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Micropipes in a 6H–SiC semiconductor wafer were studied by scanning electron and atomic force microscopy. The screw dislocations intersecting the wafer's surface were located by etch pitting, and their Burgers vectors determined by x-ray topography. The etch pits were eroded into smooth craters by ion beam etching to expose levels of dislocation line from inside the sample's bulk. There a micropipe's diameter is distant from surface relaxation effects. Hollow cores (micropipes) were observed at the base of the craters whose screw dislocations had Burgers vectors of magnitude three multiples of the c-lattice parameter and higher. Screw dislocations with 1c and 2c Burgers vectors had no associated micropipes.
The role of elastic mismatch in determining critical conditions for indentation fracture in brittle coatings on substrates of unlike modulus was investigated. A model transparent trilayer system, consisting of a glass coating layer bonded to a thick substrate of different glass or polymer by a thin layer of epoxy adhesive, facilitated in situ observations of crack initiation and propagation. A tungsten carbide sphere was used to load the layer system. Abrasion flaws were introduced into the top and bottom glass coating surfaces to control the flaw populations and to predetermine the origins of fracture: cone cracks occurred at abraded top surfaces, radial cracks at abraded bottom surfaces. Analytical relations for the critical loads are presented for each crack system in terms of elastic modulus mismatch, indenter and coating dimensions, and material fracture parameters. Implications concerning materials selection for resistance to crack initiation are considered.
Two processing routes were explored to produce crack-free amorphous Si–N–C ceramics by pyrolysis of polyureasilazane ceramic precursor. Using a warm-pressing/ pyrolysis route, a ceramic body with certain amount of open porosity was produced; densification behavior during pyrolysis was examined. A prepyrolysis/binding/pyrolysis route was also developed. Ceramics formed using this route were characterized by higher density, lower volume shrinkage during consolidation, and larger viable material size. Open porosity was essentially absent in consolidated amorphous materials produced by this second route. Recrystallization of the consolidated amorphous ceramics resulted in a Si3N4/SiC nanocomposite with both silicon nitride and silicon carbide grains in the nanometric size range.
Due to its endurance to ferroelectric fatigue, SrBi2Ta2O9 (SBT) has been extensively investigated. We report here the synthesis of Sr1−xBaxBi2Ta2O9 (x = 0.0, 0.1, 0.5, 1.0) using a solution-based route. The precursors used in this work were the salts of strontium, barium, bismuth, and tantalum ethoxide. X-ray diffraction and Raman spectroscopic studies indicated the formation of complete solid solution system for Sr1−xBaxBi2Ta2O9. This material system may provide interesting properties relevant to microwave tuning and ferroelectric memory applications, which are under investigation.
A wideband and temperature-stabilized optical isolator for 1.55-μm wavelength was developed using a new Bi-substituted holmium–ytterbium ion garnet (HoYbBiIG) single crystal as a Faraday rotator. The optical isolator features 0.34-μm bandwidth, less 0.6 dB insertion loss and over 37 dB backward loss at a wavelength of (1.55 ± 0.17) μm throughout the temperature range from −10 to 60 °C. The Faraday rotation and optical absorption loss of HoYbBiIG were investigated in the near-infrared wavelength region (λ = 0.9 to 1.7 μm). The specific Faraday rotation of Ho0.85Yb1.02Bi1.13Fe5O12 is about −767°/cm at λ = 1.55 μm. The Faraday rotation wavelength and temperature characteristics of HoYbBiIG crystals are also discussed. These results indicate that the Bi-substituted holmium–ytterbium iron garnet single crystals realize a high Faraday rotation stability against temperature and wavelength in the near-infrared region.
An in situ temperature measurement was performed during spraying of A2-tool steel, and the results were used to verify an axisymmetric two-dimensional computer simulation program, which was developed for the prediction of shape and temperature variation in a spray-forming process. A thin thermocouple was placed inside of the chamber in advance and brought to the surface of the deposit during spraying. The temperature was then recorded. The surface temperature of the deposit was also measured by an infrared video camera. The emissivity of the surface of A2-tool steel during spraying was determined to be 0.23 through comparison of the temperatures measured by the thermocouple with the ones measured by the infrared video camera. The heat transfer coefficient at the top surface was estimated by comparing the calculated results with the experimental data. The cooling curve predicted on the basis of the numerical simulation showed good agreement with the experimental data.
We examined the interfacial morphology and shear deformation of flip chip solder joints on an organic substrate (chip-on-board). The large differences in the coefficients of thermal expansion between the board and the chip resulted in bending of the 1-cm2 chip with a curvature of 57 ± 12 cm. The corner bump pads on the chip registered a relative misalignment of 10 μm with respect to those on the board, resulting in shear deformation of the solder joints. The mechanical properties of these solder joints were tested on samples made by sandwiching two Si chips with electroless Ni(P) as the under-bump metallization and 25 solder interconnects. Joints were sheared to failure. Fracture was found to occur along the solder/Ni3Sn4 interface. In addition, cracking and peeling damages of the SiO2 dielectric layer were observed in the layer around the solder balls, indicating that damage to the dielectric layer may have occurred prior to the fracture of the solder joints due to a large normal stress. The failure behavior of the solder joints is characterized by an approximate stress analysis.
Single-point diamond turning tests were carried out in two different -oriented semiconductors, InSb and Si single crystals. The analysis of the conditions in which the machining is in ductile or brittle mode indicates that the plasticity presented by semiconductor crystals during micromachining can be correlated to the value of the transition pressure. It is shown that the ductility presented by different semiconductor single crystals is inversely related to the transition pressure value of the material.
Using an interfacial force microscope, the measured elastic response of 100-nm-thick Au films was found to be strongly correlated with the films' stress state and thermal history. Large, reversible variations (2×) of indentation modulus were recorded as a function of applied stress. Low-temperature annealing caused permanent changes in the films' measured elastic properties. The measured elastic response was also found to vary in close proximity to grain boundaries in thin films and near surface steps on single-crystal surfaces. These results demonstrate a complex interdependence of stress state, defect structure, and elastic properties in thin metallic films.
A novel two-step metalorganic chemical vapor deposition process was used in this study to prepare Sr1−xBaxNb2O6 (SBN) thin films. Two thin layers of single-phase SrNb2O6 and BaNb2O6 were deposited alternately on a silicon substrate, and the solid solution of SBN was obtained by high-temperature annealing. The stoichiometry control of the SrNb2O6 and the BaNb2O6 thin films was achieved through deposition process control, according to the evaporation characteristics of double metal alkoxide. The evaporation behavior of double metal alkoxide precursors SrNb2(1-OC4H9)12 and BaNb2(1-OC4H9)12 was studied, and the results were compared with the evaporation of single alkoxide Nb(1-OC4H9)5.
Hillock formation, a stress-induced diffusional relaxation process, was studied in sputter-deposited Al films. The grain sizes in these films were small compared to those in other sputter-deposited Al films, and impurities (O, Ti, W) were incorporated during the preparation of the films. Stress and hardness measurements both indicate that the Al films were strengthened by the small grain size and incorporated impurities. We observed a new type of hillock in these Al thin films after annealing for 2 h at 450 °C in a forming gas ambient. The hillocks were composed of large Al grains created between the substrate and the original Al film with its columnar grain structure, apparently by diffusion from the surrounding area. By modifying the boundary conditions of Chaudhari's hillock formation model [P. Chaudhari, J. Appl. Phy. 45, 4339 (1974)], we have created a new model that can describe the experimentally observed hillocks. Our model seems to explain the experimentally observed abnormal hillock formation and may be applied to other types of hillock formation using different creep laws.
Commercially useful multiphase alloy Inconel 783 is made up of Ni(28%), Fe(25%), and Co(35%) with smaller amounts of Cr, Al, Nb, Mo, Ti, C, and Si. In this work, relative temperature variations of the lattice constants a(T)/a0 of the gamma (face-centered-cubic) and beta (body-centered-cubic) phases of this alloy are determined from 97 to 773 K using analyses of their x-ray diffraction (XRD) patterns. Plots of a(T)/a0 for the two phases vary from 1.0 at 97 K to 1.012 at 773 K and show that (i) for T = 500 K, thermal expansion of the β phase is larger than that of the = phase; and (ii) an abrupt jump is observed near 300 K. The appearance of new lines above 700 K in XRD representing Co2CrO4 and CoCr2O4 is interpreted in terms of the oxidation of the γ phase, whereas the β phase is oxidation resistant. The anomalous change in a(T)/a0, observed near 300 K and accompanied by a similar change in the temperature variation of the initial magnetic susceptibility, is not well understood. A brief discussion on the implications of these results is presented.
Nanocrystalline transition alumina tape casts were used as interlayers to join conventional alumina ceramic pellets. The joining experiments were performed by hot pressing at 1200–1300 °C under uniaxial pressures of 55 and 80 MPa for 1- and 5-h durations, with and without a nanocrystalline interlayer. Successful joints were enabled only above 1250 °C in the presence of the interlayer. Generally, the joint 4-point bending strength increased with the increase in joining temperature, pressure, and duration. The average bending strength of the interface joined at 1250 °C was 245 ± 65 MPa compared to the pellet strength of 268 MPa. Postjoining heat treatments at 1400 °C for 3 h caused reduction in the joint strength. The interlayer at the joint exhibited homogeneous and crack-free microstructure. The changes in the joint strength were discussed with respect to the densification and grain growth behavior of the nanocrystalline interlayer.
A systematic study was carried out on the equilibrium phases after slow solidification of the Zr41Ti14Cu12.5Ni10Be22.5 alloy. The crystalline microstructure of the slowly cooled melt of the alloy shows “polygons” and “plates” embedded in a fine-grained two-component matrix. To analyze the crystal structure of the different components, microdiffraction technique combining convergent beam electron diffraction and conventional selected-area electron diffraction were used. The stoichiometry of these phases was confirmed by field ion microscopy with atom probe and energy-dispersive x-ray analysis in a transmission electron microscope. The polygons were determined to be cubic (a = 1.185 nm) with space group Fm3m (cF116). The plates were found to be tetragonal (a = 0.37 nm, c = 1.137 nm) with space group I4/mmm (tI6). Its composition is (Cu + Ni)(Zr + Ti)2. One phase of the fine-grained two-component matrix was rich in Ti and poor in Be; the other one was rich in Be and poor in Ti. The Ti-rich phase was determined to be hexagonal (a = 0.536 nm, c = 0.888 nm) with space group P63/mmc.
Investigations of a substitutional mechanism of Pb incorporation into the crystal structure of Ba6−3xNd8+2xTi18O54 performed by x-ray diffraction analysis, scanning electron microscopy, and energy dispersive and wavelength dispersive x-ray spectroscopy revealed that Pb2+ substitutes for Ba2+ according to the formula (Ba1−zPbz)6−3xNd8+2xTi18O54. The solid solubility limit for 0.5 = x = 0.6 compositions was determined to be at 0.35 ≤ z < 0.4 (nominal composition) which, according to measurements of PbO loss occurring during the heat treatment, gives 0.30 ≤ z < 0.35 (analyses of matrix phase). Increasing the Pb2+ concentration in (Ba1−zPbz)4.5Nd9Ti18O54, results in tf decreasing from an initial positive value (80 ppm/K) to a negative value at the solid solubility limit (−25 ppm/K at z = 0.35). In the same concentration range the Q-value decreases from an initial 2000 to 1250 (z. = 0.35), measured at 4 GHz, while permittivity remains almost constant (k = 87 ± 1.5). After exceeding the solid solubility limit of Pb2+ in (Ba1−zPbz)4.5Nd9Ti18O54 the appearance of secondary phases (Nd4Ti9O24 and Pb-rich phase at grain boundaries) changes the trends of the microwave dielectric properties; permittivity decreases, Q-value remains almost constant, and Tf increases.
The seeding effects of (001) Nd123/MgO thin films and MgO single crystals were studied in isothermal solidification of YBa2Cu3Oy composite with the additions of 40 mol% Y2BaCuO5, 10 wt% Ag, and 0.5 wt% Pt. Seeding with the Nd123/MgO thin film resulted in single-domain growth of Y123 crystal with a stable growth along the “100”?direction, while seeding with MgO single crystal produced multidomain growth in which the dominant growth facet is rotated 45° about (100) plane of MgO. Multidomain growth in MgO seeded sample was suppressed by decreasing undercooling degree. The effects of undercooling degree and seed size on multidomain growth are discussed in view of classical nucleation and growth theory.
Carbon tubes were successfully produced using microwave plasma-enhanced chemical vapor deposition on silicon, quartz, and ceramic substrates. The carbon tubes, about 80–100 nm in diameter and a few tens of microns in length, were formed under methane and hydrogen plasma at 720 °C with the aid of iron oxide particles. In this approach, an average tube density of about 109 cm−2 was obtained. The crooked and nonuniform diameters of some tubes suggested that they were composed of incompletely crystallized graphitic shells due to existing defects. The characteristic of the tubes grown upward on the silicon substrate accounted for a remarkably large electron field emission current of 0.1 mA/cm2 from the surface of the tube sample at a low turn-on field of 3 V/μm.
Using a unique combination of in situ electrical and acoustical measurements and ex situ transmission electron microscopy, the phase transformations of silicon during point loading were shown to exhibit a strong dependence on the size of the deformed volume. For nanometer-size volumes of silicon, the final phase was the body centered cubic structure BC8, but for larger volumes it was amorphous. The size dependence was explained by considering how shear stress fields vary with contact size and how interfacial effects between the silicon substrate and the BC8 phase determine its stability. For both small and large contacts the presence of a nonmetallic phase (assumed to be the Rhombohedral structure R8) was observed.
The sintering and properties of cordierite–borosilicate glass composites were investigated. For the composites with ≥35% low-viscosity glass, the sintered densities decreased with the increasing sintering temperature above 850 °C. No crystallization of the glass was found. For the composites with 50–90 wt% high-viscosity glass, the sintered densities remained nearly constant (>95%) in a wide sintering temperature range. However, cristobalite crystallized from the glass phase, resulting in an undesirably high coefficient of thermal expansion. Presintering processing and a lower heating rate improved the densification of the composites with low-viscosity glass while limiting that of the composites with high-viscosity glass. This result is explained by the difference in crystallizability between these two glasses. As low- and high-viscosity glass powders were simultaneously added, the density reduction was reduced and the coefficient of thermal expansion was closer to that of Si because of the absence of cristobalite phase. The dielectric constant of all the composites was in the typical range of 4.9–5.7 at 1 MHz.
Depth-sensing indentation tests were used to determine the hardness of amorphous W–C–Co coatings deposited on different steel and copper substrates. The hardness of the film, Hf, was chosen to be always greater than the hardness of the substrate Hs and within the range Hf/Hs = 2 to 18.5. The influence of the ratio Hf/Hs on the ratio (t/hD)C between the film thickness t and the critical value of the indentation depth (hD)C, for which the substrate starts to deform plastically, was studied. Two independent methods were used to determine (hD)C values. One utilized the differential analysis of the loading part of the indentation curve, and the other was based on the plot of (Hc – Hs)/(Hf − Hs) versus t/(hD), Hc being the measured hardness of the film/substrate composite at a given indentation depth (hD). A good correlation between both methods was found.