in the shape of a human ear using synthetic biodegradable polyesters seeded with living chondrocytes isolated from a freshly sacrificed calf shoulder and implanted subcutaneously on the dorsum of athymic rats.
Abstract: This report concerns the tissue-engineered growth of new cartilage in the shape of a human ear. Using synthetic biodegradable polyesters, a porous, three dimensional device in the shape of a human ear was fabricated. The polymer matrices were seeded with living chondrocytes isolated from a freshly sacrificed calf shoulder and implanted subcutaneously on the dorsum of athymic rats. This resulted in the formation of new cartilage in the shape of a human ear of approximately the same dimensions as the original implants. Histological analysis revealed the presence of mature cartilage in all specimens.
78 citations
TL;DR: In this paper, the authors investigated the optical properties of undoped, La-doped, and Nddoped Pb(ZrxTi1−x)O3 thin films deposited on sapphire substrates by metalorganic decomposition (MOD) process.
Abstract: Optical properties were investigated for undoped, La-doped, and Nd-doped Pb(ZrxTi1−x)O3 thin films deposited on sapphire substrates by metalorganic decomposition (MOD) process. Refractive index and extinction coefficient of these films were calculated from transmission spectra in the wavelength range of 300 to 2000 nm. The packing densities of these films were calculated from the refractive index data by using the effective medium approximation. Band gap energies of these films were also reported under the assumption of direct band-to-band transition. The refractive index and band gap energy of PZT films showed a linear dependence on Zr/Ti ratio. The refractive index decreased, while the band gap energy increased with increasing zirconium content. It was also found that both La-doped and Nd-doped PZT films had higher refractive indices than those of undoped PZT films with the same Zr/Ti ratio (50/50).
68 citations
TL;DR: The effect of starting materials on the synthesis of tobermorite was studied using 29SiMAS and CPMAS NMR Spectroscopy as discussed by the authors, and three Q2 and one Q3 peak were identified during the crystallization of tobersite from C-S-H.
Abstract: The effect of starting materials on the synthesis of tobermorite was studied using 29SiMAS and CPMAS NMR Spectroscopy. Three Q2 and one Q3 peak were identified during the crystallization of tobermorite from C-S-H. C-S-H, which had a Ca/Si ratio >1, crystallized readily because this C-S-H consists of dimer and short chains of silica tetrahedra. This occurred whenever quartz was used as a starting material. C-S-H, which had a Ca/Si ratio <1 was harder to crystallize because this C-S-H consists of long and cross-linked chains of tetrahedra. The latter was observed when amorphous silica was used as a starting material.
66 citations
TL;DR: In this article, the residual stress in solution derived Pb(Zr.53Ti.O3, PZT 53:47, films was determined by measuring the bending of the substrate due to the stress.
Abstract: The residual stress in solution derived Pb(Zr.53Ti.47)O3, PZT 53:47, films was determined by measuring the bending of the substrate due to the stress. The substrate consisted of an oxidized (100) silicon wafer with 300 nm coating of platinum. In all cases the stress was tensile. Films fired at a temperature in the range where pyrochlore formation occurs (500° to 575°C) had the highest residual stresses, 200 to 350 MPa, whereas those fired at higher temperatures, 600° to 650°C, where the perovskite phase forms had stresses of 100 to 200 MPa. Stress measurements made during film firing indicate that the pyrochlore containing films had higher residual stress because their coefficient of thermal expansion was much larger than that of predominantly perovskite films. The effect of the amount of stress on ferroelectric properties was studied by making measurements on a film with and without the application of an external stress. The external stress was applied by bending a circular section of the substrate, which effectively lowered the amount of tensile stress in the film by ~30%. Decreasing the stress in this manner was found to increase the remanent polarization by ~11% and the dielectric constant by ~2%.
65 citations
IBM1
TL;DR: Spin polarized low energy electron microscopy (SPLEEM) has been developed for the high resolution imaging of surface magnetic structure as discussed by the authors, which has been modified by the incorporation of a GaAs-type spin polarized electron gun.
Abstract: Spin polarized low energy electron microscopy (SPLEEM) has been developed for the high resolution imaging of surface magnetic structure. The existing LEEM ha.s been modified by the incorporation of a. GaAs-type spin polarized electron gun. Large image contrast arises due to the spin-dependent exchange scattering, whifle the st.in-orbit contribution vanishes uniquely for the normal incidence/exit geometry used here. Pixel by pixel image subtraction for incident electron beams of opposite polarization yields precisely the spatially resolved Bragg reflection asymmetry observed in spin polarized low energy electron diffraction. The shallow electron penetration depth arising from the strong coulombic interaction is advantageous for separating surface behavior from the normally overwhelning bulk. Therefore, the use of transversally polarizedI electron beams allows the determination of in-plane surface magnetization directions. Fnrthermore, the parallel illumination and detection of SPLEEM makes it possible to image quickly with a. resolution better than 500 A in the present configuration. A useful and direct. comparison between surface magnetic, structural, and topological features is made possible by the augmentation of the unique imaging capabilities of conventional LEEM with the magnetic sensitivity of SPLEEM. In this manner, the magnetic domain structure of a Co (0001) surface and in-situ grown Co filmns on Mo(110) have been determined.
62 citations
TL;DR: In this paper, the decomposition and etch rates of polymethylmethacrylate (PMMA), polyimid (PI) and polyethylene terephthalate (PET) were determined with an incoherent excimer UV source.
Abstract: Photochemical dry etching and surface modification of different polymers, e.g., poly-methylmethacrylate (PMMA), polyimid (PI) and poly-ethylene terephthalate (PET) was achieved with an incoherent excimer UV source. Decomposition and etch rates of PMMA were determined as a function of UV intensity and exposure time at different wavelengths λ = 172 nm (Xe2*), λ = 222 nm (KrCI*) and λ = 308 nm (XeCI*). The morphology of the exposed area of PMMA was investigated with SEM. The etching of the polymers can be explained as a result of extensive photooxidation. The results are compared with data obtained from mercury lamp and excimer laser experiments.
58 citations
TL;DR: In this article, the synthesis, crystal chemistry, structure and stability of ettringite, with specific reference to use of this material in hazardous waste immobilization, are presented.
Abstract: Recent results on synthesis, crystal chemistry, structure and stability of ettringite* , with specific reference to use of this material in hazardous waste immobilization, are presented. Ettringites with selenate, chromate, borate, sulfite and carbonate replacing sulfate have been synthesized and characterized by XRD for phase purity and structure. In X-ray powder diffractograms, selenate and chromate etringites have the normal trigonal P31c ettringite structure, whereas the borate, sulfite and carbonate ettringites can be indexed on a hexagonal unit cell with a halved c parameter. In excess of 95% of the Se and B in solutions spiked with various concentrations of these elements has been removed by precipitation of ettringite. Ettringites appear to be unstable in solutions outside of the approximate pH range 11.0-12.5, and there is a potential for some oxyanion exchange between ettringites and oxyanion-containing solutions. Buffering of pH, and dissolved Ca, Al and sulfate activities by other components of ettringite-based hazardous waste forms, combined with low-reactivity C-S-H matrices, could largely mitigate these concerns.
57 citations
TL;DR: In this paper, the authors used Rutherford backscattering spectrometry (RBS), Auger electron spectrometers (AES), and transmission electron microscopy (TEM) to study Pt/Ti/SiO2/Si substrates with various thicknesses of the Ti and Pt layers.
Abstract: The deposition of the bottom electrode plays a key role in the fabrication of ferroelectric capacitors. Processing at elevated temperatures of up to 800°C can give rise to diffusion processes and thereof formation of harmful dielectric layers. p]In this paper we used Rutherford backscattering spectrometry (RBS), Auger electron spectrometry (AES) and transmission electron microscopy (TEM) to study Pt/Ti/SiO2/Si substrates with various thicknesses of the Ti and Pt layers. During heating up to about 450°C in vacuum the initial layer sequence remains unchanged. However, drastic changes occur when the electrodes are exposed to Ar/O2 atmosphere during heat treatment. Oxidation induced diffusion of Ti into Pt and oxidation of Ti were observed. A Pt electrode with a 100 nm thick Ti adhesion layer proved to be suitable for the “in-situ” deposition of PZT films.
56 citations
TL;DR: The FIBXTEM as discussed by the authors combines cross-sectional TEM sample preparation and the precise material sputtering of focussed ion beam milling (FIB) to thin samples to electron transparency.
Abstract: In the semiconductor industry, shrinking geometries and increasing process complexity have greatly increased the demand for TEM analysis of specific submicron regions. Until recently, samples of this nature have been difficult if not impossible to prepare. We have combined cross-sectional TEM sample preparation (XTEM) and the precise material sputtering of focussed ion beam milling (FIB) to thin samples to electron transparency. We call this sample preparation technique FIBXTEM. p]Three advantages of this technique are: 1) The area of interest can be analyzed in the scanning electron microscope before final thinning; 2) Any specific defect area becomes a candidate for TEM analysis, including failed sub-micron structures; and 3) Samples are generally artifact-free and of uniform thickness. p]Key elements of the FIBXTEM technique include precision planar polishing, unique holders for mounting and transferring samples between systems, and the FIB-induced deposition of a sacrificial protective layer over the area of interest during ion thinning. p]This technique extends the use of TEM analysis into new areas of semiconductor process development and failure analysis. Recent applications for materials problem solving and failure analysis are discussed.
TL;DR: In this article, a wafer curvature measurement technique has been developed which allows the contributions from density changes and plastic deformation to be distinguished and the stress dependence of the plastic deformations to be determined.
Abstract: Plastic deformation of several covalently-bound materials has been studied during ion irradiation. In all of these materials, namely crystalline and amorphous silicon, crystalline and amorphous Si0.9Ge0.1, and amorphous SiO2, the damage created by the ion beam causes density changes in the irradiated region which eventually saturate with ion dose. In the crystalline materials, the density changes were accompanied by a transformation to the amorphous phase. Superimposed on the density changes is plastic deformation which occurs during irradiation of both crystalline and amorphous materials to relieve stresses in the irradiated region. A wafer curvature measurement technique has been developed which allows the contributions from density changes and plastic deformation to be distinguished and the stress dependence of the plastic deformation to be determined. In all of the amorphous materials, the plastic deformation is Newtonian viscous shear flow, which is characteristic of solids where deformation is governed by the diffusive motion of point defects. The radiation-enhanced shear viscosity per ion was flux-independent, revealing that flow occurs rapidly, probably within the localized damaged regions created by each ion. This viscosity does not depend strongly on the material. In fact, similar viscosities were obtained during measurements of radiation-enhanced plastic deformation of crystalline covalent samples and polycrystalline aluminum films.
TL;DR: Alkali-activated cementitious materials have considerable potential, with properties ranging from very high early strength to very high long term strength and low porosity as discussed by the authors, and the role of alkalis in activating industrial byproducts such as ground granulated (glassy) blast-furnace slag, ultra-fine silica fume or other byproduct silicas, glassy fly ashes, and other amorphous materials is discussed.
Abstract: Alkali-activated cementitious materials have considerable potential, with properties ranging from very high early strength to very high long term strength and low porosity. The role of alkalis in activating industrial by-products such as ground granulated (glassy) blast-furnace slag, ultra-fine silica fume or other by-product silicas, glassy fly ashes, and other amorphous materials is discussed. These by-product materials are activated by alkali additions which break the strong silicon oxygen bonds in the silicate or alumino-silicate network, accompanied by the formation of new reaction products. The role of the different components in multi-component activated cements on the reactivity and the nature of the resultant products is discussed. Characterization of starting materials and reaction products by XRD, SEM (including environmental SEM), chemical, and particle characterization techniques has been made. Additional results include investigation of pore solution chemistry as a function of time. Some inferences for long term durability are discussed.
TL;DR: In this article, the effects of solution preparation conditions on film microstructure and ferroelectric properties were studied, and NMR spectroscopy was used to characterize chemical differences between the two precursor solutions.
Abstract: Ferroelectric PZT 53:47 thin films were prepared by two different solution deposition methodologies. Both routes utilized carboxylate and alkoxide precursors and acetic acid, which served as both a solvent and a chemical modifier. We have studied the effects of solution preparation conditions on film microstructure and ferroelectric properties, and have used NMR spectroscopy to characterize chemical differences between the two precursor solutions. Films prepared by a sequential precursor addition (SPA) process were characterized by slightly lossy hysteresis loops, with a Pr of 18.7 μC/cm2 and an Ec of 55.2 kV/cm. Films prepared by an inverted mixing order (IMO) process were characterized by well saturated hysteresis loops, a Pr of 26.2 μC/cm2 and an Ec of 43.3 kV/cm. While NMR investigations indicated that the chemical environments of both the proton and carbon species were similar for the two processes, differences in the amounts of by-products (esters, and therefore, water) formed were noted. These differences apparently impacted ceramic microstructure. Although both films were characterized by a columnar growth morphology, the SPA derived film displayed a residual pyrochlore layer at the film surface, which did not transform into the stable perovskite phase. The presence of this layer resulted in poor dielectric properties and lossy ferroelectric behavior.
TL;DR: In this paper, two separate phenomena occur during the low-load indentation of silicon which make its behavior distinctly different from that of most materials, and they are documented and discussed.
Abstract: Two separate phenomena occur during the low-load indentation of silicon which make its behavior distinctly different from that of most materials. First, silicon is one of only a very few materials whose hardness exceeds the pressure needed to transform it to a denser crystalline (or amorphous) form, and because of this, a reversible, pressure-induced phase transformation occurs during indentation. The transformation enhances the electrical conductivity of the material and creates a region around the indenter which flows like a soft metal. Second, silicon cracks when indented by a Berkovich or Vickers indenter at loads of less than 100 mN, i.e., loads typically used in nanoindentation experiments. These two phenomena, which account for a number of unusual features in the indentation load-displacement behavior, are documented and discussed.
TL;DR: In this article, a thin film of aluminum, detected by x-ray photoelectron spectroscopy, is left at the surface of aluminum nitride (AIN) substrates exposed to a high intensity excimer laser beam of UV radiation.
Abstract: A thin film of aluminum, detected by x-ray photoelectron spectroscopy, is left at the surface of aluminum nitride (AIN) substrates exposed to a high intensity excimer laser beam of UV radiation. Due to the presence of this film, there is a decrease in the surface resistivity of the substrate with increasing number of laser pulses. In addition, line profilometry shows a decrease of the surface roughness with the number of pulses. p]The thermal decomposition of AIN is assumed to take place in two stages. In the first, liquid aluminum is produced together with the evolution of gaseous nitrogen, and in the second, aluminum evaporates. Using a computer model to simulate the laser heating cycle, it is shown that the thickness of the aluminum film saturates at a given laser energy density. The saturation thickness is a strong function of the substrate absorption and reflectivity and, therefore, of the laser light frequency. The influence of the substrate roughness on the electrical resistivity of the aluminum film is discussed. p]The application of this process to direct laser writing in high density hybrid circuits is illustrated. During hole drilling by excimer laser, a thin aluminum film is continuously produced at the hole walls. This process can also be employed for substrate planarization.
TL;DR: Over the last five years, the lab has experimented with the concept of using man-made, biodegradable polymer systems as scaffolding for cell implantation devices, designed to maximize diffusion parameters allowing nutrient exchange, gas exchange, and waste exchange.
Abstract: The fields of materials science, cell and molecular biology, and surgical reconstruction are merging to create new devices for surgical transplantation and reconstructive applications. The field of artificial devices for implantation has matured over the last 40 years. Likewise, the field of transplantation and tissue reconstruction has undergone enormous change and improvements over the last 30 years. It has been proposed that these fields merge to create new tissue substitutes for functional replacement, therapy or reconstructive applications. Over the last five years, our lab has experimented with the concept of using man-made, biodegradable polymer systems as scaffolding for cell implantation devices. They have been designed to maximize diffusion parameters allowing nutrient exchange, gas exchange, and waste exchange. Vascular ingrowth occurs in the implant with subsequent resorption of the original polymer. This leaves a permanently engrafted new tissue which is a chimera of donor cells for functional replacement and recipient mesenchymal elements including blood vessels and supporting tissue. We have experimented in several model systems including hepatocyte implants, chondrocyte implants for cartilage reconstruction, urethelial implants for urinary reconstruction, and more recently small bowel and bone. Across this broad front of tissue types, much new knowledge has been gained and there continues to be hope that this will achieve clinical application.
TL;DR: A structural and chemical analogue of UO2 in spent fuel, uraninite, uO2+x, is unstable in the presence of dissolved silica (>10-3.6 mg/L) under reducing conditions and transforms into coffinite, USiO4 nH20 as mentioned in this paper.
Abstract: A structural and chemical analogue of UO2 in spent fuel, uraninite, UO2+x, is unstable in the presence of dissolved silica (>10-3.6 mg/L) under reducing conditions and transforms into coffinite, USiO4 nH20. Coffinite may incorporate numerous elements into its structure including actinides and fission products. Conditions favorable for coffinitization of UO2 in spent fuel may occur in repositories in granitic, basaltic, and tuffaceous rocks.
TL;DR: In this article, the authors investigated the mechanisms for the retardation of U(VI) migration by measuring the sorption of UO22+ by silica as a function of pH and supporting electrolyte concentration.
Abstract: This report covers the investigation of mechanisms for the retardation of U(VI) migration. It describes (1) the measurement of the solubility of UO22+as afunction of pH and equilibration time, (2) the calculation of solubility using solution thermochemical data, (3) the measurement of the sorption of UO22+by silica as a function of pH and supporting electrolyte concentration, and (4) attempts to simulate the uranium sorption isotherms in terms of the U solution thermochemical data, measured properties of the silica and proposed sorption reaction mechanisms.
TL;DR: In this article, a TEM study reveals that the spinel consists of aggregates of closely spaced spherules of 10-30 nm in diameter, which is in complete agreement with rock magnetic determinations.
Abstract: Between 20 and 50 per cent of the total iron in low-Ca fly ashes resides in low-substitution magnetite spinel with Curie temperatures in between 565 and 590°C. A TEM study reveals that the spinel consists of aggregates of closely spaced spherules of 10 – 30 nm in diameter, which is in complete agreement with rock magnetic determinations. The low-field magnetic susceptibility correlates extremely well with total iron and fairly well with specific heavy metals. Measurement of the magnetic susceptibility may serve as a rapid tool for the evaluation of the speciation of these heavy metals in fly ash.
TL;DR: In this paper, the biological response of intramedullary bone to poly-L-lactic acid (poly-Lactic acid) was examined, and the degradation rate accelerated between 6 and 12 weeks.
Abstract: Bioabsorbable synthetic polymers have been studied for their possible application in absorbable internal fracture fixation devices. The current study examines the biological response of intramedullary bone to PLLA (poly-L-lactic acid). PLLA degrades at a rate sufficiently slow to be useful for fracture fixation and undergoes hydrolytic deesterification to form metabolites normally found in the body. Nevertheless, the lactic-acid-rich degradation products have the potential to significantly lower the local pH in a closed space surrounded by bone. It is hypothesized that this acidity may tend to cause abnormal bone resorption and/or demineralization. PLLA (Medisorb, Du Pont), with a molecular weight of 1.0 × 105 daltons, was chosen for this study. PLLA coupons were made by heating the polymer pellets to 180°C and pressing them. Ultrahigh-molecular-weight polyethylene chambers with ingrowth channels running widthwise, were lined with coupons of the test material and with stainless steel (SS) and implanted in the femurs of eight skeletally mature hound dogs. Two dogs each were killed 3, 6, 12, and 24 weeks postoperatively. From an associated in vitro study, the molecular weight of the PLLA was found to decrease with immersion time to 0.92, 0.85, and 0.69 of original molecular weights by 6, 12, and 24 weeks, respectively. Ingrown bone was seen between both PLLA and SS coupons, and no resorption areas existed between bone and the chambers at 3 and 6 weeks. By 12 weeks, no bone was found ingrown in the PLLA chambers. Indeed, femoral bone around the chamber had resorbed. By contrast, the amount of bone grown into the SS chambers increased and the femoral bone was in contact with the chamber. The affinity of intramedullary bone to PLLA was greater than that of SS at the early time periods. No inflammatory cells were seen. This may explain earlier claims of osteogenic potential. Once the degradation of PLLA had started (as reflected by the molecular weight data), the affinity of PLLA to bone reduced drastically. Not only the bone ingrowth between the PLLA coupons but also the bone surrounding the chamber was resorbed. This resorption of bone is probably caused by the influence of the degradation products of PLLA that increase osteoclastic activity. The degradation rate accelerated between 6 and 12 weeks. Though the resorption of bone around the chamber was not massive in this study, it might jeopardize bone union if it were used for internal fixation of a bone fracture. These findings are significant in that experimental α-polyester-based fracture fixation systems are currently in development and clinical trials in the United States, Europe, and Japan.
TL;DR: In this article, a large number of aqueous leach tests on glass specimens doped with 237Np, 238Pu, 239pu, 241Am or 244Cm were conducted.
Abstract: Results are presented for a large number of aqueous leach tests on glass specimens doped with 237Np, 238Pu, 239pu, 241Am or 244Cm. The effects of the leaching mode (static or dynamic), temperature and environmental materials are discussed. The amount of actinides contained in the alteration film is characterized by the retention factor RF = NL(B)/NL(act). The neptunium retention factor ranged from 1 to 10 depending on the test conditions; for plutonium and americium, the retention factor varied from 6 to 1300. p]In general, the retention factor increased with the temperature. The very important influence of the environmental materials shows that there is competition for actinide retention between the alteration film and the surrounding materials, but in every case the amount of ca activity released into solution is very low.
TL;DR: In this paper, a novel wafer temperature and emissivity measurement technique for rapid thermal processing (RTP) is presented, which takes advantage of heating lamp AC ripple as the signature of the reflected component of the radiation from the wafer surface.
Abstract: A novel wafer temperature and emissivity measurement technique for rapid thermal processing (RTP) is presented. The ″Ripple Technique″ takes advantage of heating lamp AC ripple as the signature of the reflected component of the radiation from the wafer surface. This application of Optical Fiber Thermometry (OFT) allows high speed measurement of wafer surface temperatures and emissivities. This ″Ripple Technique″ is discussed in theoretical and practical terms with wafer data presented. Results of both temperature and emissivity measurements are presented for RTP conditions with bare silicon wafers and filmed wafers.
TL;DR: Low-temperature-grown (LTG) GaAs have been used in a variety of device applications to achieve record performance as discussed by the authors, such as microwave switches and photodetectors.
Abstract: Low-temperature-grown (LTG) GaAs is a unique material that has been used in a variety of device applications to achieve record performance. LTG GaAs used as a buffer layer eliminates sidegating and backgating and in GaAs integrated circuits. Record output power density (1.57 W/mm) and superior microwave-switch performance were demonstrated when LTG GaAs was used at a gate insulator in a metal-insulator-semiconductor field-effect transistor. High-speed (0.5 ps) and high-voltage (1 kV) LTG GaAs photoconductive switches have also been demonstrated. Using the same material, researchers have demonstrated highresponsivity (0.1 A/W), wide-bandwidth (∼ 375 GHz) LTG GaAs photodetectors. Devices incorporating LTG GaAs are currently being optimized for systems applications. LTG GaAs technology can enhance system performance and enable new systems for military and commercial applications in the areas of radar, communications, instrumentation, and highspeed computing.
TL;DR: In this article, the thermal polymerization reaction of divinyl siloxane bis-benzocyclobutene (DVS bis-BCB) was monitored in-situ with FT-IR spectroscopy in order to follow specific chemical changes and determine the reaction order and rate constants at temperatures from 150° to 210°C.
Abstract: The thermal polymerization reaction of divinyl siloxane bis-benzocyclobutene (DVS bis-BCB) was monitored in-situ with FT-IR spectroscopy in order to follow specific chemical changes and determine the reaction order and rate constants at temperatures from 150° to 210°C. FT-IR spectra were obtained at regular intervals throughout the reaction with a Nicolet 170SX spectrophotometer. Monomeric DVS bis-BCB contains mixed stereo and positional isomers of 1,3- bis(2-bicyclo[4.2.0]octa-1, 3, 5-trien-3-ylethenyl)-1, 1, 3, 3-tetramethyl disiloxane (CAS 117732-87-3). It polymerizes via Diels-Alder cycloaddition reactions between vinyl groups and an intermediate o -quinodimethane formed by first-order, thermally initiated ring openings of the benzocyclobutene rings. Gaseous byproducts are not produced; therefore, the cure is easier to manage than are cures for polyimides which evolve water in polycondensation reactions. The DVS bis-BCB has four reactive elements per monomer unit and, thus, polymerizes into a very highly cross linked and solvent resistant network. With the FT-IR methodology, the reaction was easily monitored through the points of gel formation and vitrification. With the exception of DSC (i.e., calorimetry) which does not sense specific chemistry, other methods were not successful in following the reaction after a gel was formed. We have found that the polymerization was first-order until vitrification occurs; the gelation alone had no apparent effect on the reaction rate. DVS bis-BCB is under development at Dow as high performance dielectric material for multilayer interconnect coating applications for the microelectronics industry. Methodology reported here is employed in developing effective cure management strategies.
TL;DR: In this article, n-dodecylammonium-montmorillonite-p-aminoazobenzene intercalation compounds were prepared by novel solid-solid reactions at room temperature.
Abstract: Homoionic-montmorillonite-p-aminoazobenzene intercalation compounds were prepared by novel solid-solid reactions at room temperature. When Na-, Ca- and Ni-montmorillonites were used as host materials, the absorption maxima of p-aminoazobenzene changed considerably upon intercalation, causing the difficulty of studying photochemical behavior of the intercalated dyes. The n-dodecylammonium-montmorillonite-p-aminoazobenzene intercalation compound gave an absorption spectrum similar to that observed for p-aminoazobenzene dissolved in benzene. The intercalated paminoazobenzene in the interlayer space of n-dodecylammoniummontmorillonite exhibited reversible cis-trans photoisomerization by irradiation of UV light.
TL;DR: The optical bowing behavior of polycrystalline thin film Culn1yGaySe2 alloys is dependent upon the Cu stoichiometry as mentioned in this paper, and the variation in optical band gap, eg, for alloys in which Cu is near stoichiometric (25 at.%) is parabolic and follows the relationship: Eg(y) = 1.011 + 0.421 y + 0244 y2 (eV), where y is the alloy parameter, [at.% Ga] / [at% Ga + at.%In].
Abstract: The optical bowing behavior of polycrystalline thin film Culn1-yGaySe2 alloys is dependent upon the Cu stoichiometry. The variation in optical band gap, Eg, for alloys in which Cu is near stoichiometric (25 at.%) is parabolic and follows the relationship: Eg(y) = 1.011 + 0.421 y + 0.244 y2 (eV), where y is the alloy parameter, [at.% Ga] / [at.%Ga + at.%In]. Contrary to this, films with Cu-poor stoichiometries (∼19 at.% Cu) exhibit little alloy bowing: Eg(y) = 1.01 + 0.733 y - 0.046 y2 (eV). The increase in Eg with Cu deficiency appears to be the result of both a structural effect associated with tetragonal lattice shrinkage, ΔV = Vstoichiometric - VCu-poor (resulting from the presence of Cu vacancies) and a chemical effect, possibly associated with a counterbalance between p-d repulsion and anion displacement effects.
TL;DR: In this paper, the authors studied the reconstructive transformation of strontium-exchanged zeolite A to Sr-anorthite, and showed that the primary coordination remains intact during conversion.
Abstract: The decomposition of zeolites is a novel route for the synthesis of aluminosilicate-based ceramic materials. Zeolites offer a number of advantages as ceramic precursors which allow the formation of dense ceramics at lower temperatures than by conventional methods. Using various cation-exchanged zeolites, ceramics and ceramic composites containing anorthite (CaAl2Si2O8). mullite (Al6Si2O11) cordierite (Mg2Al4Si5O18). celsian (BaAl2Si2O8), and s-spodumene (LiAlSi2O6) have been formed. Using Extended X-ray Absorption Fine Structure (EXAFS), we have studied the reconstructive transformation of strontium-exchanged zeolite A to Sr-anorthite and have shown that the primary coordination of strontium remains intact during conversion.
TL;DR: In this article, the authors investigated the shape memory of the pseudo-elastic hysteresis loop that accompanies the strain-induced austenitic-martensitic phase transition.
Abstract: An interesting problem within the research on shape memory is provided by the investigation of the pseudo-elastic hysteresis loop that accompanies the strain-induced austenitic-martensitic phase transition. Miller [1] suggested by thermodynamic arguments that the size of that loop should be related to the coherency energy at interfaces between martensite and austenite. That proposition received a boost when Muller & Xu [2] showed that the same thermodynamic arguments predicted the onset of internal yield and recovery inside the hysteresis loop on a well-definqd line of unstable phase equilibrium. Recently Fu, Muller and Xu [3] have reported results with improved specimens and methods of observation that may help to understand the nature of metastability inside the hysteresis loop.
TL;DR: In this paper, the microstructural changes of a simulated waste glass irradiated with doping of 238pu and 244Cm were observed by use of a preshadowed carbon replica technique in combination with scanning electron microscopy (SEM).
Abstract: The microstructural changes of a simulated waste glass irradiated with doping of 238pu and 244Cm were observed by use of a preshadowed carbon replica technique in combination with scanning electron microscopy (SEM). The irradiated glass was annealed and its microstructural changes after annealing were observed by use of the sametechnique. p]In the glass irradiated at a dose of 2.75x1025 α-decays/m3, bubbles with a radius from 0.15μrm to 0.35μm were observed. The average bubble radius and the bubble density were 0.23μm and 1x1017 bubbles/m3, respectively. Using these observed values, the volume change of the glass resulting from the bubble formation was estimated to be + 0.51 %. This valuoof + 0.51 % was close to the volume change of the irradiated glass measured in our previous study, which suggests a large portion of the volume change by α-irradiation results from bubble formation. In the glass annealed after irradiation the average bubbleradius was observed to decrease with annealing time. The bubble radius as a function of annealing time was analyzed on the basis of the helium diffusion model with two chemicalprocesses, i.e. trapping at bubbles and re-solution from bubbles into glass matrix. The values of the diffusion coefficient of helium, the trapping parameter and the re-solution parameter, which had been obtained experimentally in our previous study on the helium release, were applied to the calculation. The calculated curve was in good agreement with the observed data.