Surface erosion of TiC coatings under 4He+ and D+ ion irradiation

The surface damage and erosion of chemically vapor-deposited TiC coatings irradiated by 20, 40 and 60 keV D⁺ and ⁴He⁺ were studied for as-deposited and polished surfaces. Scanning electron micrographs of irradiated TiC reveal surface damage and erosion due to blistering and surface exfoliation. The erosion yield increases with increasing ion energy for both D⁺ and He⁺ irradiations, and it is generally larger for D⁺ than for He⁺ irradiations at a given energy. The erosion yield is larger for polished than for as-deposited surfaces for both D⁺ and He⁺ irradiations. The relationship between the blister diameter and the skin thickness agrees well with a relationship derived from a gas pressure model but disagrees with a relationship derived from a model of integrated lateral stresses as a prime mechanism for blister formation.     

Stability of photodiodes under irradiation with a 157-nm pulsed excimer laser

We have measured the stability of a variety of photodiodes exposed to 157-nm light from a pulsed excimer laser by using a radiometry beamline at the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. The intense, pulsed laser light exposed the photodiodes, whereas the low-intensity, continuously tunable light from the synchrotron source measured changes in the characteristics of the photodiodes, such as in the responsivity and the reflectance from the surface of a photodiode. Photodiodes studied include both silicon pn-junction and Schottky-barrier types. Among these photodiodes, we found that the damage mechanism for photodiodes with SiO2-based passivating layers is mainly the buildup of SiO2-Si interface trap states. The interface trap state buildup is well known for other semiconductor devices and is generally recognized as a product induced by radiation with an energy more than the 9-eV SiO2 bandgap energy rather than the 7.9-eV energy of the 157-nm radiation. Based on the generation of interface trap states, a model is proposed to describe the dependence of detector responsivity on exposure to 157-nm radiation. We also observed slow recovery in some of the damaged photodiodes, confirming that some of the interface trap states are only semipermanent. Radiation damage induced by low-power continuous 157-nm synchrotron light was also studied. As for the other photodiodes with no SiO2 layers, measurement results support the assumption that the changes in responsivity are due mainly to the deposition of thin layers on the tops of the detectors during laser irradiation.     

光控亲/疏油可逆转换的硅钛复合微纳米结构涂层研究

以简便价廉的方法制备出一种新型氟试剂修饰的SiO2-TiO2复合微纳米结构涂层。采用场发射扫描电子显微镜(FE-SEM)、X射线光电子能谱(XPS)和油接触角测试等方法对涂层进行了表征和测试。研究结果显示,该涂层具备良好的UV光驱动亲/疏油可逆转换特性,最初为疏油性(90°),光照后变得相对亲油(24°),黑暗放置后恢复疏油性,且经5次反复循环无明显衰减。与单纯纳米TiO2涂层相比,复合涂层表面油浸润性的转变因微纳米结构的存在而显著强化,且随着SiO2微球直径的增大,其亲/疏油效果得到增强。以上研究为亲/疏油可逆转换智能材料的设计提供了新的方法和借鉴。     

Coupled electronic and atomic effects on defect evolution in silicon carbide under ion irradiation

Understanding energy dissipation processes in electronic/atomic subsystems and subsequent non-equilibrium defect evolution is a long-standing challenge in materials science. In the intermediate energy regime, energetic particles simultaneously deposit a significant amount of energy to both electronic and atomic subsystems of silicon carbide (SiC). Here we show that defect evolution in SiC closely depends on the electronic-to-nuclear energy loss ratio (S_e/S_n), nuclear stopping powers (dE/dx_nucl), electronic stopping powers (dE/dx_ele), and the temporal and spatial coupling of electronic and atomic subsystem for energy dissipation. The integrated experiments and simulations reveal that: (1) increasing S_e/S_n slows damage accumulation; (2) the transient temperatures during the ionization-induced thermal spike increase with dE/dx_ele, which causes efficient damage annealing along the ion trajectory; and (3) for more condensed displacement damage within the thermal spike, damage production is suppressed due to the coupled electronic and atomic dynamics. Ionization effects are expected to be more significant in materials with covalent/ionic bonding involving predominantly well-localized electrons. Insights into the complex electronic and atomic correlations may pave the way to better control and predict SiC response to extreme energy deposition.     

Synthesis of graphitic carbon particle chains at low temperatures under microwave irradiation

Chains of graphitic carbon particles were formed by microwave irradiation of polyethylene glycol at temperatures between 160 and 220 °C for 40 min, in the absence of a catalyst. Chains were comprised of individual particles ranging in size from 340 to 620 nm; particle size increased with synthesis temperature. The D/G ratio measured by Raman spectroscopy was 0.91, indicative of a mixture of amorphous and graphitic material. SEM, TEM and TGA measurements confirmed this. Our experiments show that the chains are an intermediate product, which when heated further under hydrothermal conditions, produce MWNTs.     

高庙子天然钙基膨润土和改性钠基膨润土的热和辐射稳定性

为了筛选高放废物深地质处置库中适用的缓冲材料,以内蒙古高庙子天然钙基膨润土和由其改性制备的钠基膨润土为研究对象,分别在120℃、150℃、180℃下热老化3000h、6000h和9000h,用电子加速器在室温下进行了辐射老化,累计辐照剂量分别为1000kGy、3000kGy和5000kGy,随后对老化前后的样品用X射线衍射仪进行了分析测量。结果表明:高庙子改性钠基膨润土的高温长期热稳定性比其天然钙基膨润土好,改性钠基膨润土在大剂量率电子辐射作用下的稳定性也比其天然钙基膨润土好;因此宜选择改性钠基膨润土或者天然钠基膨润土作为处置库的缓冲材料。     

Evaluation of atomic oxygen resistant coatings for space structures

The present generation of satellites has a life expectancy limited to 3–5 years, often as a result of environmentally induced degradation of the structural materials. Far from being just inert vacuum, low Earth orbit contains reactive atomic oxygen, an increasing quantity of man made debris, natural micrometeorides, ultraviolet radiation and large temperature extremes. As a result of the synergistic effect of these factors, most polymers and polymer matrix composites degrade rapidly. Atomic oxygen resistant coatings are required to protect them. Flexible protective coatings for solar arrays, fiberglass structural elements and silver interconnects have been designated as a critical technology deficient area. In response to this need, a number of protective siloxane and carborane (siloxane) coating systems were developed and evaluated. The results of the Limited Duration Candidate Exposure flight experiment for these coatings are described. The addition of the carborane is essential in preventing cracking of the coating upon oxidation. Due to its unique structure, each carborane unit can incorporate up to 15 oxygen atoms. This gain offsets the shrinkage caused by carbon loss and densification as the upper layer of the coating turns into a silicate or borosilicate glass. No cracking of the carborane (siloxane) coating was detected, in contrast to the siloxane that cracks upon oxidation. Surface analysis of coatings exposed to the low earth orbit environment demonstrates the formation of a glassy borosilicate layer that provides excellent protection to the substrate.     

基于单分子层模型的ZnO热控涂层电子辐照光学性能退化研究

在分析电子辐照对ZnO类热控涂层光学性能退化机理的基础上,提出了单分子层电子色心产生模型,推导了"色心浓度"表达式及材料光学性能退化随辐照剂量的变化关系。用该理论对100keV电子辐照下S781白漆太阳吸收比变化Δαs的实验数据的拟合结果表明,单分子层模型能够很好地预测ZnO类热控涂层在电子辐照环境下光学特性退化趋势。该理论可扩展应用于其它热控材料在空间不同辐射环境下光学性能退化趋势的预测。     

Effects of damage rate on defect cluster formation in copper under low dose irradiation

The dependence of defect cluster formation on dose rate is an important factor in understanding the mechanisms of microstructure evolution under irradiation. In the present study, the effects of dose rate on the formation of vacancy clusters and self-interstitial atom (SIA) clusters during low dose irradiation up to 0.01 dpa were investigated. The densities of SIA clusters formed at 200 K and 458 K and vacancy clusters formed at 200 K were found to be independent of the dose rate. Vacancy cluster densities were found to be dependent on the dose rate at temperatures between 200 K and 458 K. The results can be explained by the migration and thermal stability of defects, which are determined by irradiation temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stability of nanocrystallites dispersed in Cu50Zr45Ti5 metallic glass under electron irradiation

Effect of electron irradiation on stability of nanocrystallites dispersed in a glassy matrix of a Cu50Zr45Ti5 bulk metallic glass (BMG) alloy was investigated. Microstructural evolution during electron irradiation was observed and analyzed by means of conventional and high-resolution transmission electron microscopy (CTEM and HRTEM) and X-ray energy dispersive spectroscopy (EDS). The crystalline nanoparticles of monoclinic CuZr phase formed in-situ in as-cast Cu50Zr45Ti5 alloy exhibited a high stability against electron irradiation. No obvious changes in their size, morphology, and crystal structure were observed during electron irradiation, though new crystalline nanoparticles nucleated and precipitated in the glassy matrix under electron irradiation. The new nanoparticles formed upon irradiation have a similar composition and crystal structure to those formed in-situ in the as-cast alloy. The nanocrystalline precipitates in the glassy matrix are proposed to be mainly due to electron knock-on effect under electron irradiation, which promotes atomic diffusion and assists the crystallization of the glassy phase. The present result indicates that electron irradiation to metallic glasses has potential for producing nanocrystalline structure and nanocrystalline-glassy composite structure.     

Carbon coatings with low secondary electron yield

Carbon thin films for electron cloud mitigation and anti-multipacting applications have been prepared by dc magnetron sputtering in both neon and argon discharge gases and by plasma enhanced chemical vapour deposition (PECVD) using acetylene. The thin films have been characterized using Secondary Electron Yield (SEY) measurements, Scanning Electron Microscopy (SEM), Nuclear Reaction Analysis (NRA) and X-ray Photoemission Spectroscopy (XPS). For more than 100 carbon thin films prepared by sputtering the average maximum SEY is 0.98 ± 0.07 after air transfer. The density of the films is lower than the density of Highly Oriented Pyrolytic Graphite (HOPG), a fact which partially explains their lower SEY. XPS shows that magnetron sputtered samples exhibit mainly sp² type bonds. The intensity on the high binding energy side of C1s is found to be related to the value of the SEY. In addition the initial surface concentration of oxygen has no influence on the resulting SEY, when it is below 16%. The thin films produced by PECVD have a much higher maximum SEY of 1.49 ± 0.07.Storage conditions in air, namely wrapping in aluminium foil, preserves the low SEY by more than one year. Such coatings have already been applied successfully in accelerators and multipacting test benches.     

Phase stability under irradiation

Irradiation of metals and alloys with neutrons, electrons, heavy ions, or γ-rays may introduce up to 10⁸ J/mol of energy in the form of atomic displacements. This energy, which is in the form of vacancies, self-interstitials, and cores of displacement cascades is then available to produce a range of phase changes and microstructural alterations which are not observed under thermal conditions. There exist numerous mechanisms to convert part of this displacement energy into microstructural change, including irradiation-induced solute segregation, Frenkel pair recombination at the particle: matrix interface, irradiation disordering or amorphization, and recoil resolution of atoms from precipitates. In addition, the cores of displacement cascades may act as precipitate nucleation sites and Frenkel pair recombination may trigger spinodal-like instabilities.

The theory of these mechanisms is developed in some detail, and is followed by a systematic review of experimentals observations of irradiation-altered phase stability. The observations include enhanced nucleation on displacement cascades, precipitation induced by solute segregation to defect sinks, nucleation of wrong phases, disordering and amorphization, Frenkel pair recombination driven precipitate, and inverse Ostwald ripening.     

Characterization of surfactant coatings in capillary electrophoresis by atomic force microscopy

This paper describes the adsorption mechanisms and aggregation properties of cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) surfactants that are used for dynamic coatings in capillary electrophoresis (CE). Atomic force microscopy is used to directly visualize surfactant adsorption on fused silica. It was found that the single-chained surfactant CTAB forms spherical aggregates on silica while the double-chained surfactant DDAB forms a bilayer. Aggregation at the surface occurs at approximately the same surfactant concentration in which EOF reversal is observed in CE. The nearest-neighbor distance between CTAB aggregates varies inversely with buffer pH and becomes constant at the point when the silanol groups are fully ionized. DDAB forms a flat, uniform coating independent of pH. Increasing the buffer ionic strength changes the morphology of the CTAB aggregates from spherical to cylindrical. The change in morphology can alter the surface coverage, which is related to the "normalized" EOF measured in identical buffers. The morphology of a surfactant coating is also shown to affect its ability to inhibit protein adsorption to the capillary wall. Specifically, the full surface coverage provided by DDAB proved superior in a head-to-head comparison with CTAB.     

Superconductivity under irradiation conditions

The irradiation effect of a particle flow on the electrodynamic properties of a conductor is discussed. It is established that a relationship exists between three energy quanta of different origin and high-temperature superconduction current.Notation Planck constant - c velocity of light - H magnetic intensity - E electric intensity - r radius-vector - _m , _H , _E energy quanta - l _g unit length - j electric current - A vector-potential of the electromagnetic field - F _ik field tensor - m mass of the absorbed particle - velocity - l _H ,l _E ,l _m length units - t time - e electron charge - k Boltzmann constant - T _cr critical temperature Belarus State Technological University Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 5, pp. 739–742, September–October, 1995.     

Stability of quasi-ballistic MESFETs with various buffer layer structures under irradiation with neutrons possessing different energy spectra

The effect of irradiation with neutrons possessing an average energy of 1.1 and 14 MeV on the characteristics of quasi-ballistic metal-Schottky-gate field-effect transistors (MESFETs) with various structures of the buffer layer has been studied. In the series of MESFETs with buffers based on a GaAs homostructure, AlGaAs-based heterostructure, and AlAs/GaAs superlattice, the radiation stability increases (on every passage, by a factor of 1.2–5) due to the compression of electron trajectories in the conducting channel and the gettering of radiation defects at a heteroboundary of the buffer layer.     

Deuterium and helium ion irradiation effects on TiB2 coatings

The surface damage and erosion of chemically vapor deposited TiB₂ coatings and commercial grade titanium, caused by 40, 60 and 120 keV D⁺ and ⁴He⁺ irradiation, have been studied for the as-deposited coatings and for coating surfaces that were mechanically polished prior to irradiation. Scanning electron microscopy analysis of polished TiB₂ samples irradiated with D⁺ and ⁴He⁺ to a dose of 3.1 × 10¹⁸ ions cm^-2 reveals significant surface damage due to blistering and flaking, whereas for identical irradiation conditions the as-deposited TiB₂ coatings show very little damage. For similar irradiation conditions the titanium metal samples showed blistering for the ⁴He⁺ irradiation case but no significant surface damage for the D⁺ case. Estimates of the erosion yields due to blister exfoliation in polished TiB₂ samples show an increase with increasing projectile energy for the total dose studied.     

Nanoindentation and atomic force microscopy measurements on reactively sputtered TiN coatings

Titanium nitride (TiN) coatings were deposited by d.c. reactive magnetron sputtering process. The films were deposited on silicon (111) substrates at various process conditions, e.g. substrate bias voltage (V_B) and nitrogen partial pressure. Mechanical properties of the coatings were investigated by a nanoindentation technique. Force vs displacement curves generated during loading and unloading of a Berkovich diamond indenter were used to determine the hardness (H) and Young’s modulus (Y) of the films. Detailed investigations on the role of substrate bias and nitrogen partial pressure on the mechanical properties of the coatings are presented in this paper. Considerable improvement in the hardness was observed when negative bias voltage was increased from 100–250 V. Films deposited at |V _B| = 250 V exhibited hardness as high as 3300 kg/mm². This increase in hardness has been attributed to ion bombardment during the deposition. The ion bombardment considerably affects the microstructure of the coatings. Atomic force microscopy (AFM) of the coatings revealed fine-grained morphology for the films prepared at higher substrate bias voltage. The hardness of the coatings was found to increase with a decrease in nitrogen partial pressure.