Browsing by Author "Houska, Charles R."
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- Affect of annealing on uniform and nonuniform strains in a sputtered Mo film on SiAdler, Thomas; Houska, Charles R. (American Institute of Physics, 1979)Sputtered films of 1.5 μm of Mo deposited on (111) ‐oriented Si failed either by blistering or localized eruptions after various thermal treatments. Investigations were carried out to determine the amount of strain in the film associated with this unstable mechanical behavior. Two types of measurements were employed. One employs macroscopic interferometer measurements to measure deflection and the other x‐ray diffraction. A separation is made of intrinsic and thermally induced strains. The intrinsic strains are believed to be due mainly to Ar atoms embedded during sputtering which remain throughout annealing treatments. These atoms also introduce a broadening of the diffraction lines because of the special constraints associated within films. This effect is separated from the usual line broadening due to dislocations and small particle size. Annealing treatments reconfirm that dislocation mechanisms are not as effective in relieving nonuniform microstrain in films as they are in cold‐work filings of the same material.
- Broadening of x‐ray diffraction lines from small subgrains containing gradients of spacingHouska, Charles R. (American Institute of Physics, 1978)Recent data on subgrain size and spacing gradients in the Cu‐Ni system allow a more critical examination of a technique currently used to obtain the composition profiles of small diffusion zones. These data support the assumption that the broadening from individual subgrains need not include a spacing gradient term when volume diffusion is predominant. However, care should be taken in the interpretation of the earliest stage of diffusion where grain boundaries,surface, and other defects may introduce large spacing gradients.
- Calculation of x‐ray intensity from a rough sample based on a statistical modelHwang, Bing; Houska, Charles R. (American Institute of Physics, 1988-06-01)An x‐ray intensity correction is developed which begins with a roughness model that is often used to describe real surfaces. This is based upon a normal distribution of surface asperities relative to a mean plane. Pair correlation between absorbing elements along x‐ray paths either entering or leaving the sample with respect to the signal producing element is accomplished by means of an exponential autocorrelation function. This allows the degree of roughness to be varied on a local scale to fit specific surfaces using statistical data. Equations are developed to describe x‐ray fluorescence and diffraction signals for symmetric and asymmetric beam optics. Theory is compared with experiment using a roughened, fully stabilized zirconia sample.
- Computer simulation of high fluence ion beam surface modification processesRangaswamy, Mukundhan (Virginia Polytechnic Institute and State University, 1989)Various processes that participate in ion beam surface modification are studied using phenomenological, analytical and first principle models. The processes that are modelled phenomenologically include preferential sputtering, radiation-damage induced migration and second phase precipitation. The models are based on numerical solutions of the transport equation and include the processes of ion collection, sputtering, lattice dilation or accommodation and diffusion as well. The model for preferential sputtering takes into account the depletion of the preferentially sputtered element at the surface and the atomic transport process that results from the concentration gradients caused by the depletion. Results are presented for the case of Ta implantation into Fe. ln the radiation-damage induced migration the flux of the solute atoms is coupled to the concentration gradient of the continuously introduced defects. Examples of implantation of Sn into Fe and N into Fe are modeled to demonstrate the influence of radiation-damage induced migration. The precipitation of second phases during irradiation is modelled using thermodynamic considerations but with solubility values under irradiation obtained from experiment. In the model the solute atoms in excess of the solubility limit are assumed to precipitate out. Calculations are presented for the case of N implantation into Nb. Using first principle calculation for binary collisions in solids a computer simulation code was developed to study the collisional mixing occurring during high fluence ion implantation. It is based on the Monte Carlo code TRIM, and is capable of updating the target composition as the implantation process proceeds to high fluences. The physical basis for the dynamic simulation as well as a detailed analysis on the statistics required for obtaining the profiles with a given accuracy are presented. Vectorized results in a high computational efficiency. The predicted collisional broadening of the implantation profiles is presented for Ar bombardment into a Sn-Fe target as well as Ti implantation into C-Fe. The results are compared to those of the diffusion approximation. A semi-empiricaI model based on an analytical evaluation of ion mixing at low temperatures was developed taking into account collisional mixing and thermal spike effects, as well as the thermal spike shape. The ion beam mixing parameter for the thermal spike is derived as being proportional to different powers of the damage parameter, i.e. the damage energy scaled by the cohesive energy of the matrix, dependent on the thermal spike shape and point defect density in the thermal spike regions. Three different regions of ion beam induced mixing were recognized according to different density levels of the damage parameter. An experiment was conducted to determine the effect of chemical or thermodynamic factors in the migration of C in the presence of Fe and Ti atoms. A marker layer of C in a Fe-Ti matrix was ion beam mixed using Ar. The large mixing effect is tentatively attributed to a favorable heat of mixing values.
- Dielectric properties of PFN-PFT solid solution synthesized by the molten salt methodAmanuma, Kazushi (Virginia Tech, 1991-08-09)Various compositions of PFN-PFT solid solution were synthesized by the molten salt method at 800°C and 900°C. Higher temperature was necessary to form pure perovskite of PFT than PFN. However, those synthesized at higher temperatures tended to decompose during sintering. The particle size was not uniform and increased from PFT to PEN. The distribution of grain size in the sintered pellets was bi-modal. The grain size increased as the particle size increased. Those synthesized at 900°C had quite a different behavior from those synthesized at 800°C during sintering. Each composition of PFN-PFT system showed a broad dielectric phase transition in the different temperature range. This behavior was well described by the statistical model of compositional fluctuation.
- Effect of dislocation substructure on the primary creep behavior of alpha titanium at elevated temperatureSomers, Bruce Robert, 1947- (Virginia Tech, 1976-08-15)Constant stress creep tests were performed in vacuo on alpha titanium in various thermomechanical treatments. At 500°C and 527°C annealed alpha titanium exhibits an anomalous creep arrest in the initial portion of the creep curve. After this creep arrest the creep curves swept up to a true steady state creep rate. This anomalous behavior is attributed to dynamic strain aging. The effect vanishes due to interstitial solute depletion as the interstitials are swept from the lattice by moving dislocations. The prestrained and recovered samples tested at 60006 did not show a creep arrest. This is attributed to even distribution of interstitials on the recovered dislocation substructure making dynamic strain aging less effective. Additionally, transmission electron microscopy indicates that the tilt boundaries of the recovery structure can break down during the initial portion of the creep curve releasing mobile dislocations. In steady state all conditions tested developed a dislocation subgrain substructure. The tendency for dislocation subboundaries to inhabit specific planes is not as marked as it is in the recovered structure.
- Effects of alloying elements upon austenite decomposition in high strength low alloy steelsChen, Jhewn-Kuang (Virginia Tech, 1992-10-03)The kinetics of austenite decomposition were studied in high purity Fe-0.1 C-0.4 Mn-0.3 Si-X (concentrations in weight percent, X represents 3 Ni, 1 Cr, or 0.5 Mo) steels at temperatures between 500 and 675°C. The transformation stasis phenomenon was found in the Fe-C-Mn-Si-Mo and Fe-C-Mn-Si-Ni alloys isothermally transformed at 650°C and 675°C but not in the Fe-C-Mn-Si and Fe-C-Mn-Si-Cr alloys at any of the temperatures investigated. The occurrence of transformation stasis was explained by synergistic interactions among alloying elements. The paraequilibrium model was applied to calculate the metastable fraction of ferrite in each alloy. This fraction was shown to coincide with cessation of transformation in the Mo alloy transformed at 600°C. Transformation stasis was found in both the Ni and the Mo alloys isothermally reacted at 650°C and 675°C. The interactions among Mn, Si, and Mo as well as interactions among Mn, Si, and Ni appear to decrease the threshold concentrations for occurrence of transformation stasis in Fe-C-Mn-Si systems. The segregation of Mn and Mo to the α/γ boundary assisted by Si was suggested to enhance the drag force and led to transformation stasis. In the Ni alloy, lower driving force for ferite formation by addition of Ni could be responsible for occurrence of transformation stasis.
- Grain boundary studies in ordered intermetallic compound Ni3 AlJang, Ho (Virginia Tech, 1990-06-04)Theoretical and experimental studies of grain boundary structures were carried out to understand the grain boundary properties of Ni₃Al. Using various techniques of transmission electron microscopy, the geometry of grain boundaries was determined from a number of grain boundaries to obtain a distribution of grain boundary types in Ni₃Al. Five macroscopic parameters, including a misorientation between two grains and a grain boundary plane orientation, were considered in the grain boundary type. A distribution of grain boundary types in ductile Ni₃Al contained more low Σ boundaries than brittle Ni₃Al. In a distribution of grain boundary plane orientations, the trend of having high density plane of coincident sites was maintained on low Σ boundaries up to Σ = 9 and random orientations of grain boundary planes became dominant after Σ = 11. In ductile Ni₃Al, grain boundary accommodations of dislocations in the vicinity of grain boundaries were found, which were not shown in brittle Ni₃Al. Using CBED techniques, symmetry information of a single crystal was obtained. However, an extension to bicrystals was not successful. In order to study the possible disordering and segregation near grain boundaries, two different atomistic computer simulations, namely, molecular statics simulation and two dimensional lattice gas model, were carried out. Both results showed a similar trend of disordering near the grain boundary regardless of grain boundary composition and atomic interaction energy assumptions. However, segregation behavior near grain boundaries showed strong influences from grain boundary compositions and atomic interaction energy assumptions.
- Iterative approaches describing atomic diffusion in finite single‐ and two‐phase systemsHouska, Charles R.; Unnam, Jalaiah (American Institute of Physics, 1976)Iterative solutions are given for planar one‐dimensional atomic diffusion in finite single‐ and two‐phase systems. They are usable for any continuous variation of the interdiffusion coefficient D (C) within each phase, and need not be fitted to special functions such as a power series or an exponential function. Modified integral functions similar to one first proposed by Boltzmann are used along with a conservation criterion to locate the interface position ξ. Computer time for the iterative solutions is about two to three magnitudes shorter than finite‐difference (F‐D) calculations because of the rapid convergence of the integral equations. The accuracy of these approximate forms is considered. Excellent agreement was obtained between F‐D calculations and the iterative approach for semi‐infinite single‐phase systems. Good agreement is also found for two‐phase systems; however, the accuracy varies with the solubility gap size C βα−Cαβ. The best results are obtained for gaps larger than 0.7 which includes most eutectic systems. Calculations of composition profiles which are based upon the maximum solid solubilities for the Cu‐Ag system are within 1% of the F‐D calculations.
- Least‐squares analysis of x‐ray diffraction line shapes with analytic functionsHouska, Charles R.; Smith, Terence M. (American Institute of Physics, 1981)This is a second paper of a sequence that provides a useful analytic function which is based upon the Warren‐Averbach line shape analysis. Once the Fourier coefficients are interrelated in terms of a minimum number of parameters, the rather lengthy Fourier series can be evaluated by reducing it to a convolution of two known functions. One of these functions includes the particle size distribution, strain, and the Cauchy‐like contribution to the instrumental broadening. The second includes another strain parameter and the Gaussian contribution to the instrumental broadening. The resultant convolution integral is readily carried out using a nine‐point Gauss‐Legendre quadrature. Instrumental parameters are obtained from a separate convolution of Cauchy and Gaussian functions. This procedure reduces the computer time to one‐tenth the time required to synthesize the Fourier series and makes it feasible to carry out a least‐squares fitting of the profile data. Examples are given for Mo films and for an InSb film.
- The measurement of elastic stresses and energy in cubic single‐crystal films by x‐ray diffractionRao, Satish I.; Houska, Charles R. (American Institute of Physics, 1981)Anisotropicelasticity calculations have been made for use in conjunction with strain measurements by x‐ray diffraction for sputtered single‐crystal films. Only the cubic case has been treated. Data from InSb films with (100) and (111) orientations on similarly oriented GaAs substrates are given. It was found that nearly alike planar strains ε yield lower planar stresses σ′ 1 and σ′ 2 and stored energy density U for the (100) orientation. The (100) films exhibit a relatively large strain perpendicular to the film ε3.
- Near surface wear structure of ceramic componentsHwang, Bing-Hwai (Virginia Polytechnic Institute and State University, 1987)X-ray techniques were used extensively in this work to examine ceramic samples. The well known techniques including : (1) identification of phases and chemical species, (2) peak separation, (3) quantitative phase analysis, (4) intensity band simulation, and (5) line shape analysis, were applied to ceramic materials of silicon nitride, zirconia toughened alumina, fully and partially stabilized zirconia. A theoretical derivation of the x-ray intensity from a rough sample based on a statistical roughness model was carried out. The statistical model was then combined with the intensity band simulation technique to simulate the intensity band from a rough sample. The modified technique for intensity band simulation was used to determine the strain profile in the near surface regions of a flat and a rough fully stabilized zirconia samples. The results show that a compressive zone is present in the near surface region of each sample. A partially stabilized zirconia disk was examined using symmetrical and asymmetrical diffraction optics after a prolonged pin on disk wear test. The different diffraction optics provided different probe depths and revealed a depth gradient of the phase distribution. A general picture of the wearing process of ceramic components is described based on the examinations carried out in this work.
- On a point defect inside an idealized elastic sphereLing, Chih B.; Houska, Charles R. (American Institute of Physics, 1983)This paper presents a method of solution for the displacement, stress, and strain due to a point defect located inside a sphere. The solution is represented by a Love stress function in spherical coordinates, which is biharmonic in character. Two axisymmetric types of the point defect are considered. One is treated as a center of dilatation and the other as a double force without moment, or a doublet, oriented axisymmetrically. The Love stress function for the point defect in an infinite solid is specified in each case by a single biharmonic function. The residual tractions on the surface of the sphere left by this function are annulled by superposing two series of biharmonic functions. When the Love stress function is determined, the displacement, stress, and strain can be derived straightforwardly.
- Optical property studies and metalorganic chemical vapor deposition of ferroelectric thin filmsPeng, Chien-Hsiung (Virginia Tech, 1992-08-01)Ferroelectric lead zirconate titanate thin films, Pb(ZrxTi 1-x)0 3 or PZT, have aroused considerable interest in recent years for the application in nonvolatile electronic memories because of their excellent ferroelectric properties. In this research, PZT thin films were studied from two aspects: the scientific aspect and the technical aspect. The optical properties of PZT solid solutions and the structure development in PZT films were extensively investigated in the scientific aspect. The PZT films used in this part of study were prepared by metalorganic decomposition (MOD) process. The envelope method, with consideration of light intensity loss from the back surface of the substrate, was demonstrated to be a simple and convenient tool for obtaining the optical properties of the PZT films in the medium and weak absorption regions. In the near optical band gap region, both the transmission and reflection spectra were used to successfully calculate the optical constants of the films. The film thickness derived from the envelope method was cross checked by a computer simulation method and was found to have an accuracy better than 2%. An effective, versatile, and nondestructive optical method was developed for the study of the structure development in MOD PZT films. Also, the models for the structure development were proposed and were verified by this optical method. Using this method, the characteristic temperatures (i.e., the initiation and completion temperatures) of each phase can be easily identified. In addition, the volume fraction of the perovskite phase in the pyrochlore-perovskite phase transformation region was obtained from this optical method. From the technical point of view, ferroelectric PZT films were successfully and reproducibly deposited for the first time by hot-wall metalorganic chemical vapor deposition (MOCVD). One of the problems associated with the MOCVD technique is the availability of the precursors. After intensive studies searching for the most suitable precursors for MOCVD PZT thin films, the safe and stable precursors, namely lead tetramethylheptadione [Pb(thd)2)], zirconium tetramethylheptadione [Zr(thd)4], and titanium ethoxide [Ti(OEt)4] were chosen. The films were deposited at temperatures as low as 55QOC and had pure perovskite phase in the as-deposited state. Also, the films were smooth, specular, crack-free, uniform, and adhered well on the substrates. The stoichiometry of the films can be easily controlled either by varying the individual precursor temperature and/or the flow rate of the carrier gas. Auger electron spectroscopic (AES) depth profile showed good uniformity through the thickness of the films. The AES spectra also showed no carbon contamination in the bulk of the films. As-deposited films were dense and showed uniform and fine grains. The film (Pb/Zr/Ti = 50/41/9) annealed at 6QQOC showed a spontaneous polarization of23.3 pC/cm² and a coercive field of 64.5 kV /em.
- A Post-Treatment Evaluation of the Combined Effects of Imipramine Pharmacotherapy and Brief Psychotherapy in the Treatment of Childhood Anxiety DisordersPorter, Daniel B. III (Virginia Tech, 1998-02-01)This study evaluated a treatment program for anxiety disordered children, ages five to twelve years, utilizing both qualitative and quantitative methodologies. The treatment program integrated Imipramine pharmacotherapy and brief psychotherapy. The participants' nuclear and extended family histories were examined in terms of the occurrence of psychopathology and endemic transactional patterns. The examination of family patterns utilized Murray Bowen's Generational Model, as well as the T-F-A model of Hutchins and Cole, as a means of explaining the transmission of anxiety in the family. Ten children suspected of experiencing anxiety disorders were referred by pediatric physicians for treatment. Following an initial diagnostic assessment, children were placed on 25 milligrams of Imipramine per day for four to six weeks, while participating in weekly conjoint psychotherapy with their mothers for a six to eight-week period. A post-treatment evaluation was conducted by selecting ten prototypic participants. Selection was based upon age, diagnosis of overanxious disorder or separation anxiety disorder in childhood, and a time interval of no more than one year or less than one week following treatment. Semi-structured interviews were conducted with mother-child pairs separately to evaluate participants' perceptions of pre- and post-treatment symptom levels and family dynamics. DSM-III diagnostic criteria, Bowenian and T-F-A models served as the frameworks for organizing and evaluating qualitative data. All child participants experienced a dramatic and lasting resolution of both OAD and SAD symptomology. A quantitative analysis was performed utilizing the Wilcoxon sign rank to compare pre- and post-treatment symptom levels, with a significant effect by treatment occurring at the .005 level of significance. Cross-validation of treatment outcome was achieved through review of medical records, original psychotherapy notes, and videotapes of the interviews. Qualitative data regarding transgenerational medical and psychological disorders and family dynamics was generated. The data supported the Unitary model of generational family pathology. Six of seven Bowenian constructs were confirmed in this sample. The T-F-A model was used to demonstrate a cyclical pattern of reassurance, anxiety reduction, and child dependency between anxious children and their mothers. These results were discussed to provide a better understanding of the etiology and treatment of childhood anxiety disorders (OAD and SAD). The term "anxogenic family" was suggested to convey the interaction of genetics and generational learning in the families of anxiety disordered children. Implications for future research and modification of the DSM-IV regarding childhood anxiety disorders were discussed.
- Preparation and characterization of Mo/Al layered thin filmsGiridhar, Jayaramachandran (Virginia Tech, 1987-04-15)The fabrication of bilayer and multilayer Mo/Al thin films using a conventional multi-target rf-diode sputter deposition system was studied. The films were deposited on glass and Si( 100) substrates. The as-deposited films were characterized with respect to their structure and composition profiles using X-Ray diffraction, AES, SEM, TEM, and RBS techniques. The as-deposited bilayer films were of good quality with an interface thickness of about 200 Ã . They were annealed at different temperatures in the range of 300-600°C for different times. Diffusional intermixing and phase changes were monitored and diffusivity measurements were made. The growth characteristics of intermetallic compounds were analysed. Mo/Al multilayer thin films with layer thicknesses less than 200 Ã were also deposited. An assessment of structural and compositional modulations in these multilayer films revealed the need for the conversion of the conventional diode sources to magnetron sources for improvement of film quality. Also presented are a few preliminary theoretical calculations for high-energy ion-beam mixing of the Mo/Al bilayer thin films.
- Quantitative analysis of fiber texture in cubic filmsRao, Satish I.; Houska, Charles R. (American Institute of Physics, 1983)The method of Roe and Krigbaum for determining fiber texture has been extended to thin film applications. It is often desirable to make i n s i t u pole density measurements of a film on a thick substrate. This does not permit complete data to be collected by x‐ray transmission techniques. If the data are restricted to the range 0≤χ≤75°, obtained by reflection, incomplete pole density plots are obtained, and it is necessary to devise a self‐consistent extrapolation technique that extends the pole density data to 90°. This requires least‐squares fitting over the range from χ=0 to 75° and an iterative procedure for extrapolating with functions consistent with a single orientation function. The example of a 1.14‐μ Mo film on a (111) Si substrate requires an expansion of the symmetry relations to order 46. The procedure established herein is readily extended to include the effect of static displacements resulting from embedded gas atoms that are associated with sputtered films.
- Residual strain gradients in a fully stabilized zirconia sampleHwang, Bing; Houska, Charles R.; Ice, Gene E.; Habenschuss, Anthony (American Institute of Physics, 1988-06-01)Polished and severely ground fully stabilized zirconia samples are examined using primarily x‐ray diffraction(XRD). The XRD (111) profile reflections from both samples were broadened asymmetrically compared to that of an annealed sample. The asymmetry results from a d‐spacing gradient extending from the free surface into undisturbed bulk material. There are two possible origins of this depth gradient, i.e., variations in residual strain or chemical composition. The latter is eliminated by means of x‐ray photoelectron spectroscopy which did not reveal a chemical gradient. d‐spacing profiles for both samples are obtained nondestructively using a trial and error fitting procedure. A maximum compressive strain of ∼4% is obtained at the surface of the ground sample which decreases gradually to zero at greater depths. The overall zone is ∼1–2 μm. A similar but smaller compressive zone is found in the polished sample which is followed by a zone of tension. The maximum compressive strain at the surface is ∼5% and the overall zone of residual strain is ∼0.1 μm.
- Simplifications in the x‐ray line‐shape analysisAdler, Thomas; Houska, Charles R. (American Institute of Physics, 1979)It is shown that a Fourier series associated with the Warren‐Averbach line‐shape analysis can be fitted with only five parameters to a pair of peaks. These interrelate the Fourier coefficients and thereby provide a simplified series which has been applied to the study of a Mo film on a Si crystal. The parameters include the average particle size, the first neighbor rms strain, a term which gives the variation in rms strain with cell separation, and two instrumental broadening coefficients. Although considerable simplification is possible, equivalent information can be obtained as compared with the original analysis and the ’’hook effect’’ is eliminated in the fitted coefficients.
- A simplified procedure for obtaining relative x‐ray intensities when a texture and atomic displacements are presentHe, Baoping; Rao, Satish I.; Houska, Charles R. (American Institute of Physics, 1994-05-01)The orientation problem in polycrystalline cubic materials has been simplified, using fundamental relationships, so that the determination of a quantitative interrelationship between the various Bragg peak intensities is no longer a formidable task. This is demonstrated with a cubic Cu-Be-Co alloy having a fiber texture, and a conventional focusing diffractometer. Because data are required which extend over a larger range in d spacings, extinction, thermal, and static atomic displacements must be included into the analysis of intensities. The displacement terms and the extinction parameters may be of primary interest or used as a correction. Seventeen diffraction peaks are used in the example. These must be internally consistent with a crystallite orientation function, the cubic symmetry of the sample, extinction effects influencing the two strongest peaks, and attenuation due to atomic displacements. Tabulated coefficients are presented which greatly reduce the task of calculating the orientation function. A correction is given for instrumental smearing which should be considered for stronger textures than the intermediate case examined or for intermediate textures and nonfocusing instrumental conditions.