Impurity control in sapphire whiskers

The effect of selective chemical removal of impurity elements on the structure and high temperature stability of sapphire whiskers has been investigated. For Compagnie Thomson Houston (CTH) sapphire whiskers (with an as-grown impurity concentration of 6% silicon and approximately 2% (sodium + potassium + calcium)), treatment in a 20% H₂SO₄-20% HF solution (Solution A) effectively removed the grown-in second phase particles and formed a surface reaction product. The reaction product could be removed from the whiskers with distilled water and the cleaned whisker annealed at 1100° C for 17 h (in high purity argon) without attendant whisker disintegration. A similar effect was noted for Thermokinetic Fibers Inc. (TFI) whiskers (approximately 0.2% silicon) after treatment in Solution A. In both cases the improvement in high temperature stability is correlated with a reduction in silicon concentration to < 0.15%.An alternative treatment in H₃PO₄ (Solution B) resulted in preferential removal of (sodium + potassium + calcium) and aluminium from CTH whiskers. The grown-in second particles were retained in the whiskers and coarsened into a variety of configurations during an 1100° C/17 h anneal, which may be attributed to the geometry produced by differential whisker thinning.     

Stability of nickel coated sapphire whiskers

The effect of high temperature annealing (1100 to 1300° C) on the stability of nickel coated purified sapphire whiskers has been investigated. It was found that the initially coherent coating spheroidized to form a series of partially adherent nickel particles, which increased in size with continued time at temperature. The time for complete adhesion of the particles, which was established from the product of the number and volume of particles per unit area of surface, decreased from 72 h at 1100° C to 3 h at 1300° C, giving an activation energy of 70 kcal/mol. In addition at 1300° C, after 8 h, there was evidence for whisker breakdown from a sapphire-nickel reaction.     

The stability of sapphire whiskers in nickel at elevated temperatures

Composites of 1 to 20 vol % sapphire whiskers contained in a nickel matrix were produced by roll-bonding and by hot-pressing. The composites were examined using transmission electron microscopy, X-ray and electron diffraction, optical microscopy and mass spectrographic analysis. Composites were annealed in vacuum (1.3×10^–3 N m^–2), in low-pressure air (13.3 N m^–2) and in dried hydrogen (101.3 N m^–2) in the temperature range 1100 to 1400° C for times up to 3800 h. Whiskers in situ and the whisker/matrix interface were observed by transmission electron microscopy; matrix dislocations were associated with whiskers and were stable after annealing at 1400° C. Whiskers extracted from annealed composites showed significant morphological changes. These were attributed to: (i) ovulation from the tips of whiskers by interfacial diffusion, (ii) waisting from surface undulations, (iii) Ostwald ripening and (iv) constant-volume shape changes.     

The tensile strength of sapphire whiskers at elevated temperatures

Tensile tests have been carried out on 124 sapphire whiskers at temperatures in the range 350 to 1810° C. The effects of crystallographic orientation, growth process, purity and surface perfection have been investigated.The strength-temperature-dependences of different categories of whisker are compared with that to be expected for a Griffith fracture mechanism, and it is shown that such a mechanism may apply up to 1000° C to A-type () whiskers grown by the wet-hydrogen process [1]. Chemically-polished whiskers of this type show a strength-temperature-dependence more consistent with a dislocation-induced fracture mechanism. There is some evidence that chemically-polished C-type (<0001>) whiskers grown by a chloride oxidation process [2] behave in the same manner. For other categories of whisker there are insufficient data to ascribe fracture mechanisms.     

Thermally induced structural changes in Nomex fibres

Thermally aged Nomex fibres manifest several residual effects viz. reduction in X-ray crystallinity, weight loss and deterioration in tensile characteristics. Surface damages in the form of longitudinal openings, holes, material deposits etc have also been observed. Based on the data from thermally exposed fibres, the time needed for states of zero tensile strength and modulus have been predicted.     

Size-strength effects in sapphire and silicon nitride whiskers at 20° C

Tensile tests at 20° C have been carried out on seventy-three sapphire whiskers and on seventeen silicon nitride whiskers. The sapphire whiskers were of 0001, 1¯120, 10¯10, and 10¯11 orientations, while the silicon nitride whiskers were 0001, 11¯20, and 10¯13. Tensile strengths were in the range 45 to 1500 kg/mm², and deformation was found to be purely elastic. The tensile strength data have been analysed and fitted to empirical equations describing the effect of size on strength for different orientations. These empirical equations have been used to deduce possible fracture nucleation mechanisms. It is concluded that, in the case of 0001 sapphire whiskers, fracture nucleation may be due to dislocation pile-ups or interactions, while in the other cases a Griffith flaw mechanism is probably applicable.     

The effects of chemical polishing on the strength of sapphire whiskers

Tensile tests at 20 C have been carried out on forty-four sapphire whiskers after chemical polishing in hot orthophosphoric acid. The orientations tested were 0001, 11¯20, 10¯10, and 10¯11. The results show that chemical polishing increases the strength of large whiskers by a factor of up to 10, but not the strength of small ones. Good correlation is obtained between fracture strength, _f, and whisker diameter,d. The relevant size-strength equations, _f=Kd ^m l ⁿ (wherel is gauge length, andK, m, andn are constants depending on whisker orientation), predict strengths in good agreement with the theoretical strength of sapphire at unit-cell dimensions and with the measured strengths of macroscopic flame-polished crystals.The observations are contrasted with those for unpolished sapphire whiskers [1]. They show a transition in the fracture nucleation mechanism of unpolished whiskers at a certain stress.In unpolished, A-type (11¯20 and 10¯10) whiskers, with _f<1000 kg/mm², fracture initiates at surface flaws, and strength is dependent on surface area. But, for whiskers with _f>1000 kg/mm₂, and for all polished whiskers (both A and C type), fracture is due to dislocation pile-ups or interactions, and strength is dependent only upon diameter. In unpolished, C-type (0001) whiskers, however, with _f<800 kg/mm₂, fracture initiates at surface flaws which are related to whisker diameter; while, for _f> 800 kg/mm², it occurs at dislocation pile-ups or interactions and is again related to diameter. In contrast, therefore, to A-type whiskers, the strength of C-type whiskers is always diameter-dependent, although there is a clear transition in the size-strength curve at _f800 kg/mm².     

Thermally induced structural and morphological changes of CdSe/CdS octapods

Branched nanostructures are of great interest because of their promising optical and electronic properties. For successful and reliable integration in applications such as photovoltaic devices, the thermal stability of the nanostructures is of major importance. Here the different domains (CdSe cores, CdS pods) of the heterogeneous octapods are shown to have different thermal stabilities, and heating is shown to induce specific shape changes. The octapods are heated from room temperature to 700 °C, and investigated using (analytical and tomographic) transmission electron microscopy (TEM). At low annealing temperatures, pure Cd segregates in droplets at the outside of the octapods, indicating non-stochiometric composition of the octapods. Furthermore, the tips of the pods lose their faceting and become rounded. Further heating to temperatures just below the sublimation temperature induces growth of the zinc blende core at the expense of the wurtzite pods. At higher temperatures, (500-700 °C), sublimation of the octapods is observed in real time in the TEM. Three-dimensional tomographic reconstructions reveal that the four pods pointing into the vacuum have a lower thermal stability than the four pods that are in contact with the support.     

Crystallinity and structural changes in HA plasma-sprayed coatings induced by cyclic loading in physiological media

In this study hydroxyapatite plasma-sprayed Ti-6Al-4V was characterized by X-ray diffraction (XRD) after being cyclically loaded, while immersed in two different testing environments: Hank's balanced salt solution (HBSS) and an isotonic saline solution (0.15 m NaCl). An original method was developed to test these systems under cyclic bending, while immersed in the testing solution. The crystallinity of the coating was calculated, before and after testing. Crystallinity changes were related to coating dissolution rates, measured by atomic absorption spectroscopy (AAS) and by a photocolourimetric method using ascorbic acid. Structural changes induced either by the plasma-spraying procedure or due to corrosion-fatigue tests, namely a preferential dissolution of the coating's amorphous phase, were also found in X-ray diffraction patterns. Long-term immersion tests, without any applied load, were carried out for assessing cyclic loading effects. An immersion for 2 years in static conditions was found to be equivalent to a cyclic test of 27.8 h in the same solution, as measured by the changes on the coating structure and by the amount of coating material lost to the solution.This paper was accepted for publication after the 1995 Conference of the European Society of Biomaterials, Oporto, Portugal, 10–13 September.     

Nonsteady changes in structural thermophysics

An experimental method is proposed for determining the damping of a harmonic temperature wave passing through the model of a real building construction. The rates of temperature changes at the inner surface of the wall are measured. Characteristic conditions and results of the experiment and the principle of operation of a specially designed thermal-conductivity meter are described.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 1, pp. 104–109, January, 1978.     

Laser-induced acoustic waves and index of refraction changes in Ti- doped sapphire

Acoustic waves in Ti³⁺-doped sapphire have been observed after optical pumping with a pulsed 532 nm Nd:YAG laser. Photothermal beam-deflection spectroscopy has been used to measure the resulting change in the index of refraction. The effect of the acoustic wave on the index of refraction is discussed. It is shown that the energy of the acoustic wave is negligible compared with the increase of internal energy produced by the optical pumping. The resulting sound velocity of the acoustic wave was calculated to be c_s=9500±500 m/s.     

Temperature dependence of the electrical conduction in discontinuos silver films on sapphire substrates

The temperature dependence of the electrical conduction in discontinuous silver films on sapphire substrates was examined in the temperature range 120–300 K. The experimental dependences were approximated by functions taken from some theoretical models. The best approximation was found using the phenomenological model presented previously by the present author.     

蓝宝石光纤端面上ZnO薄膜的温变特性

为了能利用半导体材料ZnO对光谱曲线随温度变化而移动的原理设计一种光纤温度传感器,因此有必要对生长在光纤端面上的ZnO材料的温变特性的进行研究。基于ZnO材料的测温原理,利用电子束蒸发技术在蓝宝石光纤端面上生长ZnO薄膜,并设计了一种具有低插入损耗的光纤检测系统用于光学性能的分析。光谱测试结果显示,蒸镀在蓝宝石光纤端面上的ZnO薄膜具有陡峭的光学吸收边,随着温度的升高(室温～450℃),ZnO薄膜的禁带宽度减小,吸收边向长波长方向移动,符合半导体材料的温变特性。同时随着温度的升高,平均的透射率有一定的减小。ZnO薄膜温变特性的研究,将为进一步研制以ZnO镀膜为敏感材料的新型光纤温度传感器打下良好的基础。     

Surface structural changes,surface energy and antiwear properties of polytetrafluoroethylene induced by proton irradiation

In this work, the polytetrafluoroethylene (PTFE) surface was modified with 25 keV proton beam irradiation in vacuum condition. Multiple characterization techniques including X-ray photoelectron spectroscopy, Raman spectroscopy and infrared spectroscopy were employed for research on microstructure changes in the PTFE surface. The changes in the surface energy and antiwear properties of PTFE were evaluated using contact angle analysis and a ball-on-disk tribometer, respectively. Experimental results showed that the surface energy of PTFE obviously increased from 13.17 mJ/m² to 33.73 mJ/m² and the wear rate decreased from 8.9 × 10^− 3 mm³/Nm to 5.8 × 10^− 4 mm³/Nm after proton irradiation for 15 min. Moreover, TRIM simulation indicated that the H⁺ ions cannot penetrate through the PTFE block and only stop at a depth of about 730 nm from the material surface. Proton irradiation has been proved to be a simple, rapid and effective measure for the surface modification of PTFE with distinctly improved surface energy and antiwear properties, and the possible reaction mechanism taking place in PTFE was also discussed in this paper.     

Gamma-ray irradiation induced structural and optical constants changes of thermally evaporated neutral red thin films

Neutral red (NR) is polycrystalline in powder form, it transforms to nanocrystallite phase upon thermal deposition. Gamma-ray irradiation with doses 1.25–6 KGy induced partial transformation of nanocrystallite phase to amorphous structure. The changes of optical constants with γ-ray doses were calculated using spectrophotometer measurements of transmittance and reflectance at normal incidence of light over spectral range 200–2500 nm. The complex refractive index of NR film is highly influenced by exposure to γ-ray irradiation, the onset and optical energy gaps decrease with increasing γ-ray doses, and Urbach tail increases linearly with increasing irradiation dose. The type of electronic transition, oscillator, and electric dipole strengths and dispersion parameters were determined before and after irradiation. The spectral behavior of dielectric constant with γ-ray doses was also estimated.     

Creep-induced structural changes in Ni-Si-B amorphous alloy

The influence of the stress annealing on the reversible structural relaxation of a Ni-Si-B amorphous ribbon was studied. Creep-induced structural changes in the amorphous structure were derived from anisothermal DSC and dilatometric experiments. It is demonstrated that considerable enthalpy and specimen length variations associated with the reversible structural relaxation are observed after previous creep at higher temperatures. __________ Translated from Problemy Prochnosti, No. 1, pp. 24–27, January–February, 2008.