Ion-implantation technology for improved GaAs MESFETs performance

Electrical properties of Si-implanted and co-implanted with Mg or Be in semi-insulating GaAs was studied. The Si-implanted MESFETs with and without buried p-layer (formed by Mg or Be) have been fabricated and characterized by their d.c. and r.f. performance. The experimental results showed that the device with a buried p-layer can effectively suppress the substrate leakage current (thus good pinch-off characteristic) and obtained higher gain linearity than these without a buried p-layer. For 1 m×100 m MESFETs device with co-implantation of Si (8×10¹² cm^–2) and Be (6×10¹¹ cm^–2) demonstrated uniform transconductance (g_m) of 115 mS mm^–1 with the gate voltage ranging from –1 to 1 V and reduced pinch-off voltage compared to those with co-implantation of Si and Mg (6×10¹¹ cm^–2). The measured f_T and f_max of a 1 m×25 m MESFET with co-implantation of Si and Be are 10 and 39 GHz, respectively. However, FETs with increased Mg dose (from 6×10¹¹ cm^–2 to 2×10¹² cm^–2) in a buried p-layer can obtain higher transconductance and saturation current.     

Synthesis of Fe-substituted Al-mordenites by hydrothermal method

Al-mordenite and Fe-mordenite were synthesized with the tetraethylammonium as atemplate by hydrothermal method at 150°C in the Na₂O–Al₂O₃–SiO₂–H₂O and Na₂O–Fe₂O₃–SiO₂–H₂O system, respectively. Synthesis of several Al-mordenites substituted with Fe, Al/Fe ratio = 75/25, 50/50, 25/75, were also attempted in the Na₂O–Al₂O₃–Fe₂O₃–SiO₂–H₂O system under the same conditions by the hydrothermal method. The continuous solid solution of Fe in Al-mordenite was successfully obtained in mordenites with various Al-Fe molar ratios. Al-mordenite crystal was tablet-like with approximately 20–30 m in diameter and 5–10 m in thickness. Fiber-like Fe-mordenite grew up to 20–30 m in length and 5 m in diameter. The morphology of Fe-substituted Al-mordenite was cubic-like with 5–10 m in size. The size of Fe-substituted Al-mordenite decreased with the increase of Fe-content.     

Absorption Spectrum of ZnO Precipitates in ZnSe

The absorption spectrum of ZnO precipitates in ZnSe saturated with oxygen is studied in the spectral range 500–2000 cm^–1 and is shown to correlate with the transmission spectrum of single-crystal ZnO. Saturation of ZnSe with oxygen in the course of growth leads to ZnO precipitation during cooling. The precipitates give rise to narrow absorption bands in the range 5.8–7.1 m, which correlate with the components of the multiphonon absorption spectrum of ZnO, formed by combinations of LO and TO phonons. The 2LO mode defines the long-wavelength transmission edge of ZnO. In addition, the spectrum shows strong absorptions due to the LO + TO(10.2 m) and 2TO (11.3 and 12.7 m) modes. The possible role of SeO₂ is discussed. It is suggested that ZnO with a high carrier concentration may act as a gray filter in the spectral range 2–9 m.     

Ferroelectric Lead Zirconate Titanate Films Prepared by Spray Pyrolysis of Carboxylate Solutions

Ferroelectric PbTi_0.6Zr_0.4O₃films 0.5–1.5 m in thickness were produced on platinum substrates by spray pyrolysis of carboxylate solutions. The optimized compositions of the precursor solutions, containing methacrylic acid and ethylene glycol, are stable under normal conditions, allow the annealing temperature to be reduced, and lead to higher quality film surfaces and large grains. The film exhibit the following electrical properties: T _C= 360–460°C, _max= 1750 at T _C, tan = 0.02–0.1 at 1 kHz and room temperature, P _{s max} = 18 C/cm², P _{r max} = 15 C/cm²at 50 Hz, and E _c= 42–120 kV/cm.     

Preparation of Si3N4 whiskers by reaction of wheat husks with NH3

-Si₃N₄ whiskers that are 1–10 mm long and 0.5–1.1 m thick were obtained by the reaction of wheat husks with NH₃ at 1250–1450 °C. A maximum whisker yield of about 30% was obtained at 1450 °C with the addition of an iron impurity. Whiskers with 1.3–2.2 m thickness (average 1.6 m) were obtained by the addition of an H₂S impurity. Thin whiskers with periodic thick and thin diameters were also obtained.     

Deposition from the vapour phase during induction plasma treatment of alumina powders

A study was carried out of the induction plasma melting of alumina powders (particle mean diameter, ¯d _p=24.5 m), (Ar/H₂ or Ar/N₂ plasma, plate power, 40 kW) under reduced pressure conditions (400 torr). The results reveal that in the process, partial vaporization of the alumina powders takes place in the hot region of the discharge. As the molten particles cool down and solidify, the deposits from the vapour phase was formed with the spheroidized particles. In all treatments with the Ar/H₂ and Ar/N₂ plasmas, a condensate of ultrafine alumina fume (d _p<200 nm) was obtained. The fine particles consisted essentially of metastable -, - and -phases. Needle-like crystals(0.1–0.3 m diameter, by 5–15 m long) were observed when operating with an Ar/N₂ plasma at powder feed rates exceeding 10 g min^–1. Electron diffraction analysis revealed that the needles were whiskers, whose structure was very similar to - or -aluminas with an hexagonal close-packed oxygen lattice. The change of morphology is related to the degree of supersaturation in the vapour phase.     

Coating of hydroxyapatite on various substrates via hydrothermal reactions of Ca(edta)2- and phosphate

Hydroxyapatite was coated on various substrates such as 12 mol % ceria-doped tetragonal zirconia (12Ce-TZP), 3 mol % yttria-doped tetragonal zirconia (3Y-TZP), alumina, monetite coated titanium (Ti/CaHPO₄) and calcium titanate coated titanium (Ti/CaTiO₃) via hydrothermal reactions of Ca(edta)^2- and 0.05 M NaH₂PO₄ at initial pH 6 and 160–200 °C for 0.5–6 h. Rod-like particles of hydroxyapatite precipitated to form film on the substrates above 160 °C. The morphology of the film changed significantly depending on the characteristics of substrate, i.e. hydroxyapatite entirely coated the surfaces of 12Ce-TZP, Ti/CaHPO₄ and Ti/CaTiO₃ plates, but sparsely deposited on 3Y-TZP and Al₂O₃ plates. Film thickness increased with time (ca. 20 and 90 m on 12Ce-TZP plates for 0.5 and 6 h, respectively, at pH 6 and 200 °C). The adhesive strength of the film for the substrate was in the order, 12Ce-TZP/hydroxyapatite(28 MPa) > Ti/CaTiO₃/hydroxyapatite (22 MPa) > Ti/CaHPO₄/hydroxyapatite (9 MPa). © 2001 Kluwer Academic Publishers     

Rheology of stimulated whole saliva in a typical pre-orthodontic sample population

The dynamic viscosity () of stimulated whole saliva in a typical pre-orthodontic sample population was characterized as a function of temperature (T). Samples were collected from 30 adolescents or young adults, after screening for factors that are known to have an effect on salivary viscosity. Using a cone and plate viscometer, 1.5 ml of stimulated whole saliva was evaluated at a constant shear rate of 450 s^-1 from T=20°C to T=40°C. Data from the -T plots showed a negative dependence of the form, =a–bT, over a range of from 1.08 to 2.45 centipoise (cps) at 34°C. Most of the samples fell into a narrow envelope, where the mean of the saliva samples ranged from 2.42±0.61 cps at 20°C to 1.57±0.32 cps at 37°C. With regard to sample stability, viscosity-time plots indicated that a small but predictable decrease in occurred during the 3 h period. The -T plots generated from fresh and frozen saliva samples demonstrated an appreciable change in as a result of refrigeration. With regard to sample reproducibility, viscometric data obtained from a typical pre-orthodontic patient over a 1-week period fluctuated within a fairly broad envelope of values.Presented, in part, at the 21st Annual Meeting and Exhibition of the A.A.D.R., Boston, Massachusetts, March, 1992.     

Study of light absorption in n-type and p-type GaInAs and the possibility of making 1.55-μm GaInAs/InP Bragg mirrors

Effects of impurity doping on optical properties of epitaxially grown Ga_0.47In_0.53As semiconductors lattice-matched to InP have been studied. The absorption coefficient decreased and the photoluminescent peak energy increased at very high doping levels, due to a Burstein–Moss shift. Absorption in n-type Ga_0.47In_0.53As was negligible at a wavelength of 1.55 m when the donor (Si) concentration was higher than 7.7 × 10¹⁸cm^–3, but was higher than 4000 cm^–1 at = 1.3 m. Absorption in p-type Ga_0.47In_0.53As was 2600 cm^–1 and 7000 cm^–1 at 1.55 and 1.3 m, respectively, when the acceptor (Be) concentration was 3.4 × 10¹⁹cm^–3. Emission from n-type Ga_0.47In_0.53As experienced a maximum shift of 170 meV towards higher energies relative to emission from undoped Ga_0.47In_0.53As, while emission from p-type Ga_0.47In_0.53As shifted 45 meV. The Burstein–Moss effect was used in growth of 1.55-m distributed Bragg mirrors having 20 pairs of quarter-wave n–Ga_0.47In_0.53As/InP layers. Reflectivity greater than 95% and a stop-band width of 60 nm were demonstrated.     

Synthesis of (Pb,La) (Zr,Ti)O3 films using a diol based sol–gel route

Morphotropic phase boundary compositions of lead zirconate titanate (PZT) modified with 2, 5 and 10 mol% lanthanum (PLZT) have been prepared using a diol based sol–gel route. Thin films of these PLZT compositions were fabricated on platinized silicon substrates by a spin coating technique. The effects of firing temperature and lanthanum modifications were investigated with regard to phase development, microstructure, and ferroelectric and dielectric characteristics. A strong 1 1 1 orientation developed as the amount of lanthanum doping increased. The results indicate that the values of remanent polarization, P _r, and dielectric constant, _r, decrease, relative to unmodified PZT, for films modified with 2 and 5 mol% lanthanum. The 5 mol% La films for example had a P _r of 14 C cm^–2 and an _r value of 700 compared to 31 C cm^–2 and 1480 for undoped PZT films. At these La concentrations there was also an improvement in the leakage current density by two orders of magnitude compared with unmodified PZT. The 10 mol% La sample did not exhibit any switchable polarization behaviour.     

State of Radiorhodium in High-Level Liquid Waste from Regeneration of Spent Nuclear Fuel

The state of radiorhodium in liquid waste from processing of spent nuclear fuel was analyzed in detail by ESR and NMR spectroscopy on several nuclei. The most probable oxidation state of rhodium in nitric acid solutions is +3. Three procedures for preparing nitric acid solutions of rhodium were studied. The composition of Rh(III) complexes in these solutions is similar but not identical. The method for identifying the ionic composition of similar objects was proposed. The speciation of rhodium in nitric acid solutions with the acid concentration ranging from 2×10^{- 2} to 15 M was studied. Polynuclear oligomers with (-ONO₂)₂ bridges, mainly tetramers, are formed in solutions with CRh > 2 M, [NO₃ ^-] 8 M, and [H⁺] 0.7 M. At CRh = (1-2)×10^{- 2}, [NO₃ ^-] = 1-4 M, and [H⁺] = 0.4-4 M, the (-OH, -ONO₂) dimer and trimer prevail; at [NO₃ ^-] < 1 M and [H⁺] < 0.2 M, the (-OH, -ONO₂) dimer dominates. No Rh(III) aqua ion and no monomeric nitrate complexes were detected in the quasiequilibrium system at any concentrations of the components. In nitrate-nitrite solutions, both subnitrated (-OH, -ONO₂) Rh(III) oligomers and mononuclear Rh(III) nitroaqua complexes exist. The equilibrium between these species is attained very slowly and depends on the equilibrium concentration of nitrite anion. Forecast was given on isolation of rhodium from nitric acid solutions of SNF by extraction, ion-exchange, electrolytic, and precipitation procedures.     

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals

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     

On the serrated yielding in Cu-14.1 at%AI polycrystals

Cu-14.1 at %Al polycrystals with three different grain sizes (76,113 and 157 m) have been tested for the Portevin-LeChatlier effect under various conditions of temperature (–196 to 200°C) and strain rate (2.78 × 10^–5 to 5.56 × 10³ sec^–1). In the above range of strain rate, serrated yielding was observed in the temperature range 60 to 160° C. The strain rate dependence of the onset of serrations is most probably due to the diffusion of vacancy-solute atom pairs, as indicated by the low value of the activation energy for migration (0.77 eV). The correlation governing the test variables at the onset of serrations appears to be: = (const)₀ ^{2.2 ± 0.2 –0.87 + 0.03}, where , ₀ and are strain rate, critical strain for the onset of serration and grain size, respectively. The onset of serrations is most probably due to dynamic strain ageing, although the possibility of short range ordering is not ruled out.On leave from Mechanical Engineering Department, Ferdowsi University, Mashhad, Iran.     

The growth of non-stoichiometric apatites using the constant composition method

The kinetics of the crystal growth of calcium-deficient hydroxyapatites with different stoichiometry (Ca₅-(HPO₄)(PO₄)₃-(OH)₁-) have been investigated at 37°C using the constant composition method. The growth was performed in solutions supersaturated only with respect to Ca₅(PO₄)₃(OH) (HAp) by inoculating with well-characterized seed crystals. The stoichiometry of the grown apatites was consistent with values of 00.185. The deviation from HAp stoichiometry of the growing apatite increased with increasing supersaturation degree (S). The constant composition method also provides relevant information about the solubility behaviour of the growing phase with a definite composition. From the decrease of the normalized growth rate j with decreasing S, an estimate could be made of the composition of the solution for which the growth ceases. The determined solubility product of the grown apatite (4.28×10^-54 M⁹) was higher than the value obtained by the equilibration of the seed material. The results were interpreted on the basis of differences in crystal lattice perfection.This paper was accepted for publication after the 1995 Conference of the European Society of Biomaterials, Oporto, Portugal, 10–13 September.     

High-Purity As2S1.5Se1.5 Glass Optical Fibers

Core–clad optical fibers were fabricated from high-purity As₂S_1.5Se_1.5 glass, and their properties were studied. The arsenic sulfo-selenide was prepared by melting a mixture of high-purity arsenic monosulfide, arsenic, and selenium. Optical fibers with core/clad diameters of 300/400 and 200/400 m were fabricated by the double-crucible method. The minimum loss was found to be 60 ± 20 dB/km at 4.8 m and 200–300 dB/km between 4 and 6 m. The numerical aperture of the fibers was 0.28. A 1.5-m-long section of the fiber transmitted 6-W CO laser radiation. The average bending strength of the 400-m-diameter fibers was 0.8 GPa.     

Bone formation following implantation of fibrous calcium compounds (β−Ca(PO3)2, CaCO3(aragonite)) into bone marrow

Bone formation around three types of fibrous calcium-containing crystals has been examined histologically using rats. The implanted materials are (i) calcium metaphosphate (–Ca(PO₃)₂) fibers having aspect ratios of 15–80 with 2–20 m in diameter, (ii) –Ca(PO₃)₂) fibers surface-modified using dilute NaOH and (iii) calcium carbonate (CaCO₃; aragonite phase) whiskers having aspect ratios of 15–40 with 0.5–3 m in diameter. –Ca(PO₃)₂ fibers show a mechanically high strength with a low modulus of elasticity, and the surface-modified fibers have a thin layer consisting of a calcium orthophosphate phase. CaCO₃ whiskers were used for comparison reasons. The materials were implanted for 4, 8, and 12 weeks into bone defects created in the bone marrow of rat tibiae. Cancellous bone formation was observed around –Ca(PO₃)₂ fibers, the surface-modified fibers and CaCO₃ whiskers after implantation for 12, 4 and 4 weeks, respectively. CaCO₃ whiskers were scarcely observed after 12 weeks for resorbing. The calcium phosphate fibrous materials show combined advantages of mechanically high strength for toughening a matrix phase and biological activities; thus, these materials may prove to be useful for novel applications in the biomedical field.     

Thermoelectric properties of fine-grained sintered (Bi2Te3)25-(Sb2Te3)75 p-type solid solution

P-type semiconductor alloy compacts of the composition (Bi₂Te₃)₂₅-(Sb₂Te₃)₇₅ with grain size (L) in the range 30 > L > 20 m, 20 > L > 15 m, 15 > L > 10 m, 10 > L > 5 and L < 5 m were prepared by cold press at a pressure of 77 × 10⁷ Nm^–2. The samples were sintered at 673 K. Measurements of the Seebeck coefficient, electrical resistivity and thermal conductivity were carried out. The experimental results show that the Seebeck coefficient increases, but not much from the single crystal. The electrical resistivity increases in particular for the size L < 5 m with a reduction in grain size. The total thermal conductivity seriously decreases as grain size decreases. It is concluded that the figure of merit of the compacted alloy would be significantly improved through the use of fine-grained powders of size 30–10 m.     

The role of Coble creep and interface control in superplastic Sn-Pb alloys

The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260m. In Sn-2 wt% Pb with grain sizes larger than 50 m, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses ( 0.1 MPa) and strain rates ( 10^–10 sec^–1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.     

Synthesis of ceramic membranes

Non-supported -alumina films are prepared from a boehmite colloidal suspension. After calcination at 600°C, the microstructure is characterized by a mean pore diameter of 3 nm, a porosity of 50% and a tortuosity of 5.5. The structure is formed by card packed, plate-shaped particles, giving rise to slit-shaped pores. Supported -alumina films, made by a slipcasting process using the same boehmite precursor, have a similar structure as the non-supported films. The slipcasting process is very sensitive to support characteristics and frequently yields defect films. Polyvinylalcohol (PVA) (molecular weight 72000 g mol^–1) is added to the boehmite precursor (0.25 gg^–1 film) to improve this process. This addition results in a less critical and better controllable drying and calcining procedure. The addition of PVA is necessary to slipcast defect free -alumina films on supports with pore diameters of 0.4 m or smaller and on multi-layer supports. It has no significant effect on the resultant microstructure of the -alumina film, provided all PVA is removed by appropriate thermal treatment. The slipcasting rate is slower, resulting in thinner films of 3–5 m at identical slipcasting times.     

Method of measuring vibrational temperatures in thermodynamically nonequilibrium gas flows

A numerical calculation is made of the integral radiation of CO₂ molecules in the 2.7- and 4.3-m bands and of CO molecules in the 4.7-m band when the vibrational temperatures significantly exceed the rotational and translational temperatures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 3, pp. 411–419, March, 1980.