Showing papers in "Journal of Materials Research in 1993"

Mechanical properties of nylon 6-clay hybrid

Abstract: Various nylon 6-clay hybrids, such as molecular composites of nylon 6 and silicate layers of montmorillonite and saponite, NCH's and NCHP's, respectively, have been synthesized. To estimate the mechanical properties of these hybrids, tensile, flexural, impact, and heat distortion tests were carried out. NCH was found superior in strength and modulus and comparable in impact strength to nylon 6. The heat distortion temperature (HDT) of NCH (montmorillonite: 4.7 wt. %) was 152 °C, which was 87 °C higher than that of nylon 6. In NCHP, saponite had a smaller effect on the increase of these mechanical properties. The modulus and HDT of NCH and NCHP increased with an increase in the amount of clay minerals. It was found that these properties were well described by the contribution of the constrained region calculated from the storage and loss modulus at the glass transition temperature. According to the mixing law on elastic modulus, the following expression was obtained between the modulus E at 120 °C and the fraction of the constrained region C, En = Ecn = C, where the values of n and Ec (modulus of the constrained region) were 0.685 and 1.02 GPa, respectively.

Synthesis of nylon 6-clay hybrid

Abstract: It was found that montmorillonite cation exchanged for 12-aminolauric acid (12-montmorillonite) was swollen by ∊-caprolactam to form a new intercalated compound. Caprolactam was polymerized in the interlayer of montmorillonite, a layer silicate, yielding a nylon 6-clay hybrid (NCH). The silicate layers of montmorillonite were uniformly dispersed in nylon 6. The carboxyl end groups of 12-aminolauric acid in 12-montmorillonite initiated polymerization of ∊-caprolactam, and as 12-montmorillonite content became larger, the molecular weight of nylon was reduced. From the result of end-group analysis, carboxyl end groups were more than amino end groups. The difference between the carboxyl and the amino end groups was attributed to ammonium cations (-NH3+) of nylon molecules, because the difference agreed with the anion site concentration of the montmorillonite in NCH. It is suggested that the ammonium cations in nylon 6 interact with the anions in montmorillonite.

Swelling behavior of montmorillonite cation exchanged for ω-amino acids by ∊-caprolactam

Abstract: Natural Na-montmorillonite was cation exchanged for the ammonium cations of various ω-amino acids [H3N+(CH2)n−1 COOH, n = 2, 3, 4, 5, 6, 8, 11, 12, and 18]. X-ray diffraction (XRD) results suggested that the chain axes of ω-amino acids with a carbon number of eight or less were parallel to the silicate layers, and that the chain axes of those with a carbon number of 11 or more were slanted to the layers. The cation-exchanged montmorillonites form intercalated compounds with ∊-caprolactam at 25 °C. The montmorillonites intercalated with both ω-amino acid and ∊-caprolactam were studied by XRD measurement at room temperature and 100 °C. We propose a model where amino acid molecules were arranged perpendicular to silicate layers and ∊-caprolactam molecules filled the space between them. When the ∊-caprolactam was melted at 100 °C, the basal spacing for the montmorillonite increased, in which the carbon number exceeds 11. This phenomenon will be applicable to obtaining the nylon 6-clay hybrid, a molecular composite of nylon 6 and montmorillonite.

A review of catalytically grown carbon nanofibers

Abstract: Carbon nanofibers (sometimes known as carbon filaments) can be produced in a relative large scale by the catalytic decomposition of certain hydrocarbons on small metal particles. The diameter of the nanofibers is governed by that of the catalyst particles responsible for their growth. By careful manipulation of various parameters it is possible to generate carbon nanofibers in assorted conformations and at the same time also control the degree of their crystalline order. This paper is a review of the recent advances made in the development of these nanostructures, with emphasis both on the fundamental aspects surrounding the growth of the material and a discussion of the key factors which enable one to control their chemical and physical properties. Attention is also given to some of the possible applications of the nanostructures which center around the unique blend of properties exhibited by the material.

A simple predictive model for spherical indentation

Abstract: A simple model is described with which the entire force versus penetration behavior of indentation with a sphere, during loading and unloading, may be simulated from knowledge of the four test material parameters, Young's modulus, Poisson's ratio, flow stress at the onset of full plastic flow and strain hardening index, and the elastic properties of the indenter. The underlying mechanisms are discussed and the predictions of the model are compared with data produced by an ultra low load, penetration measuring instrument.

Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process

Abstract: A two-step seeding process has been developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio. Previous study has shown that nucleation is the rate-limiting step for the perovskite formation. Therefore, any process that enhances the kinetics of nucleation is likely to decrease the transformation temperature. In this process, a very thin (45 nm) seeding layer of PbTiO3, which has a low effective activation energy for perovskite formation, was used to provide nucleation sites needed for the low temperature perovskite formation. In this study, we have shown that the pyrochlore-to-perovskite phase transformation temperature of PbZrxTi1−xO3 films of high Zr/Ti ratio (e.g., x = 53/47) can be lowered by as much as 100 °C. The grain size of these films can also be substantially modified by this two-step approach.

The yield stress of polycrystalline thin films

Abstract: In recent experiments it has been shown that the yield stress of polycrystalline thin films depends separately on the film thickness and the grain size. It was also shown that the grain size dependence varies as the reciprocal of the grain size. In this paper an analysis is presented which leads to these results and provides a more detailed understanding of the origins of the observed behavior.

Stress development and relaxation in copper films during thermal cycling

Abstract: The reliability of integrated-circuit wiring depends strongly on the development and relaxation of stresses that promote void and hillock formation. In this paper an analysis based on existing models of creep is presented that predicts the stresses developed in thin blanket films of copper on Si wafers subjected to thermal cycling. The results are portrayed on deformation-mechanism maps that identify the dominant mechanisms expected to operate during thermal cycling. These predictions are compared with temperature-ramped and isothermal stress measurements for a 1 μm-thick sputtered Cu film in the temperature range 25–450 °C. The models successfully predict both the rate of stress relaxation when the film is held at a constant temperature and the stress-temperature hysteresis generated during thermal cycling. For 1 μm-thick Cu films cycled in the temperature range 25–450 °C, the deformation maps indicate that grain-boundary diffusion controls the stress relief at higher temperatures (>300 °C) when only a low stress can be sustained in the films, power-law creep is important at intermediate temperatures and determines the maximum compressive stress, and that if yield by dislocation glide (low-temperature plasticity) occurs, it will do so only at the lowest temperatures (<100 °C). This last mechanism did not appear to be operating in the film studied for this project.

Hydrothermal synthesis and sintering of ultrafine CeO2 powders

Abstract: Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of almost-equal-to 10-15 nm, were prepared under hydrothermal conditions of 10 MPa at 300-degrees-C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1-10-degrees-C/min up to 1350-degrees-C. The undoped CeO2 started to sinter at almost-equal-to 800-900-degrees-C and reached a maximum density of 0.95 of the theoretical at 1200-degrees-C, after which the density decreased slightly. Isothermal sintering at 1150-degrees-C produced a sample with a relative density of almost-equal-to 0.98 and an average grain size of almost-equal-to 100 nm. The samples sintered above 1200-degrees-C exhibited microcracking. The decrease in density and the microcracking above 1200-degrees-C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution Of O2 gas. Doping with Y2O3 produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO2. Sintered samples of the doped CeO2 showed no microcracks. The CeO2 doped with up to 3 mol % Y2O3 was sintered to almost full density and with a grain size of almost-equal-to 200 nm at 1400-degrees-C.

The frictional component of the indentation size effect in low load microhardness testing

Abstract: The role of friction between the microhardness indenter and the test specimen is addressed through the analysis of dry (unlubricated) and lubricated tests on iron by Atkinson and Shi. Quantitative evaluation through a proportional specimen resistance model accurately describes the results. It suggests that friction is a major portion of the observed hardness increase at low test loads, the indentation size effect. The ISE is related to the surface-area-to-volume ratio of the indentation, which is inversely related to the indentation dimension.

Electrical and optical characterization of Sb : SnO2

Abstract: Films of Sb : SnO2 have been formed by vacuum e-beam evaporation. The structural, electrical, and optical properties of these films have been investigated with respect to annealing time and temperature. After heat treatment in an oxygen atmosphere, thin films with a peak transmittance of 98% and 4–9 × 10−3 Ωcm resistivity have been obtained. The barrier heights and energy band diagrams of Sb : SnO2/Si n-n and p-n heterojunctions have been determined by C-V measurements.

Dielectric properties of aerogels

Abstract: We have measured the real (dielectric constant) and imaginary (loss factor) components of the complex relative permittivity at 298 K using microwave frequencies (2, 10, and 18–40 GHz) for bulk SiO2-aerogels and for two types of organic aerogels, resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF). Measured dielectric constants are found to vary linearly between values of 1.0 and 2.0 for aerogel densities from 10 to 500 kg/m3. For the same range of densities, the measured loss tangents vary linearly between values of 2 × 10−4 and 7 × 10−2. The observed linearity of the dielectric properties with density in aerogels at microwave frequencies shows that their dielectric behavior is more gas-like than solid-like. The dielectric properties of aerogels are shown to be significantly affected by the adsorbed water internal to the bulk material. For example, water accounts for 70% of the dielectric constant and 70% of the loss at microwave frequencies for silica aerogels. Because of their very high porosity, even with the water content, the aerogels are among the few materials exhibiting such low dielectric properties. Our measurements show that aerogels with greater than 99% porosity have dielectric constants less than 1.03; these are the lowest values ever reported for a bulk solid material.

An investigation of ductility and microstructural evolution in an Al−3% Mg alloy with submicron grain size

Abstract: A submicrometer-grained (SMG) Al−3% Mg solid solution alloy, with an initial grain size of ∼0.2 μm, was produced by intense plastic straining. Experiments show that tensile specimens of the SMG alloy exhibit high elongations to failure at low testing strain rates at the relatively low temperature of 403 K. The stress exponent is high (∼7–8) and calculations show deformation is within the region of power-law breakdown. The initial microstructure of the alloy consists of diffuse boundaries between highly deformed grains. At strain rates of ∼10−4 s−1 and lower, plastic deformation leads to dynamic recrystallization and the formation of highly nonequilibrium grain boundaries that gradually evolve into a more equilibrated configuration.

High-temperature oxidation of ion-plated TiN and TiAlN films

Abstract: The high-temperature oxidation of TiN, Ti0.9Al0.1N, and Ti0.6Al0.4N films which were deposited onto stainless steel substrates using an arc ion-plating apparatus was studied at temperatures ranging from 923 to 1173 K for 0.6 to 60 ks in air. The oxidation rate obtained from mass gain as a function of time was found to fit well to a parabolic time dependence. From their temperature dependence, the apparent activation energies of oxidation were determined. With increasing Al contents, the oxidation rate decreased, and the activation energies of oxidation reaction increased. Formed oxide layers were analyzed by XRD, SEM, and EPMA. With increased Al content in TiAlN films, the rate-determining step changes from oxygen ion diffusion in formed rutile to oxygen or aluminum ion diffusion in the formed Al2O3 layer.

Phase evolution in boron nitride thin films

Abstract: Boron nitride (BN) thin films were deposited on monocrystalline Si(100) wafers using electron beam evaporation of boron with simultaneous bombardment by nitrogen and argon ions. The effect of film thickness on the resultant BN phase was investigated using Fourier transform infrared (FTIR) spectroscopy and high resolution transmission electron microscopy (HRTEM). These techniques revealed the consecutive deposition of an initial 20 A thick layer of amorphous BN, 20–50 A of hexagonal BN having a layered structure, and a final layer of the polycrystalline cubic phase. The growth sequence of the layers is believed to result primarily from increasing biaxial compressive stresses. Favorable surface and interface energy and crystallographic relationships may also assist in the nucleation of the cubic and the hexagonal phases, respectively. The presence of the amorphous and hexagonal regions explains why there have been no reports of the growth of 100% cubic boron nitride on Si.

Microwave-hydrothermal processing for synthesis of electroceramic powders

Abstract: Microwave-hydrothermal processing has so far been used only to dissolve inorganic solids for chemical analysis. We report herein the use of microwave-hydrothermal processing to synthesize various ceramic powders in binary and polynary systems. We describe the synthesis of some electroceramic powders such as BaTiO3, SrTiO3, Sr0.5Ba0.5TiO3, PbTiO3, BaZrO3, SrZrO3, Pb(Zr0.52Ti0.48)O3, and pyrochlore phases with the Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 compositions by this novel microwave-hydrothermal processing technique.

Fatigue of ferroelectric PbZrxTiyO3 capacitors with Ru and RuOx electrodes

Abstract: The fatigue behavior of ferroelectric PZT capacitors with Ru and RuOx electrodes was studied. These capacitors show no sign of fatigue out to 1 X 1011 cycles, in sharp contrast to the degradation typically observed with Pt electrodes. Compared to Pt electrodes, the initial polarization was lower, but after fatiguing the polarization was comparable to or larger than that for Pt electrodes. Differences in polarization in response to switching and nonswitching pulses greater than 10 μC/cm2 were observed at 1 X 1011 cycles.

Observation, Analysis, and Simulation of the Hysteresis of Silicon Using Ultra-Micro-Indentation with Spherical Indenters

Abstract: The recently reported hysteretic behavior of silicon under indentation (Clarke et al.1 and Pharret al.2-5) is investigated using an ultra-micro-indentation system with an 8.5 μm spherical-tipped indenter. The onset of “plastic” behavior during loading and hysteresis during unloading was readily observed at loads in excess of 70 mN. Cracking about the residual impression was observed only at loads of 350 mN and higher. An analysis of the data is presented that estimates the following: (1) the initial onset of deformation occurs at a mean pressure of 11.8 ± 0.6 GPa, (2) the mean pressure at higher loads is 11.3 ± 1.3 GPa, and (3) the hysteretic transition on unloading occurs at mean pressures between 7.5 and 9.1 GPa. These values are in good agreement with the accepted literature values for the known silicon transformation pressures. A simulation of the force-displacement data based on the analysis and model is presented and is found to fit the observations very well.

Tribological properties of quasicrystalline coatings

Abstract: Coatings of three different compositions (Al65Cu20Fe15, Al64Cu18Fe8Cr8, and Al67Cu9Fe10.5Cr10.5Si3) were realized by various thermal deposition techniques. They were studied in the as-deposited state and after annealing. In view of potential applications, these quasicrystalline coatings were examined from the point of view of tribology: friction and wear. Some basic components of friction such as roughness, plowing, and adhesion have been studied in scratch testing. The friction resistance of the coating is strongly dependent on its inherent porosity, hardness, and thickness. The damage of the coatings is essentially brittle though some ductile behavior is observed. Static indentation hardness is in the range 500–600 HV0.03 (5–6 GPa), whereas the scratch hardness varies from 1.4 to 2.4 GPa depending on the percentage of porosity. Friction coefficients (measured at constant load of 20 N) were found to be typically 0.07 and 0.20 for diamond (tip radius R = 0.79 mm) and AISI 52100 (radius R = 0.79 mm) indenters, respectively.

Nanocrystalline α-Fe, Fe3C, and Fe7C3 produced by CO2 laser pyrolysis

Abstract: Nanocrystalline α–Fe, Fe3C, and Fe7C3, particles with narrow size distributions were produced by CO2 laser pyrolysis of vapor mixtures of Fe(CO)5 and C2H4. Details of the synthesis procedure are discussed. Mossbauer spectroscopy and x-ray diffraction were used to identify the structural phases and the former was used also to study the magnetism of the nanoparticles. All the nanoparticles were observed to be ferromagnetic in this size range. If excess C2H4 appears in the reactant gas mixture, several monolayers of pyrolytic carbon were observed to form on the particle surface, as deduced from transmission electron microscopy and Raman scattering studies. Results of thermo-gravimetric analysis/mass spectroscopy studies of this carbon coating indicate it is gasified in hydrogen at temperatures T ∼ 250 °C.

Analysis of buried oxide layer formation and mechanism of threading dislocation generation in the substoichiometric oxygen dose region

Abstract: The structure of SIMOX wafers implanted at 180 keV with doses of 0.1 × 1018-2.0 × 101816O+ cm−2 at 550 °C, followed by annealing over the temperature range of 1050–1350 °C, has been investigated using cross-sectional transmission electron microscopy and a chemical etching. With doses of 0.35 × 1018-0.4 × 1018 cm−2, a continuous buried oxide layer having no Si island inside is formed by high-temperature annealing at 1350 °C. At a dose of 0.7 × 1018 cm−2, multilayered oxide striations appear in the as-implanted wafer. These striations grow into multiple buried oxide layers after annealing at 1150 °C. The multiple layers lead to a discontinuous buried oxide layer, resulting in the formation of a number of Si micropaths between the top Si layer and the Si substrate when the wafer is annealed at 1350 °C. These Si paths cause the breakdown electric field strength of the buried oxide layer to deteriorate. With doses of 0.2 × 1018-0.3 × 1018 cm−2 and of higher than 1.3 × 1018 cm−2, an extremely high density of threading dislocations is generated in the top Si layer after annealing at 1350 °C. The dislocation density is greatly reduced to less than 103 cm−2 when the oxygen dose falls in the range of 0.35 × 1018-1.2 × 1018 cm−2. Here we propose a mechanism that accounts for the threading dislocation generation at substoichiometric oxygen doses of less than 1.2 × 1018 cm−2.

Highly oriented, textured diamond films on silicon via bias-enhanced nucleation and textured growth

Abstract: Highly oriented diamond films were grown on single-crystal silicon substrates. Textured films were first nucleated by a two-step process that involved the conversion of the silicon surface to an epitaxial SiC layer, followed by bias-enhanced nucleation. The nucleation stage, which produced a partially oriented diamond film, was immediately followed by a (100) textured growth process, thus resulting in a film surface where approximately 100% of the grains are epitaxially oriented relative to the silicon substrate. The diamond films were characterized by both SEM and Raman spectroscopy. Structural defects in the film are discussed in the context of their potential effect on the electrical characteristics of the resulting film.

A new method for measuring the strength and ductility of thin films

Abstract: A new method of measuring the mechanical strength of thin films is described. We prepare miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long, and 2.2 μm thick, using deposition, patterning, and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a rectangular silicon frame. Thirty-six such specimens are produced on a single wafer. After a specimen frame is mounted, its vertical sides are severed without damaging the specimens. The load is applied by micrometers through a special tension spring. Tensile properties of a 2.2 μm thick Ti–Al–Ti film were determined.

Diamond/Al metal matrix composites formed by the pressureless metal infiltration process

Abstract: Diamond particles are unique fillers for metal matrix composites because of their extremely high modulus, high thermal conductivity, and low coefficient of thermal expansion. Diamond reinforced aluminum metal matrix composites were prepared using a pressureless metal infiltration process. The diamond particulates are coated with SiC prior to infiltration to prevent the formation of Al4C3, which is a product of the reaction between aluminum and diamond. The measured thermal conductivity of these initial diamond/Al metal matrix composites is as high as 259 W/m-K. The effects of coating thickness on the physical properties of the diamond/Al metal matrix composite, including Young's modulus, 4-point bend strength, coefficient of thermal expansion, and thermal conductivity, are presented.

High refractive index films of polymer nanocomposites

Abstract: Solutions of PbS particles and gelatin were used for the preparation of nanocomposites by a spin-coating process. This allows for the preparation of nanocomposite films with controlled thickness, e.g., between 40 nm and 2 μm for a film containing 45 wt.% PbS. Surface roughness and film thickness were investigated by surface profilometry and scanning electron microscopy (SEM). The refractive index at 632.8 nm can be expressed by a linear function of the volume fraction of PbS in the range of 0 to 55 vol. % PbS. In this range, the refractive index increases from 1.5 to 2.5 with increasing PbS ratio and belongs, therefore, to the highest refractive indices known for polymeric composite materials.

A new method for tensile testing of thin films

Abstract: A new method for tensile testing of thin films is presented. The strain is measured directly on the unsupported thin film from the displacement of laser spots diffracted from a thin grating applied to its surface by photolithography. The diffraction grating is two-dimensional, allowing strain measurement both along and transverse to the tensile direction. In principle, Young’s modulus, Poisson’s ratio, and the yield stress of a thin film can be determined. Cu, Ag, and Ni thin films with strong ⟨111⟩ texture were tested. The measured Young moduli agreed with those measured on bulk crystals, but the measured Poisson ratios were consistently low, most likely due to slight transverse folding of the film that developed during the test. The yield stresses of the evaporated Cu and Ag thin films agreed well with an extrapolation of the Hall-Petch relation found for bulk materials. Ni thin films are known to deviate from a bulk Ni Hall–Petch relation for submicron grain sizes, and sputtered Ni films show much higher yield stresses than electrodeposited or vapor-deposited films of similar grain size. Our sputtered Ni films had higher yield stresses than other sputtered films from the literature.

Mechanism of MoSi2 pest during low temperature oxidation

Abstract: The pest of monolithic poly- and single-crystalline molybdenum disilicide, as well as its composites, has been investigated by low temperature oxidation in air at temperatures ranging from 350 to 700 °C. Pest phenomenon (i.e., disintegration from bulk into powders) was consistently observed from samples oxidized at temperatures between 375 and 500 °C. In contrast, samples oxidized at 550 °C exhibited only severe cracking, and samples oxidized at 350 °C or at/above 600 °C were intact. The pested samples resulted in powdery products consisting of MoO3 whiskers, SiO2 clusters, and residual MoSi2. The MoO3 whiskers exhibited protruding characteristics, and were highly concentrated at microstructural heterogeneous sites, such as interparticle boundaries, grain boundaries, and cracks. Blisters were observed and found to be formed predominantly on the grain boundary and interparticle boundary interfaces of tested samples. These blisters were often found to be erupted, indicating that significant vapor pressure was built up underneath the sample surface. Interrupted oxidation tests at 500 °C revealed that a substantial volume change also occurred. The development of the pest of MoSi2 was found, macroscopically, to consist of nucleation and growth. In most cases, it initiated from some local microstructural inhomogeneities, and propagated throughout the samples. Based on the morphologies of disintegrated powders, two different kinetic processes were identified to be responsible for the pest reaction. According to the results from single crystals, the pest of MoSi2 appeared to be kinetically controlled by the formation and volatilization of MoO3. The mechanism of MoSi2 pest is proposed and discussed. Thermodynamic analyses are also presented to substantiate the experimental observations.

Wear resistance of arc ion-plated chromium nitride coatings

Abstract: Wear resistance of arc ion-plated chromium nitride films has been studied. It has been found that texture and phases composing the films depend much on bias voltage and nitrogen gas pressure at the deposition. A phase diagram was constructed as a function of these two parameters, which indicated that three categories exist: CrN single, CrN and Cr2N dual, and CrN and Cr dual phased regions, respectively. Results of Falex No. 2 test showed that the wear resistance of CrN single phased films is superior to others, especially when (220) preferred orientation is developed. Since hardness and surface morphology do not differ much between these films, a high toughness of CrN single phased film is considered to make a difference by suppressing abrasion wear.

Structure property relationships in core-shell BaTiO3—LiF ceramics

Abstract: A sintering, microstructural development and dielectric property study of BaTiO3–LiF ceramics was performed to assess the potential application of low-fired multilayer capacitors. Not only does LiF allow for sintering below 1000 °C, it also allows for the manipulation of dielectric properties and interfaces within BaTiO3–LiF ceramics. Using mixing laws, a model of the dielectric properties of the core-shell microstructures is presented that agrees well with the observed experimental data.

Synthesis of stable quasicrystalline particle-dispersed Al base composite alloys

Abstract: We report the preparation of the quasicrystalline particle-dispersed Al base composite alloys for the first time. Icosahedral AlCuFe quasicrystalline (i-AlCuFe) particles were homogeneously dispersed in a crystalline Al matrix using a novel process which combined mechanical alloying and hot press techniques. Hardness of the composite alloy increases with increasing volume fraction of the i-AlCuFe particle, i.e., increases from 25 kg/mm2 for pure Al to about 120 kg/mm2 for a 25 vol. % i-AlCuFe particle dispersed alloy.