Showing papers in "Journal of Materials Research in 1994"

The radiation-induced crystalline-to-amorphous transition in zircon

Abstract: A comprehensive understanding of radiation effects in zircon, ZrSiO4, over a broad range of time scales (0.5 h to 570 million years) has been obtained by a study of natural zircon, Pu-doped zircon, and ion-beam irradiated zircon. Radiation damage in zircon results in the simultaneous accumulation of both point defects and amorphous regions. The amorphization process is consistent with models based on the multiple overlap of particle tracks, suggesting that amorphization occurs as a result of a critical defect concentration. The amorphization dose increases with temperature in two stages (below 300 K and above 473 K) and is nearly independent of the damage source (α-decay events or heavy-ion beams) at 300 K. Recrystallization of completely amorphous zircon occurs above 1300 K and is a two-step process that involves the initial formation of pseudo-cubic ZrO2.

Thermal properties of organic and inorganic aerogels

Abstract: Aerogels are open-cell foams that have already been shown to be among the best thermal insulating solid materials known. This paper examines the three major contributions to thermal transport through porous materials, solid, gaseous, and radiative, to identify how to reduce the thermal conductivity of air-filled aerogels. We found that significant improvements in the thermal insulation property of aerogels are possible by (i) employing materials with a low intrinsic solid conductivity, (ii) reducing the average pore size within aerogels, and (iii) affecting an increase of the infrared extinction in aerogels. Theoretically, polystyrene is the best of the organic materials and zirconia is the best inorganic material to use for the lowest achievable conductivity. Significant reduction of the thermal conductivity for all aerogel varieties is predicted with only a modest decrease of the average pore size. This might be achieved by modifying the sol-gel chemistry leading to aerogels. For example, a thermal resistance value of R = 20 per inch would be possible for an air-filled resorcinol-formaldehyde aerogel at a density of 156 kg/m3, if the average pore size was less than 35 nm. An equation is included which facilitates the calculation of the optimum density for the minimum total thermal conductivity, for all varieties of aerogels.

Orientation of rapid thermally annealed lead zirconate titanate thin films on (111) Pt substrates

Abstract: The nucleation, growth, and orientation of lead zirconate titanate thin films prepared from organometallic precursor solutions by spin coating on (111) oriented platinum substrates and crystallized by rapid thermal annealing was investigated. The effects of pyrolysis temperature, post-pyrolysis thermal treatments, and excess lead addition are reported. The use of post-pyrolysis oxygen anneals at temperatures in the regime of 350–450 °C was found to strongly affect the kinetics of subsequent amorphous-pyrochlore-perovskite crystallization by rapid thermal annealing. The use of such post-pyrolysis anneals allowed films of reproducible microstructure and textures [both (100) and (111)] to be prepared by rapid thermal annealing. It is proposed that such anneals and pyrolysis temperature affect the oxygen concentration/average Pb valence in the amorphous films prior to annealing. Such changes in the Pb valence state then affect the stability of the transient pyrochlore phase and thus the kinetics of perovskite crystallization.

Size dependent thermal vibrations and melting in nanocrystals

Abstract: A simple model for the size-dependent amplitude of the atomic thermal vibrations of a nanocrystal is presented which leads to the development of a model for the size dependent melting temperature in nanocrystals on the basis of Lindemann's criterion. The two models are in terms of a directly measurable parameter for the corresponding bulk crystal, i.e., the ratio between the amplitude of thermal vibrations for surface atoms and that for interior ones. It is shown that the present model for the melting temperature offers not only a qualitative but even an excellent quantitative agreement with the experimentally observed size-dependent superheating, as well as melting point suppression in both the supported and embedded metallic and semiconductor nanocrystals.

ARXPS studies of SiO2-SiC interfaces and oxidation of 6H SiC single crystal Si-(001) and C-(001) surfaces

Abstract: The main puzzle in oxidation of hexagonal SiC is the slower rate of the Si-terminated surface as compared to the C-terminated surface, which is blamed on an unknown interface compound. ARXPS is a unique method to identify minor amounts of interface compounds, especially for smooth surfaces. Our ARXPS analysis of oxidized Si-(001) and C-(001) surfaces of 6H SiC reveals the interface oxide Si4C4−xO2 (x < 2), likely a reaction product of a peroxidic O2-bond to a SiC double layer. Si4C4−xO2 occurs in larger thickness (≃1 nm) at the slowly oxidizing Si-(001) surface, whereas the C-(001) surface shows smaller amounts, diminishing fast with oxidation above 1000 K. Evidence is presented that with increasing amount of Si4C4−xO2 the oxidation of SiC to SiO2 is reduced. ARXPS is consistent with a layer of SiO2 containing less than 3% Si4C4O4 being an oxidation product of Si4C4−xO2. At the surface of SiO2, graphite and some Si4C4O4 exist, aside from standard adsorbates.

Nanowires formed in anodic oxide nanotemplates

Abstract: A simple electrochemical method is described for producing metal or semiconductor nanowires with diameters in the continuous range 10 to 200 nm. The technique involves a three-step process that begins with the electrochemical generation of an aluminum oxide template with uniform nanometer-sized pores, followed by the deposition of metal or semiconductor in them. The nanowires are then exposed for study or device fabrication by etching back the oxide matrix. Examples of cadmium nanowires fabricated by this technique are shown.

A new procedure for measuring the decohesion energy for thin ductile films on substrates

Abstract: A novel testing technique has been developed capable of measuring the interfacial fracture resistance, Γi, of thin ductile films on substrates. In this technique, the thin film on the substrate is stressed by depositing onto the film a second superlayer of material, having a large intrinsic stress, such as Cr. Subsequent processing defines a precrack at the interface between the film and the substrate. The strain energy available for driving the debond crack is modulated by varying the thickness of the Cr superlayer. Spontaneous decohesion occurs for superlayers exceeding a critical thickness. The latter is used to obtain Γi from elasticity solutions for residually stressed thin films. The technique has been demonstrated for Cu thin films on silica substrates.

On the role of ions in the formation of cubic boron nitride films by ion-assisted deposition

Abstract: We have investigated how ion irradiation can selectively promote the formation of dense sp3-bonded cubic boron nitride (cBN) over the graphite-like sp2-bonded phases. We have conducted a series of experiments using ion-assisted pulsed laser deposition in which either the ion mass (mion) or ion energy (E) was varied in conjunction with the ratio of ion flux to depositing atom flux (J/a). For a fixed ion energy and mass, there is a critical J/a above which cBN formation is initiated, a window of J/a values in which large cBN percentages are obtained, and a point at which J/a is so large that the resputter and deposition rates balance and there is no net film deposition, in agreement with Kester and Messier. As do Kester and Messier, we find that cBN formation is controlled by a combination of experimental parameters that scale with the momentum of the ions. However, unlike Kester and Messier, we do not find that cBN formation scales with the maximum momentum that can be transferred in a single binary collision, as either incorrectly formulated by Targove and Macleod and used by Kester and Messier, or as correctly formulated. Instead we observe that cBN formation best scales with the total momentum of the incident ions, (mionE)1/2. We also consider the mechanistic origins of this (mionE)1/2 dependence. Computer simulations of the interaction of ions with BN show that cBN formation cannot be simply scaled to parameters such as the number of atomic displacements or the number of vacancies produced by the ion irradiation. A critical examination of the literature shows that none of the proposed models satisfactorily accounts for the observed (mionE)1/2 dependence. We present a quantitative model that describes the generation of stress during ion-assisted film growth. The model invokes a kinetic approach to defect production and loss. We apply a simplified version of the model to cBN synthesis, and find that it predicts an approximate (mionE)1/2 dependence for cBN formation.

Deformation and fracture of mica-containing glass-ceramics in Hertzian contacts

Abstract: The Hertzian indentation response of a machinable mica-containing glass-ceramic is studied. Relative to the highly brittle base glass from which it is formed, the glass-ceramic shows evidence of considerable “ductility” in its indentation stress-strain response. Section views through the indentation sites reveal a transition from classical cone fracture outside the contact area in the base glass to accumulated subsurface deformation-microfracture in the glass-ceramic. The deformation is attributed to shear-driven sliding at the weak interfaces between the mica flakes and glass matrix. Extensile microcracks initiate at the shear-fault interfaces and propagate into the matrix, ultimately coalescing with neighbors at adjacent mica flakes to effect easy material removal. The faults are subject to strong compressive stresses in the Hertzian field, suggesting that frictional tractions are an important element in the micromechanics. Bend-test measurements on indented specimens show that the glass-ceramic, although weaker than its base glass counterpart, has superior resistance to strength degradation at high contact loads. Implications of the results in relation to microstructural design of glass-ceramics for optimal toughness, strength, and wear and fatigue properties are discussed.

Preparation of fine multicomponent oxide ceramic powder by a combustion synthesis process

Abstract: An important, rather novel procedure for the synthesis of submicron crystalline multicomponent oxide ceramic powders has been studied. The synthesis of CuFe2O4 powder, a ferrite material, has been used as a model system for understanding the synthesis process. The effect of the fuel content, powder packing, and surface heat loss has been investigated in terms of the maximum reaction temperature and reaction period, phase formation, and particle size and morphology. It has been shown that the maximum temperature and reaction period can be tailored to produce different phases. The submicron features of the synthesized powders are indicated by the large surface area values obtained from BET measurement.

Electrical-resistance of a Carbon Nanotube Bundle

Abstract: The first direct electrical resistance measurements performed on a single carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependence of the resistance above 2 K, it is shown that some nanotubes exhibit a semimetallic behavior akin to rolled graphene sheets with a similar band structure, except that the band overlap, DELTA almost-equal-to 3.7 meV, is about 10 times smaller than for crystalline graphite. In contrast to graphite which shows a constant low-temperature resistivity, the nanotubes exhibit a striking increase of the resistance followed by a broad maximum at very low temperatures. A magnetic field applied perpendicular to the sample axis decreases the resistance. Above 1 K, this behavior is consistent with the formation of Landau levels. At lower temperatures, the resistance shows an unexpected drop at a critical temperature which increases linearly with magnetic field. These striking features could be related to the unique quasi-one-dimensional structure of the carbon nanotubes.

Alloy thermodynamics in nanostructures

Abstract: The importance of the interactions between alloy atoms and topological defects for the thermodynamic properties of nanostructured alloys is pointed out. The McLean model for grain boundary segregation is extended to yield an expression for the total Gibbs free energy of an alloy polycrystal. This provides a simple conceptual basis for a qualitative discussion of the thermodynamic properties of nanocrystalline alloys. It is demonstrated that certain alloy poly- or nanocrystals may reach a metastable state, where the alloy is stable with respect to variation of its total grain boundary area.

Template synthesis of metal microtubule ensembles utilizing chemical, electrochemical, and vacuum deposition techniques

Abstract: Microtubules are an interesting type of microstructure that resemble miniature drinking straws. Such tubular microstructures are found in nature. In addition, we and others have been investigating strategies for making synthetic analogs. We are especially interested in the idea of making metal microtubules. Four procedures for preparing metal microtubules are described in this paper. The general approach, called template-synthesis, entails using the pores in a microporous membrane as templates for forming the tubules. Microporous anodic aluminum oxide membranes and nuclear track-etch membranes are used as the template membranes. Gold and silver microtubules are made with outer diameters as small as 200 nm. These microstructures are characterized by scanning electron microscopy.

Current-voltage characteristics of ultrafine-grained ferroelectric Pb(Zr, Ti)O 3 thin films

Abstract: Room-temperature current-voltage dependence of ultrafine-grained ferroelectric Pb(Zr, Ti)O3 thin films has been investigated. Both strong varistor type behavior and space charge limited conduction (SCLC) were observed. Differences in the current-voltage characteristics are attributed to differences in the nature of the grain boundaries resulting from variations in processing conditions. The strong varistor type behavior is believed to be due to the presence of highly resistive grain boundaries and thus may be termed grain boundary limited conduction (GBLC). A double-depletion-layer barrier model is used to describe the origin of high resistivity of the grain boundaries. It is suggested that the barrier height varies significantly with the applied field due to the nonlinear ferroelectric polarization, and that the barrier is overcome by tunneling at sufficiently high fields. In some other cases, the resistivity of the grain boundaries is comparable to that of the grains, and therefore the intrinsically heterogeneous films degenerate into quasi-homogeneous media, to which the SCLC theory is applicable. As such, a unified grain boundary modeling reconciles different types of conduction mechanisms in the ultrafine-grained ferroelectric thin films. This grain boundary modeling also well accounts for some other dc-related phenomena observed, including abnormal current-voltage dependencies, remanent polarization effect, electrode interface effect, and unusual charging and discharging transients. In addition, many other electrical properties of the ferroelectric films may be better understood by taking the effect of grain boundaries into account.

Hardness and Young's modulus of amorphous a -SiC thin films determined by nanoindentation and bulge tests

Abstract: Due to its interesting mechanical properties, silicon carbide is an excellent material for many applications. In this paper, we report on the mechanical properties of amorphous hydrogenated or hydrogen-free silicon carbide thin films deposited by using different deposition techniques, namely plasma enhanced chemical vapor deposition (PECVD), laser ablation deposition (LAD), and triode sputtering deposition (TSD). a-SixC1−x: H PECVD, a-SiC LAD, and a-SiC TSD thin films and corresponding free-standing membranes were mechanically investigated by using nanoindentation and bulge techniques, respectively. Hardness (H), Young’s modulus (E), and Poisson’s ratio (v) of the studied silicon carbide thin films were determined. It is shown that for hydrogenated a-SixC1−x: H PECVD films, both hardness and Young’s modulus are dependent on the film composition. The nearly stoichiometric a-SiC: H films present higher H and E values than the Si-rich a-SixC1−x: H films. For hydrogen-free a-SiC films, the hardness and Young’s modulus were as high as about 30 GPa and 240 GPa, respectively. Hydrogen-free a-SiC films present both hardness and Young’s modulus values higher by about 50% than those of hydrogenated a-SiC: H PECVD films. By using the FTIR absorption spectroscopy, we estimated the Si-C bond densities (NSiC) from the Si-C stretching absorption band (centered around 780 cm−1), and were thus able to correlate the observed mechanical behavior of a-SiC films to their microstructure. We indeed point out a constant-plus-linear variation of the hardness and Young’s modulus upon the Si-C bond density, over the NSiC investigated range [(4–18) × 1022 bond · cm−3], regardless of the film composition or the deposition technique.

Contribution of electrodes and microstructures to the electrical properties of Pb(Zr0.53Ti0.47)O3 thin film capacitors

Abstract: Pb(Zr0.53Ti0.47)O3 (PZT) thin film capacitors have been fabricated with four electrode combinations: Pt/PZT/Pt/SiO2Si, RuO2/PZT/Pt/SiO2/Si, RuO2/PZT/RuO2/SiO2/Si, and Pt/PZT/RuO2/SiO2/Si. It is shown that polarization fatigue is determined largely by the electrode type (Pt vs RuO2), and microstructure has only a second-order effect on fatigue. If either the top or bottom electrode is platinum, significant polarization fatigue occurs. Fatigue-free capacitors are obtained only when both electrodes are RuO2. In contrast, the bottom electrode is found to have a major effect on the leakage characteristics of the PZT capacitors, presumably via microstructural modifications. Capacitors with bottom RuO2 electrodes show high leakage currents (J = 10−3-10−5 A/cm2 at 1 V) irrespective of the top electrode material. Capacitors with Pt bottom electrodes have much lower leakage currents (J = 10−8 A/cm2 at 1 V) irrespective of the top electrode material. At low voltage, the I-V curves show ohmic behavior and negligible polarity dependence for all capacitor types. At higher voltages, the leakage current is probably Schottky emission controlled for the capacitors with Pt bottom electrodes.

Physical properties of A-C:N films produced by ion beam assisted deposition

Abstract: Carbon films with up to 32 at. % of nitrogen have been prepared with ion beam assisted magnetron, using a N[sup +][sub 2]/N[sup +] beam at energies between 50 and 300 eV. The composition and density of the films vary strongly with the deposition parameters. EELS, SXS, XPS, and IR studies show that these a-C:N films are mostly graphitic and have up to 20% [ital sp][sup 3] bonding. Nitrogen is mostly combined with carbon in nitrile (C[equivalent to]N) and imine (C=N) groups. RBS and NDP show that density goes through a maximum as the average damage energy per incoming ion increases. Positron annihilation spectroscopy shows that the void concentration in the films goes through a minimum with average damage energy. These results are consistent with a densification induced by the collisions at low average damage energy values and induced graphitization at higher damage energy values. These results are similar to what is observed for Ar ion assisted deposition of a-C films. The mechanical properties of these films have been studied with a nanoindenter, and it was found that the hardness and Young's modulus go through a maximum as the average damage energy is increased. The maximum of mechanical properties correspondsmore » to the minimum in the void concentration in the film. Tribological studies of the a-C:N show that the friction coefficient obtained against diamond under dynamic loading decreases strongly as the nitrogen composition increases, this effect being more pronounced at low loads.« less

Order-disorder in a2m3+m5+o6 perovskites

Abstract: Using x-ray and neutron diffraction data, the degree of order of the octahedral site cations has been determined for the perovskites Sr2AlNbO6 and Sr2AlTaO6, which have been prepared by several different methods and annealed at temperatures up to 1690 °C. The degree of order generally increases with increasing synthesis temperature. The amount of cation ordering is, therefore, primarily controlled by kinetic processes and not by thermodynamic equilibrium considerations. Increased order obtained with increased heating time confirms this general kinetic limitation on the degree of order. However, annealing Sr2AlNbO6 in the highest temperature region resulted in some decrease in order, presumably due to thermodynamic considerations. The cubic edge of both compounds decreases significantly with increasing order. Ordered domains are separated by antiphase boundaries which occur in high concentrations. The cubic cell edge within the ordered domains is significantly smaller than the overall cell edge when the concentration of antiphase boundaries is high. The antiphase boundaries cause significant lattice strain which generally decreases as the concentration of antiphase boundaries decreases. Results on other A2M3+M5+O6 systems are briefly presented.

Mechanical properties of compositionally modulated au-ni thin films : nanoindentation and microcantilever deflection experiments

Abstract: The mechanical properties of compositionally modulated Au-Ni films were investigated by submicrometer depth-sensing indentation and by deflection of micrometer-scale cantilever beams. Films prepared by sputter deposition with composition wavelengths between 0.9 and 4.0 nm were investigated. Strength was found to be high and invariant with composition wavelength. Experimental and data analysis methods were developed to provide more accurate and precise measurements of elastic stiffness. Large enhancements in stiffness (the “supermodulus effect”) were not observed. Rather, relatively small but significant minima were observed at a composition wavelength of about 1.6 nm by both techniques. These variations were found to be strongly correlated with variations in the average lattice parameter normal to the plane of the film. Both structural and mechanical property variations are consistent with a simple model in which the film consists of bulk-like Au and Ni layers with interfaces of constant thickness.

Crystallization of nanosized titania particles prepared by the sol-gel process

Abstract: Sols of titania were obtained by the sol-gel method and their size profile was followed by dynamical light scattering. In the early stages of the reaction an unstable behavior was detected. After this unstable regime the particle size reaches a steady state where the sols have a constant size while increasing in number. Once the sol concentration reaches its overlap value, the gelation regime takes place. For samples prepared in this way Raman spectra and x-ray diffractometry were used to characterize the kinetics of crystallization of the material.

Oxygen availability in mixed cerium/praseodymium oxides and the effect of noble metals

Abstract: Oxyreduction studies of mixed Ce/Pr oxides have been carried out. Temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO) were used to study the uptake and release of the oxygen. Large amounts of oxygen, exceeding those in ceria, are accessible in the mixed metal oxides at moderate temperatures. The addition of small amounts of noble metals to the mixed oxides shifts the accessibility of the “stored” oxygen to still lower temperatures with the effect of Pd being more pronounced than that of Pt. In a sample containing 45 mol % ceria and 55 mol % praseodymia, a small addition of Pd (0.24 mol %) was found to lower the reduction temperature by more than 100 °C. The addition of Pt had a lesser effect. Similarly, in pure praseodymia (Pr6O11) Pd influences the reduction much more strongly than Pt. In the mixed samples, whether doped with a noble metal or not, the whole oxyreduction effect can be accounted for by the change in oxidation state of the praseodymium ions solely. This notwithstanding, the reduction of the mixed oxides, without noble metals or doped by Pt, is more facile than that of praseodymia. Only the incorporation of Pd makes the reduction of praseodymia proceed at a temperature below that registered for a mixed ceriapraseodymia sample.

Microwave sintering of hydroxyapatite ceramics

Abstract: Hydroxyapatite ceramics have been fabricated by microwave sintering in a 500 W microwave oven. Circular-plate specimens of various green densities were sintered in the oven at 1200 and 1300 °C, for 5, 10, and 20 min, respectively. Ceramics with density up to 97% of the theoretical were obtained. Density, grain size, microstructure, and strength of the ceramics sintered by microwave and by conventional methods were compared. The results show that microwave sintering of hydroxyapatite is not only highly efficient in saving time and energy, but can also improve the microstructure and thus enhance mechanical strength of the ceramics.

Effects of electronic and recoil processes in polymers during ion implantation

Abstract: It has been shown that ion implantation produces remarkable improvements in surface-sensitive mechanical properties, as well as other physical and chemical properties in polymers. To understand mechanisms underlying such property changes, various polymeric materials were subjected to bombardment by energetic ions in the range of 200 keV to 2 MeV. The magnitude of property changes is strongly dependent upon ion species, energy, and dose. Analysis indicated that hardness and electrical conductivity increased by employing ion species with larger electronic cross sections and with increasing ion energy and dose. The results showed that electronic stopping or linear energy transfer (LET, energy deposited per unit track length per ion) for ionization was the most important factor for the enhancement of hardness, while nuclear stopping or linear energy transfer for displacement generally appeared to reduce hardness.

High temperature oxidation of ion-plated CrN films

Abstract: The high-temperature oxidation of CrN films which were deposited onto stainless steel substrates using an arc ion plating apparatus was studied at temperatures ranging from 1023 to 1173 K for 0.6 to 480 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. Formed oxide layers were analyzed by XRD, SEM, and SAM. An activation energy of the oxidation of CrN was slightly lower than that of the self-diffusion coefficient of Cr ion in Cr2O3. It is concluded that the oxidation of CrN is controlled by the outward diffusion of Cr ions through the Cr2O3 layer formed on each CrN grain.

Scanning tunneling spectroscopy of carbon nanotubes

Abstract: Calculations predict that carbon nanotubes may exist as either semimetals or semiconductors, depending on diameter and degree of helicity. This communication presents experimental evidence supporting the calculations. Scanning tunneling microscopy and spectroscopy (STM-S) data taken in air on nanotubes with outer diameters from 17 to 90 A show evidence of one-dimensional behavior; the current-voltage (I-V) characteristics are consistent with a density of states containing Van Hove type singularities for which the energies vary linearly with inverse nanotube diameter.

Effect of intergranular glass films on the electrical conductivity of 3Y-TZP

Abstract: The electrical conductivity of 3Y-TZP ceramics containing SiO2 and Al2O3 has been investigated by complex impedance spectroscopy between 500 and 1270 K. At low temperatures, the total electrical conductivity is suppressed by the grain boundary glass films. The equilibrium thickness of intergranular films is 1-2 nm, as derived using the “brick-layer” model and measured by HRTEM. A change in the slope of the conductivity Arrhenius plots occurs at the characteristic temperature Tb at which the macroscopic grain boundary resistivity has the same value as the resistivity of the grains. The temperature dependence of the conductivity is discussed in terms of a series combination of RC elements.

Heteroepitaxial growth of chemically derived ex situ Ba2YCu3O7−x thin films

Abstract: Heteroepitaxial growth of Ba2YCu3O7−x (BYC) thin films prepared by postdeposition annealing on (001) LaAlO3 was characterized by TEM and x-ray diffraction studies of specimens rapidly cooled from various points in the growth heat treatment Heteroepitaxial nucleation of BYC occurred between 720 and 770 °C during heating at 25 °C/min to the annealing temperature of 830 °C The c-axis normal BYC rapidly coalesced into a continuous film with nearly complete coverage of the substrate surface after growth of a film of several unit cells thickness The experimental results were not consistent with purely solid phase heteroepitaxial nucleation and growth or epitaxial grain growth, mechanisms for microstructural evolution of other chemically derived epitaxial oxide thin films The nature of the transformation and the microstructure of the final superconducting films were consistent, instead, with growth of epitaxial BYC from a liquid phase that is present transiently during the anneal This hypothesis was supported by thermal analysis results obtained from the precursor material of which the films are composed prior to transformation to BYC

Preparation, characterization, and sinterability of well defined silica/yttria powders

Abstract: Dispersions of uniform submicron spherical particles consisting of silica cores and yttria coatings, or vice versa, were prepared by a precipitation technique. The overall size of the particles and the thickness of the shells could be varied over a wide range. Such powders were used to form green bodies by sedimentation, centrifugation, or pressure filtration, and the density and the pore size distribution of the resulting solids were evaluated. The green bodies were sintered and the changes in density, phases, and microstructure were followed with temperature. In general, the coated powders exhibited enhanced densification. On processing composite solids at temperatures <1000 °C, the formation of Y2Si2O7 took place, which caused a pronounced shrinkage of the samples. Powders of coated particles having the same silica/yttria ratios sintered at lower temperatures when the shell was composed of silica rather than of yttria. When either silica or yttria were in molar excess in the coated particles, the sintered products had a mixed composition of Y2Si2O7 and the component in excess. By terminating the sintering process before the grain growth started, the solids displayed a well-defined microstructure with a uniform distribution of areas of one phase in the matrix of the matter in excess. This property was mainly due to the uniformity of initial powders in terms of the particle size and the coating.

Vapor phase synthesis of al-doped titania powders

Abstract: The role of aluminum as dopant in gas phase synthesis of titania powders was experimentally investigated in an aerosol flow reactor between 1300 and 1700 K. Titania was produced by vapor phase oxidation of titanium tetrachloride in the presence of dopant aluminum trichloride vapor. The presence of aluminum altered the particle morphology from polyhedral to irregular crystals. Energy dispersive analysis and transmission electron microscopy indicated that the powders were mixtures of crystalline titania and amorphous alumina. Analysis by XPS indicated significant enrichment of aluminum on the particle surface. Some aluminum titanate (up to 17% by volume) was formed at 1700 K when a high concentration of AlCl3 was used (AlCl3/TiCl4 ≥ 0.07). Measurements of lattice parameters by x-ray diffraction indicated that aluminum formed a solid solution in titania. While titania synthesized in the absence of aluminum was about 90% anatase, the introduction of aluminum resulted in pure rutile at AlCl3/TiCl4 = 0.07. The effects of aluminum on titania phase composition and morphology are explained by the creation of oxygen vacancies in the titania crystallites and by the enhancement of the sintring rate of titania grains.

Formation kinetics of PbZrxTi1−xO3 thin films

Abstract: The pyrochlore to perovskite transition in sputtered PZT thin films has been studied using SEM and XRD. The films were annealed in the temperature range between 350 °C and 750 °C, and the transition temperature for pyrochlore to perovskite transition was found to be around 525 °C. Isothermal annealing was used to study the nucleation and growth kinetics of the perovskite phase. The results showed a linear growth rate for the perovskite phase, thereby indicating an interface controlled process. Also, the growth was found to be isotropic in two dimensions parallel to the plane of the substrate. The nucleation of the perovskite phase was found to be random. The effective activation energy of the perovskite transition was found to be 494 kJ/mol using Avrami’s approach.