High-temperature stability multilayers for extreme-ultraviolet condenser optics

We investigate the thermal stability of Mo/SiC multilayer coatings at elevated temperatures. Transmission electron microscopy and x-ray diffraction studies show that, upon annealing, a thermally induced structural relaxation occurs that transforms the polycrystalline Mo and amorphous SiC layers in as-deposited multilayers into an amorphous Mo-Si-C alloy and crystalline SiC, respectively. After this relaxation process is complete, the multilayer is stable at temperatures up to 400 degrees C.     

High-temperature properties and microstructural stability of hot-work tool steels

Indexable insert tools for machining operations are in service exposed to high temperatures and cyclic mechanical loads. Secondary hardening steels such as hot-work steels are commonly used for tools subjected to thermal exposure. However, these steels, highly alloyed with strong carbide forming elements as Cr, V and Mo, are generally difficult to machine and machining represents a large fraction of the production cost of a tool. Thus, the present study concerns the development of a new steel with improved machinability and meeting the requirements for high-temperature properties.Softening resistance of the THG2000 and QRO90 tool steels, commonly used in hot-work applications, and a newly developed tool steel MCG2006 with lower alloying content of carbide forming elements, was investigated by tempering and isothermal fatigue testing. Mechanisms of high-temperature softening of the tested tool steels were discussed with respect to their microstructure and high-temperature mechanical properties. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis.No difference in softening behaviour was found among the QRO90 and MCG2006 regarding hot hardness measurements. The THG2000 indicated some stabilization of the hardness between 450 and 550 °C and a considerable hardness decrease at higher temperatures.The short-time cyclic softening in isothermal fatigue was controlled by dislocation rearrangement and annihilation. The alloying composition of the steels presently tested had no influence on the dislocation density decrease.The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance. The QRO90 with greater molybdenum and lower chromium contents than in the THG2000 show the best resistance to softening among the tested grades at all temperatures. The MCG2006, leaner alloyed with the carbide forming elements and alloyed with 4 wt% nickel, has better temper resistance than THG2000 at higher temperatures and longer tempering times.     

High-temperature high-dose implantation of N+ and Al+ ions in 6H-SiC

A series of experimental and theoretical investigations has been initiated for 6H-SiC samples sequentially implanted with high doses of N⁺(65 keV) + N⁺(120 keV)+Al⁺(100 keV)+Al⁺(160 keV) ions at temperatures between 200 and 800 °C. Nitrogen and carbon distribution profiles are measured by ERD and structural defect distributions are measured by Rutherford backscattering with channeling. A comparison between the experimental data and the results of computer simulation yields a physical model to describe the relaxation processes of the implanted SiC structure, where the entire implanted volume is divided into regions of different depth, having different guiding kinetics mechanisms. Pis’ma Zh. Tekh. Fiz. 23, 6–14 (August 26, 1997)     

High-temperature stability of Nicalon under Ar or O2 atmosphere

The thermal stability of Nicalon NL400 at 1573, 1673 and 1773 K has been studied by TGA and XRD. Under an Ar atmosphere, Nicalon pyrolysed severely to crystallize into -SiC involving the generation of both SiO and CO. Under an O₂ atmosphere, the oxide film which formed around Nicalon retarded the pyrolytic reaction of the core. Nicalon coated with a silica film by the previous oxidation treatment pyrolysed hardly at all under an Ar atmosphere, because the film restricted the escape of SiO and CO. Nicalon which was oxidation-treated at 1773 K retained 63% of its original strength after heating at 1773 K under an Ar atmosphere. The amorphous silica film was found to resist rapid thermal cycling between room temperature and elevated temperatures.     

Type,stability, and acidity of hydroxyl groups of HNaK-erionites

The concentration and acid strength of the hydroxyl groups of a series of HNaK erionites have been investigated by means of temperature programmed activation (t.p.a.), temperature programmed desorption (t.p.d.) of ammonia, and transmission absorption i.r. spectroscopy. With an increasing degree of NH₄⁺ exchange, primarily Na⁺, at first, and, subsequently, K⁺ cations are replaced. During activation of these materials, NH₃ desorbs in a temperature interval from 540 to 900 K, leading to hydroxyl groups. However, it is impossible to decompose NH₄⁺ quantitatively without dealumination, if the degree of NH₄⁺ exchange is higher than 85%. Five types of hydroxyl groups are present, characterized by i.r. bands at approximately 3741, 3691, 3660, 3618-3608, and 3566 cm⁻¹. The assignment of these bands is discussed. Only the OH groups represented by the bands at 3660, 3618–3608, and 3566 cm⁻¹ adsorb ammonia under the conditions employed. The concentration and strength of the bridging hydroxyls (3618-3608 cm⁻¹), associated with framework Al³⁺, increase with increasing degree of ion exchange.     

High-temperature stability of low-oxygen silicon carbide fiber heat-treated under different atmosphere

The effects of heat-treating atmosphere on the thermal stability of low-oxygen silicon carbide fiber were investigated. Heat-treatment of EB-cured PCS fiber were conducted at 1573 K in argon, nitrogen or vacuum of 10^–6 atm. Subsequently the fibers were exposed to 1873 K in argon. The strength of fibers were strongly influenced by the heat-treating atmosphere. When heat-treated in nitrogen, the fibers absorbed nitrogen. High-temperature exposure caused severe degradation of strength owing to the decomposition of silicon oxycarbonitride phase. When heat-treated in vacuum, the fiber surface was smooth and pore-free, minimizing the degradation of strength at high temperature.     

磁控溅射制备SiC薄膜的高温热稳定性

采用磁控溅射方法在Si基底上制备SiC薄膜,研究了SiC薄膜经不同温度和气氛条件高温退火前后结构、成份的变化.结果表明,薄膜主要以非晶为主,由Si-C键,C-C键和少量Si的氧化物杂质组成;在真空条件下经高温退火后,薄膜C-C键的含量减少,而Si-C键的含量增加,真空退火有利于SiC的形成;在800℃空气中退火后,薄膜表面生成一层致密的SiO2薄层,阻止了氧气与薄膜内部深层的接触,有效保护了内部的SiC.在空气条件下,SiC薄膜在800℃具有较好的热稳定性.     

高羟基含量苯丙乳液聚合工艺稳定性研究

鉴于某种特殊功能涂料的应用,采用预乳化工艺和半连续种子乳液聚合技术,以过硫酸铵为引发剂,合成了羟基含量在48%左右的St-MMA-BA-AA-HEMA五元共聚苯丙乳液,重点考察了乳化剂的种类和用量、种子单体用量、单体滴加工艺、预乳化单体滴加速率和搅拌速率对乳液聚合工艺稳定性的影响.     

High-temperature thermal stability and in-plane compressive properties of a graphene and a boron-nitride nanosheet

The structural performance of graphene and boron-nitride nanosheet (BNNS) with zigzag and armchair types, when subjected to high temperatures, is investigated through molecular dynamics simulations. It is found that the degree of structure distortion is related to chirality; materials at high temperature of 3500 K, the zigzag nanosheet always exhibits less distortion than the armchair for the same material, and the BNNS exhibits less distortion than graphene for the same chirality. Graphene and BNNS with different in-plane compressive strains are optimized by using the Universal Force Field (UFF) method. It is found that there are two entirely different buckling modes, i.e., the lateral buckling of graphene begins to occur at the middle part, whereas buckling of BNNS begins to occur at near both ends and shows lateral deformation in two opposite directions. The coefficient of elasticity of graphene is slightly smaller than that of BNNS for the same chirality, the coefficient of elasticity of zigzag is slightly bigger than that of armchair for the same material, buckling strain of zigzag nanosheet is larger than that of armchair for the same material, and buckling strains of graphene are always larger than those of BNNS. These phenomena are also analyzed on the basis of radial distribution function (RDF) and system energy. The results indicate that there are thermal expansion anisotropy and planar stress anisotropy in a graphene and a BNNS. Among these materials, zigzag graphene has the highest resistance to compressive buckling but zigzag BNNS can have the highest resistance to distortion at high-temperature distortion and have high compression elasticity.     

Effects of hydroxyl group distribution on the reactivity, stability and optical properties of fullerenols

We investigate the impact of hydroxyl groups on the properties of C(60)(OH)(n) systems, with n = 1, 2, 3, 4, 8, 10, 16, 18, 24, 32 and 36 by means of first-principles density functional theory calculations. A detailed analysis from the local density of states has shown that adsorbed OH groups can induce dangling bonds in specific carbon atoms around the adsorption site. This increases the tendency to form polyhydroxylated fullerenes (fullerenols). The structural stability is analyzed in terms of the calculated formation enthalpy of each species. Also, a careful examination of the electron density of states for different fullerenols shows the possibility of synthesizing single molecules with tunable optical properties.     

Thermal stability of β-tricalcium phosphate doped with monovalent metal ions

The effect of doping β-tricalcium phosphate with monovalent metal ions (lithium, sodium, and potassium ions) on its thermal stability was evaluated using the formation ratio of α-tricalcium phosphate and the rate constant for β-α transformation. The thermal stability of β-tricalcium phosphate doped with monovalent metal ions was higher than that of pure β-tricalcium phosphate, and increased with the amount of metal ions. The increase in stability was attributed to the occupation of all calcium sites including vacancy in β-tricalcium phosphate structure by the calcium ions and monovalent metal ions and the resultant crystal stabilization. However, the thermal stability of β-tricalcium phosphate doped with monovalent metal ions was lower than that of β-tricalcium phosphate doped with magnesium ions. These results indicate that the thermal stability of β-tricalcium phosphate is influenced by the difference in the structural stabilization caused by doping metal ions into different calcium sites in the crystal structure.     

High-temperature pairing in stripes

That the superconducting transition in underdoped high temperature superconductors is controlled by phase ordering, implies that pairing is a local phenomenon that occurs on an intermediate length scale, and typically at a temperature aboveT _c . The discovery of local stripe order in the LSCO family of high temperature superconductors, along with the theoretical suggestion that such structures are a general feature of doped antiferromagnets lead us further to propose that in fact pairing occurs in the vicinity of an individual stripe. The transition to a superconducting state then follows at lower temperature due to Josephson coupling between stripes. In this paper we review our microscopic model of high temperature pairing on individual stripes.     

High-temperature metallizing

The method of making strong metal-ceramic seals to alumina and beryllia ceramics by first sintering a coating containing a molybdenum or tungsten powder on the ceramic and then brazing to it, is reviewed. Evidence in the literature shows that the strongest seals are generally formed on debased aluminas when glass migrates from the alumina into the metallized layer and forms a dense glass/metal structure. A hypothesis, based on a capillary flow mechanism between two porous beds, is proposed which describes how the experimental variables affect the glass migration. The relative importance of the different variables is predicted for varying circumstances.     

High-temperature thermal stability and axial compressive properties of a coaxial carbon nanotube inside a boron nitride nanotube

The structural performance of double-walled C(5, 5)@BN(10, 10) and C(5, 5)@C(10, 10) nanotubes subject to high temperatures is investigated through molecular dynamics simulations. It is found that the inner tube C(5, 5) in the C(5, 5)@BN(10, 10) exhibits less distortion than that in the C(5, 5)@C(10, 10) at annealing temperatures of 3500 and 4000 K. The C(5, 5)@BN(10, 10) and C(5, 5)@C(10, 10) models with different axial compressive strains are optimized using the universal force field (UFF) method. It is found that the critical buckling strains of the inner tubes in the C(5, 5)@BN(10, 10) and C(5, 5)@C(10, 10) are 12.74% and 9.1%, respectively. The critical buckling strain of the former is larger than that of the latter; although the former exhibits greater deformation and energy loss after buckling than does the latter. These phenomena are also analyzed on the basis of the radial distribution function (RDF) and system energy. The results of this study indicate that the outer tube boron nitride nanotube (BNNT) has a better protective effect on the inner tube than does the outer tube carbon nanotube (CNT) under both high-temperature and lower compressive strain conditions. In these cases, the thermal stability and compressive resistance properties of the C(5, 5)@BN(10, 10) are superior to those of the C(5, 5)@C(10, 10).     

High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite

A unidirectional solidification method was investigated to manufacture Al2O3/YAG eutectic composites with high-temperature resistance that would make them usable at very high temperatures. We were successful in manufacturing a single-crystal Al2O3/single-crystal YAG eutectic composite with a dimension of 40 mm in diameter and 70 mm in length containing no colonies or pores. This composite also displayed excellent high-temperature strength characteristics. The flexural strength was in the range 350400 MPa from room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. During tensile tests above 1923 K, the eutectic composite showed evidence of plastic deformation occurring by dislocation motion, and a yield phenomenon similar to many metals was observed. In addition, the microstructure of the composite was extremely stable: after 1000 h of heat treatment at 1973 K in an air atmosphere there was no growth. The above superior high-temperature characteristics are caused by such factors as the eutectic composite having a single-crystal Al2O3/single-crystal YAG structure, the formation of a compatible interface with no amorphous phase and thermal stability, and the combined effect of a YAG phase with superior high-temperature characteristics. © 1998 Chapman & Hall     

Equilibria of hydroxyl defects in MgO

Isochronal annealing studies were carried out on slow-cooled MgO single crystals. The annealing behaviour of the hydroxyl defects is consistent with a defect model in which brucite precipitates dissolve in MgO forming OH-V-OH defects with a heat of solution equal to 0.45 eV. These OH-V-OH defects further dissociate into V _OH ^– defects and hydroxyl ions with a dissociation energy of 0.82 eV.