Agglomeration problems in the formation of FeSi2 from Fe-Si thin-film couples

The agglomeration process that occurs during annealing of thin Fe films (<200 nm) on Si substrates has been studied. Agglomeration occurred on uncapped films prior to silicide formation. Capping layers of SiO₂, or more appropriately Si-SiO₂, have been used to minimize agglomeration effects. Continuous thin films of -FeSi₂ have been grown on 11 1 and 100 oriented substrates. Preferred growth of (202) and (220) suicide planes on 111 Si has been obtained, while preferential growth was not observed on 100 Si.     

The role of starting powder size on the compressive response of stand-alone plasma-sprayed alumina coatings

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 m, 19 m and 32 m alumina; the resulting effective lamella diameters were 10 m, 28 m, and 55 m, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 m powder exhibited greater moduli than tubes sprayed from either 19 or 32 m powders. This difference was attributed to the greater percentage of unmelted -Al₂O₃ in the coating.     

Sputtered Films of PdMn for Thermometry and Bolometry

Measurements of the magnetic sensitivity of thin sputtered films of PdMn alloy demonstrate the viability of this material for high resolution thermometry. The thinnest films (thickness 1.0 m) show significant domain scale noise below the Curie Temperature, T_c, while thicker films (thickness 10 m) show reliable non-hysteretic behavior throughout the temperature range of interest. The thin films show the effects of demagnetization with the field perpendicular to the surface, but a fine screen in this orientation shows good response with no evidence of saturation and a manageable degree of demagnetization.     

Vacuum deposited amorphous Se-Te photoconductor films: fractionation effects and charge transport in homogeneous films

A series of a-Se:Te photoconductor films of thickness 60 m were prepared by conventional vacuum deposition onto heated Al substrates to study fractionation effects. There is a Te-rich surface region, a plateau of relatively uniform bulk region, and a Se-rich zone near the substrate. For a source material of Se:12.5% Te (all compositions in wt %), the substrate surface of the film was about 5.5–6.2% Te and the top surface was Te-rich up to about 25% Te content. The plateau region is between approximately 10 m to 40 m from the substrate and has about 10% Te. The existence of the plateau region provides a means of preparing relatively homogeneous films by suitably shuttering the substrate during the evaporation to capture this plateau region. A first-order fractionation model of the evaporation process was developed to account for the observed compositional variation across the films. The fractionation model applies concepts from mass transport to the melt by considering a surface region (SR) of a fixed number of atoms in which rapid diffusional changes maintain a relatively uniform Te composition that changes over the course of the evaporation process. The model is able to explain the general features of the experimental results and is in reasonable quantitative agreement with the experiments. A series of nearly homogeneous a-Se_1-xTe_t films (composition homogeneous to within 10%) were prepared by suitably shuttering the vacuum deposition process to capture the plateau region. Charge transport in both homogeneous and fractionated films was studied using the conventional time-of-flight (TOF) and interrupted time-of-flight (IFTOF) transient photoconductivity experiments. As a-Se is alloyed with Te, the hole drift mobility _h decreases rapidly with the Te content up to about 5% Te beyond which the decrease in the mobility becomes small. Both hole and electron drift mobility has been found to depend on the electric field E as E ⁿ where n is a small index that increases with the Te content in agreement with earlier work on probably fractionated samples. Consequently, the power law dependence is not an artifact of the fractionation effect. Hole lifetime in Cl-doped a-Se:Te films from IFTOF experiments was found to increase with the Te composition, nearly in proportion to the decrease in the drift mobility, in agreement with the shallow-trap-controlled model proposed for hole transport in a-SeTe films.     

Germanium selective solar absorber surfaces

Thin Ge films, produced by evaporation through 0.3, 1 and 3 torr pressures of argon onto polished aluminum substrates were found to exhibit selective absorption behaviour. Maximum absorptance to emittance ratios of 131 were measured, with values of 81 for films having solar absorptances of 0,9. Film structures were significant in determining collection efficiency. Films deposited at 0.3 torr showed better overall properties due to a larger particle size of about 1 m compared with an approximate 0.1 m size measured with films deposited at 1 and 3 torr.On leave from Australian Atomic Energy Commission, Sydney, Australia.     

Discrete β-Ta2O5 crystallite formation in reactively sputtered amorphous thin films

The crystallization of thin amorphous TaOx films formed by d.c. reactive sputtering was investigated at temperatures from 500–700 °C. The films remained amorphous for times up to 100 h at 500 °C. The formation of discrete, single crystallites of the orthorhombic -Ta2O5 phase was observed after annealing at 600 °C for times from 8–108 h. The crystallites were 0.35 m×0.35 m after 8 h and grew to approximately 2.5 m×2.0 m after 108 h. A (2 0 0) fibre texture with a 6° spread was observed. More rapid in-plane growth in the [0 1 0] direction resulted in a near-rectangular shape and is attributed to a ledge growth mechanism. Higher temperature anneals at 650 and 700 °C produced less-textured polycrystalline films with remnant amorphous regions. © 1998 Kluwer Academic Publishers     

Sputtered lead scandium tantalate thin films: a microstructural study

Lead scandium tantalate (PST) thin films have been deposited on a platinized silicon substrate with and without a buffer layer of MgO at the temperature of 525°C. It was found that PST films on the substrate without a buffer layer were strongly (1 1 1) oriented perovskite, whilst films on the substrate with a buffer layer showed the presence of second-phase pyrochlore, and the films were (1 1 1) and (1 1 0) oriented. These structural differences were believed to result from the structural differences between the platinum layers immediately below the respective PST layers. The lines which divide PST into network of islands were found to be no more than wider grain boundaries, rather than cracks as believed previously. Micro-beam diffraction and energy dispersive X-ray analysis showed that grain boundaries were tantalum-rich and lead-deficient compared to perovskite grain centres. Electrical properties, such as relative permittivity and dielectric loss, for the films were also measured.     

Simultaneous Double-Sided Deposition of Large Size HTSC Thin Films

An apparatus for simultaneous deposition of large size double-sided YBCO thin films by sputtering from single inverted cyclindrical target was constructed. Double-sided YBCO thin films on LaAlO₃ (100) substrate up to 30 mm in diameter were prepared to test the performance of the apparatus. The microwave surface resistance, R _s (77 K, 10 GHz, 0 T), of the YBCO thin films on both sides of the wafer ranged from 500 to 800 . Values of R _s (75 K, 145 GHz, 0 T) below 60 m were reached over the area of 20 mm × 15 mm on the wafer. The majority of the wafer area given in percent has R _s (75 K, 145 GHz, 0 T) values in the range of 21.5– 45.5 m. Lateral homogeneity of R _s values in the whole wafer was good enough to well meet the microwave application.     

Single-Mode As–S Glass Fibers

Single-mode As–S glass fibers with a core diameter from 3 to 20 m and a clad diameter of 125 m are prepared by the double-crucible method. The cutoff wavelength of the fibers is 0.9–6 m. The lowest transmission losses in the fibers at 2.2–2.3 m are 100 dB/km, and their mean bending strength is 800–1000 MPa.     

Size effects on the mechanical behavior of gold thin films

The Membrane Deflection Experiment was used to test the mechanical response of freestanding thin film gold specimens. We present stress-strain curves obtained on films 0.3, 0.5, and 0.1 m thick. Elastic modulus was consistently measured in the range of 53–55 GPa. Several size effects on the mechanical properties were observed including yield stress variations with membrane width and film thickness. It was observed that a width of 2.5 m and a thickness of 0.5 m correspond to major transitions in the material deformation behavior.     

Microstructure and crystallographic texture of rolled polycrystalline Fe3Al

An imperfectly B₂ ordered Fe₃Al aggregate was cast, thermomechanically hot rolled and finally annealed at 870 K. Subsequently, the specimen was rolled at 800–830 K to a strain of 80%. The microstructure and the crystallographic texture of the rolled polycrystalline sample was investigated within the range =20–80%. The microstructure consisted of flat, elongated grains. In numerous grains straight slip lines were detected. Even after =80% recrystallization was not observed. The rolling texture of Fe₃Al considerably deviates from that of non-ordered body centered cubic (b.c.c.) alloys and pure b.c.c. metals. The {111}uvw texture fibre (7-fibre) was very pronounced, while the {hkl}110 fibre (-fibre) was very weak. The {112}110 orientation which represents the strongest texture component in non-ordered b.c.c. alloys did not occur at all. The textures are discussed in terms of the {110}111, {112}111, {112}111 and {123}111 slip systems. The contribution of crystallographic slip of the various types of potential slip systems was simulated by means of the Taylor theory.     

On the formation of thermally sprayed alumina coatings

A model for the fomation of thermally sprayed alumina coatings is proposed. The spreading and crystallization of liquid droplets on impact with the substrate are analysed and the thermal history of individual particles related to the kinetics of nucleation of -Al₂O₃ to other forms. The results suggest that under the usual spraying conditions undercooling of the liquid droplets is such that -Al₂O₃ nucleates in preference to -Al₂O₃ and the cooling rate after solidification is sufficiently rapid to prevent transformation to -Al₂O₃ or -Al₂O₃. Transformation of initially formed -Al₂O₃ to -Al₂O₃ appears to be possible only if the lamellae formed on impact are thicker than about 10 m if the substrate is heated to about 1000° C, or if the thickness is greater than about 20 m on an unheated substrate. The -Al₂O₃ generally observed in thermally sprayed coatings is the result of crystallization from pre-existing nuclei arising from incomplete melting of the feed material.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

Hydrothermal synthesis of barium titanate fine particles from amorphous and crystalline titania

Barium titanate fine particles of cubic system were synthesized hydrothermally from aqueous barium hydroxide solutions with fine particles of either amorphous or crystalline (rutile) titania in suspension. The mean size of barium titanate particles prepared from amorphous titania ranged from 0.03 m to 0.11 m, depending on hydrothermal conditions. The particle size approximately agreed with the crystallite size (0.04–0.09 m). On the other hand, the mean size of barium titanate particles prepared from rutile titania ranged from 0.2 to 0.7 m, which was about six times as large as the crystallite size. The difference of sizes of barium titanate particles prepared might be ascribed to the difference in dissolution rates of amorphous or crystalline titania particles.Nomenclature mean particle size, m - BT barium to titanium molar ratio - d crystallite size, m - K shape factor appearing in Equation 3 - half value breadth of diffraction peak - lattice strain - Bragg diffraction angle, degree - wave length of X-ray, urn - _g geometric standard deviation defined by 84.1%-size/50%-size in a log normal size distribution     

Diaphyseal fractures treated by polylactide and hydroxyapatite pins. Experimental study in rat

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO₃)₂ + 6(NH₄)₂HPO₄ + 8NH₄OH), could produce dense coatings when sintered at 875–1000°C. Single EPD coatings cracked during sintering owing to the 15–18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the valleys in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be 12 MPa on a titanium substrate and 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since -316L (20.5 m mK^-1) > -HAp (14 m mK^-1), resulting in residual compressive stresses in the coating, whereas -titanium (10.3 m mK^-1) < -HAp, resulting in residual tensile stresses in the coating. © 1999 Kluwer Academic Publishers     

Preparation of barium titanate ultrafine particles from amorphous titania by a hydrothermal method and specific dielectric constants of sintered discs of the prepared particles

Barium titanate ultrafine particles were synthesized from amorphous titania by a hydrothermal method. The mean size of the barium titanate particles prepared at a hydrothermal treatment time of 4 h, was nearly equal to 0.04 5 m in the range of barium-to-titanium molar ratio (BT) 2, and approximately agreed well with the crystallite size. At a BT molar ratio of 1.0, the mean particle size increased to 0.2 m, while the crystallite size remained constant at 0.045 m. When the particle size ranged from 0.12–0.20 m, prepared for the BT molar ratio of 1.0–1.4, the specific dielectric constant for a sintered disc composed of these particles attained a value of 5000 or more. As the BT molar ratio increased to exceed 1.5, when the mean particle size decreased from 0.13 m to 0.045 m, the specific dielectric constant for the sintered disc was decreased greatly. The specific dielectric constant for the sintered disc can be correlated well with the size of the composing particles.     

Measurement of the Micrometer Diameters of Focused Laser-Beam Radiation with a Thin-Wire Bolometer

The profile distributions of a focused Gaussian beam of 63-m laser radiation for E- and H-polarizations were recorded by a thin-wire bolometer 3 m in diameter. The profiles were used to determine the beam diameters, which were D ^E = (8.16 ± 0.12) m and D ^H = (8.00 ± 0.12) m upon elimination of the dominant systematic errors.     

Laser surface melting of Ti-6Al-4V alloy

Surface hardening of Ti-6Al-4V alloy with laser surface melting (LSM) in a nitrogen atmosphere has been studied. In LSM, hardness increased almost three-fold in comparison to that of the substrate, the latter having a Vickers hardness of 350, by the formation of TiN in the range of 100m of melt depth. Hardness, then, decreased slowly and reached a minimum of 580 VHN at a maximum melt depth of 750m. -Ti was formed in the heat-affected zone (HAZ) with a VHN of 450. Ageing treatments were performed for all specimens at 450 °C and different ageing times (1–20h). Short ageing treatments increased the hardness in the melted zone as well as in the HAZ (1–3h). Long ageing treatments (7–20h) resulted in uniform hardness distribution in the melted zone.     

Particle size control of 1,3,5-triamino-2,4,6-trinitrobenzene by recrystallization from DMSO

Batches of up to 46g of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been recrystallized from DMSO in an effort to prepare larger particle-size material for recycling previously-used TATB and also for use in special formulations. The first part of the study investigated the conditions required to shift the particle-size distribution maximum from 50–70 m to several hundred micrometres in diameter. Distributions peaking at 200–246 m were successfully produced by varying the cooldown rate and degree of agitation during cooling. The second part of the study emphasized regeneration of the standard 50–70 m distribution from submicron size (ultrafine TATB) particles. The distributions peaking at 76–88 m, 27–31 m, and 15–17 m, successfully bracketed the target particle sizes, were grown by changing the degree of solution saturation. The choice of saturation temperature for the TATB/DMSO solution was based on earlier small-scale recrystallization and solubility work.     

Preparation of Thick LaCu1 – x Ni x O2.5 + δ Films Exhibiting a High-Temperature Metal–Semiconductor Transition

The conditions for producing thick LaCu_{1 – x} Ni _x O_{2.5 +} films on different substrates were optimized. The effects of the heat-treatment conditions, substrate material, and the nature of the liquid organic binder on the composition, structure, and properties of the films were studied. Single-phase coatings obtained on MgO, ZrO₂, BaSO₄, and Ni substrates 50 to 200 m in thickness were close in properties to bulk LaCu_{1 – x} Ni _x O_{2.5 +} and exhibited a metal–semiconductor transition at about 500°C.