Microstructure and properties of MoSi2–MoB and MoSi2–Mo5Si3 molybdenum silicides

MoSi₂-based intermetallics containing different volume fractions of MoB or Mo₅Si₃ were fabricated by hot-pressing MoSi₂, MoB, and Mo₅Si₃ powders in vacuum. Both classes of alloys contained approximately 5 vol.% of dispersed silica phase. Additions of MoB or Mo₅Si₃ caused the average grain size to decrease. The decrease in the grain size was typically accompanied by an increase in flexure strength, a decrease in the room temperature fracture toughness, and a decrease in the hot strength (compressive creep strength) measured around 1200 °C, except when the Mo₅Si₃ effectively became the major phase. Oxidation measurements on the two classes of alloys were carried out in air. Both classes of alloys were protected from oxidation by an in-situ adherent scale that formed on exposure to high temperature. The scale, although not analyzed in detail, is commonly recognized in MoSi₂ containing materials as consisting mostly of SiO₂. The MoB containing materials showed an increase in the scale thickness and the cyclic oxidation rate at 1400 °C when compared with pure MoSi₂. However, in contrast with the pure MoSi₂ material, oxidation at 1400 °C began with a weight loss followed by a weight gain and the formation of the protective silica layer. The Mo₅Si₃ containing materials experienced substantial initial weight losses followed by regions of small weight changes. Overall, the MoB and Mo₅Si₃ additions to MoSi₂ tended to be detrimental for the mechanical and oxidative properties.     

大气等离子喷涂MoSi2-30Al2O3电热涂层的组织结构及性能

以MoSi₂-30Al₂O₃混合粉末为原料, 利用大气等离子喷涂技术制备MoSi₂-Al₂O₃体系电热涂层。采用XRD、SEM、通电测试、热重-差热分析等对涂层的相组成、组织形貌和热稳定性进行表征。结果表明：MoSi₂-30Al₂O₃电热涂层体系组织均匀致密, 添加Al₂O₃能改善MoSi₂的电阻率及低温抗氧化性; MoSi₂-30Al₂O₃涂层电热性能优异, 在循环加热测试中, 能稳定地加热到320 ℃并长时间保温, 辊面温度分布均匀, 中部温差控制在25 ℃之内; 循环加热过程中的氧化及热应力的弛豫会导致涂层产生裂纹及孔隙进而导致涂层电阻率升高。     

The effect of Si3N4 on the thermal expansion behavior of MoSi2

The effect of Si₃N₄ particulates on the thermal expansion coefficient (CTE) of MoSi₂ was investigated. It was observed that as the volume percent of Si₃N₄ increases, the CTE of the MoSi₂-Si₃N₄ composites decreases. In the temperature range 1000–1500 °C, typical of that required for glass melting, about 30–35 vol% Si₃N₄ particulates are needed in the MoSi₂-Si₃N₄ composites such that the CTE of the composite matches the CTE of Mo.     

Porous SnO2 sputtered films with high H2 sensitivity at low operation temperature

Undoped and Pd-doped SnO₂ films were deposited at various substrate temperatures and discharge gas pressures using reactive magnetron sputtering. Structural factors of the films, such as crystallite size, grain size, and film density, were systematically investigated. The main objectives of this study are to clarify the operation temperature dependence of the H₂ sensitivity of these films as well as to clarify the dominant structural factor in the determination of the sensitivity. The operation temperature at which the sensitivity defined by (R_a−R_g)/R_g, where R_a and R_g are the resistances before and after exposure to H₂, showed a maximum decreased with decreasing film density. The highest sensitivity of 4470 was obtained for a Pd-doped film with the lowest density of 3.1 g/cm³ at 100 °C. It was found that the sensitivity correlated with film density rather than with crystallite size and grain size. The high sensitivity of a Pd-doped porous film at a low temperature was discussed in relation to the Schottky-barrier-limited transport as well as the chemical and electronic effects of Pd.     

二硅化钼喷涂粉末的制备及其涂层组织结构

以粒度为1~2μm的MoSi₂粉末为原料,采用喷雾干燥和真空烧结制备了喷涂用MoSi₂团聚粉末. 分别用平均粒度为9.68μm的MoSi₂粉末和团聚造粒MoSi₂粉末(38~72μm)为喂料, 大气等离子喷涂制备了二硅化钼涂层,分析了涂层的微观组织结构. 研究结果表明,喷雾干燥造粒后的近球形粉末在1300℃真空烧结1h后,粉末流动性和松装密度分别为17.1s/50g和2.16g/cm³,比烧结前分别增加了57.0%和46.0%.平均粒度为9.68μm 的MoSi₂粉末制备的涂层主要是由Mo和Mo₅Si₃等富钼相组成. 团聚粉末制备的涂层主要由MoSi₂相组成,涂层较致密, 内部出现了类似“网状”的组织结构.     

Degradation of NiCr/CuNiMn/NiCr films on alumina substrates

This paper addresses the aging behaviour of NiCr/CuNiMn/NiCr triple layers on Al₂O₃ ceramics at temperatures up to 200°C for film thicknesses d0.5 μm. Investigations of the film structure and the increase of resistance and its temperature coefficient during the annealing process and studies of the dependence of this aging drift on both the film thickness and the storage temperature have been carried out. Furthermore, the film stress and the effect of substrate bending on resistance have been measured. The results can be explained by the irregular film structure (columns and small bridges between them), which causes stress and current concentrations as well as local creeping, cracking and oxidation processes in the micro-bridges. They are compared with such for structurally homogeneous films on silicon wafers.     

Thin film interaction between low-k dielectric hydrogen silsesquioxane (HSQ) and Ti barrier layer

The interaction between low-k dielectric hydrogen silsesquioxane (HSQ) and Ti barrier layer has been studied using four-point-probe sheet resistance measurement, X-ray diffraction, conventional Rutherford backscattering spectrometry (RBS), nuclear resonance analysis (NRA), elastic recoil detection (ERD), secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). The conventional intermetal dielectrics SiO₂ and plasma-enhanced tetraethylorthosilicate (PETEOS) have been studied also for the purpose of comparison with HSQ. In the low temperature regime (300–550°C), a considerable amount of oxygen atoms, from various sources, diffuses into Ti film to form a Ti(O) solid solution, raising the resistivity of Ti significantly and causing the expansion of the Ti lattice. A good correlation between the oxygen composition in the Ti film, the sheet resistance variation of Ti and the change of Ti lattice parameter C₀ have been observed. At the same temperature, there are more oxygen atoms incorporated into the Ti film in Ti/HSQ than those for Ti/PETEOS, suggesting that additional HSQ-related oxygen sources, such as the moisture uptake and the conversion reaction of HSQ, may be attributed to this. In the high temperature regime (550–700°C), HSQ reacts with Ti to form a final TiO/Ti₅Si₃/HSQ stack structure. It is assumed that a few competing reactions occur in this regime. At 550–650°C, HSQ reacts directly with Ti; in the meantime, part of HSQ undergoes conversion reactions, with the reaction products SiO₂ and SiH₄ reacting with Ti to form Ti silicide. At 650–700°C, HSQ is almost completely converted into SiO₂, so the dominant mechanism is Ti reaction with SiO₂. Before HSQ is completely turned into SiO₂, the Ti/HSQ system is more reactive than both Ti/PETEOS and Ti/SiO₂. The initiating temperature for the Ti/HSQ reaction exhibits no obvious Ti thickness dependence.     

Synthesis and formation mechanisms of molybdenum silicides by mechanical alloying

The synthesis and formation of MoSi₂, Mo₅Si₃, and Mo₃Si compounds by the mechanical alloying of MoSi powder mixtures has been investigated. Ball-milling experiments were conducted for the composition range of 10–80 at.% Si. The formation of molybdenum silicides, especially MoSi₂, during mechanical alloying and the relevant reaction rates markedly depended on the powder composition. The spontaneous formation of MoSi₂ during mechanical alloying at 67 at.% Si (MoSi₂ stoichiometry) proceeded by a mechanically-induced self-propagating reaction (MSR), the mechanism of which is analogous to that of the self-propagating high-temperature synthesis (SHS). At the compositions of 54 and 80 at.% Si, however, the formation of MoSi₂ proceeded by the gradual formation of both the and /gb phases instead of the MSR mode. The formation of Mo₅Si₃ during mechanical alloying was characterized by a slow reaction rate as the reactants and product coexisted over a long period. The milling of Mo-rich powder mixtures up to 150 h did not lead to the direct formation of Mo₃Si. The Mo₃Si phase appeared only after brief annealing at temperatures of 800°C and above.     

Sensitivity enhancement for H2 gas detection using a SnO2–Ag2O–PdOx nanocrystalline system

Thick film H₂ sensors were fabricated using SnO₂ loaded with Ag₂O and PdO_x. The composition that gave highest sensitivity for H₂ was in the wt.% ratio of SnO₂:Ag₂O:PdO_x as 93:5:2. The nano-crystalline powders of SnO₂–Ag₂O–PdO_x composites synthesized by sol–gel method were screen printed on alumina substrates. Fabricated sensors were tested against gases like H₂, CH₄, C₃H₈, C₂H₅OH and SO₂. The composite material was found sensitive against H₂ at the working temperature 125 °C, with minor interference of other gases. H₂ gas as low as 100 ppm can be detected by the present fabricated sensors. It was found that the sensors based on SnO₂–Ag₂O–PdO_x nanocrystalline system exhibited high performance, high selectivity and very short response time to H₂ at ppm level. These characteristics make the sensor to be a promising candidate for detecting low concentrations of H₂.     

The dependence of hardness on the density of amorphous alumina thin films by PECVD

Smooth, uniform and transparent aluminum oxide films were deposited at low temperature (≤ 500°C) by r.f. plasma enhanced chemical vapor deposition (PECVD) with AlCl₃, H₂ and CO₂ as the reactants. The measurements of mechanical and optical properties demonstrate a potential application of PECVD aluminum oxide film as an optical protective coating. The variation of film hardness and density with deposition parameters is apparent and shows a similar dependence while the composition stoichiometry is not so influenced. For amorphous thin films incorporating hydrogen and/or microvoid with no apparent variation on composition stoichiometry, the film hardness is dominated by density and a function near linear can be constructed as energy density is taken into account as an apparent character of hardness.     

Structural, chemical, and electrical characterisation of reactively sputtered WSx thin films

WS_x films were sputter-deposited on Si, SiO₂/Si, and glass substrates from a WS₂ target in an Ar/H₂S atmosphere. Their structure, morphology, chemical composition, and electrical properties were investigated as a function of deposition parameters such as working pressure and H₂S fraction. Films could be grown in the composition range WS_0.3−WS_3.5. Crystallisation was achieved at substrate temperatures T_s > 70 °C and compositions 0.7 ≤ x ≤ 1.95. While the first 5–50 nm near the interface exhibited a basal orientation (c), further growth resulted in the formation of edge-oriented platelets (c) giving rise to a porous, lamellar microstructure. The crystalline structure was mainly turbostratic, while some degree of ordered stacking was present in samples grown at high substrate temperature (600 °C). Resistivity measurements showed a semiconductor-type temperature dependence characterised by activation energies up to 95 meV. Sheet resistance was found to be nearly independent of film thickness, suggesting that the main carrier transport takes place in an interfacial layer of about 20 nm in thickness.     

Effect of minor alloying with Al on oxidation behaviour of MoSi2 at 1200°C

Effect of Al alloying in small concentrations on oxidation behaviour of molybdenum di-silicide (MoSi₂) at 1200°C has been investigated. MoSi₂–2.8 and 5.5 at.% Al alloys possessed 0.5, and 2.5 at.% Al in solid solution, respectively, and dispersoids of -Al₂O₃. On the other hand, MoSi₂–9 at.% Al alloy possessed 3.1 at.% in solid solution in MoSi₂ and Mo(Si,Al)₂ phase, besides -Al₂O₃ dispersoids. The kinetics of oxidation of all the alloys followed a parabolic rate law. The oxidation rate was higher in the MoSi₂–Al alloys in comparison to MoSi₂, with weight gain values varying by an order of magnitude. The MoSi₂–5.5 and 9 at.% alloys demonstrated closely related oxidation characteristics and proved to be more resistant to oxidation than MoSi₂–2.8 at.% Al alloy. The oxide scale comprised of SiO₂ in MoSi₂, mixture of SiO₂ and -Al₂O₃ in MoSi₂–2.8 at.% Al alloy, and -Al₂O₃ in case of MoSi₂–5.5 and 9 at.% Al alloys. The mechanism of oxidation has been analysed using thermodynamic and kinetic considerations.     

Elastic stiffness and thermal expansion coefficients of various refractory silicides and silicon nitride films

The elestic stiffness parameter E_f/(1−ν_f) and the thermal expansion coefficient _f were obtained for four different silicides (TiSi₂, TaSi₂, MoSi₂ and WSi₂) and for two different nitrides (chemically vapor-deposited Nitrox Si₃N₄ and r.f. plasma SiN) from stress-temperature measurements on identical films deposited on two different substrate materials. The values determined for _f and E_f/(1−ν_f) were quite similar for all silicides and averaged 15 ppm °C⁻¹ and 1.1 × 10¹² dyncm⁻² respectively. The thermal mismatch of these silicides is such that, once safely formed, the silicide film should be able to withstand high temperature processing steps without cracking. For the nitrides the values were essentially the same (approximately 1.5 ppm°C^-1), although the larger value of E_f/(1−ν_f) chemically vapor-deposited Si₃N₄ film (3.7 × 10¹² as opposed to 1.1 × 10¹² dyn cm^-2) indicates that it is somewhat stiffer than the SiN film.     

Perturbation method for analyzing mass sensitivity of planarmultilayer acoustic sensors

A perturbation method is developed to analyze the mass loading sensitivity of planar composite acoustic gravimetric sensors. The sensitivity formulas are obtained in explicit forms for the two lowest sagittal (D₁ and D₂) modes, the lowest shear horizontal (SH₀) mode and high-order SH_m modes in a two-layer isotropic composite plate sensor. The composite plate consists of a plate of thickness b coated by a film of thickness h on which the mass loading layer of infinitesimal thickness is deposited. This coating can be a chemically selective film which is assumed to be acoustically thin (h≪λ), where λ is the acoustic wavelength. For Love modes supported by a film coated on a semi-infinite substrate and for Rayleigh modes on a semi-infinite substrate, the sensitivity formulas are expressed in analytical form. These formulas specify the contribution of each material parameter in the substrate and film, and of the elasticity of the mass loading layer for each planar sensor, and provide a general guide for enhancing the sensor sensitivity     

Effects of oxidation on surface stresses and mechanical properties of liquid phase pressureless-sintered SiC–AlN–Y2O3 ceramics

The understanding of the oxidation mechanism of 50 wt% SiC–50 wt% AlN composites obtained by means of pressureless sintering without the protective powder bed and with Y₂O₃ as sintering-aid were significantly improved by means of Raman spectroscopy. These analyses put in evidence that “amorphous carbon” started to be formed at 1300 °C as main effect of active oxidation of SiC. At higher temperature the crystallization process began and it was completed at 1500 °C when only graphite could be recognized. On the basis of these new evidences, oxidation effects on the mechanical properties of SiC–AlN–Y₂O₃ composites were defined. First of all, heat treatment in air was able to induce a compressive surface stress due to the volume gain associated to the oxidation of the intergranular phase. As a consequence apparent fracture toughness showed a value of 6.6 MPa m^1/2 after a heat treatment at 1300 °C, while at higher temperature effects of active oxidation caused a decreasing up to 4.7 MPa m^1/2. This toughening mechanism was also used to improve the resistance to thermal shock, which was evaluated by performing quenching tests. Furthermore, passive oxidation induced the healing of superficial flaws by means of the formation of -cristobalite. This phenomenon was assumed to be responsible for the increasing of the flexural strength.     

Heteroepitaxial growth of tungsten oxide films on sapphire for chemical gas sensors

The performance of chemiresistive gas sensors made from semiconducting metal oxide films is influenced by film stoichiometry, crystallographic structure, surface morphology and defect structure. To obtain well-defined microstructures, heteroepitaxial WO₃ films were grown on r-cut and c-cut single crystal sapphire substrates using rf magnetron Ar/O₂ reactive sputtering of a W target. On r-cut sapphire, an epitaxial tetragonal WO₃ phase is produced at a 450°C deposition temperature whereas 650°C growth stabilizes an epitaxial monoclinic WO₃ phase. On c-cut sapphire, a metastable hexagonal WO₃ phase is formed. RHEED and X-ray diffraction indicate that the films have a ‘polycrystalline epitaxial structure’ in which several grains are present, each having the same crystallographic orientation. STM analysis of the film surfaces reveals morphological features that appear to be derived from the substrate symmetries. The monoclinic phase has a step/terrace growth structure, has the smallest mosaic spread in XRD rocking curves and exhibits the highest degree of reproducibility suggesting that it is the best suited for sensor applications. Measurements of film conductivity versus temperature indicate that the charge transport mechanisms are also dependent on the crystallographic phase and microstructure of the WO₃ films.     

Al2O3基多孔隔热材料表面Al2O3/MoSi2涂层的制备及其性能

采用刷涂法在Al₂O₃基多孔隔热材料表面制备Al₂O₃/MoSi₂涂层,涂层以硅溶胶作为粘结剂,纳米Al₂O₃与Al₂O₃纤维作为耐高温组分,MoSi₂为高发射率组分。通过SEM、XRD对Al₂O₃/MoSi₂涂层微观表面结构、物相组成进行分析。研究纳米Al₂O₃与Al₂O₃纤维的质量比和MoSi₂含量对Al₂O₃/MoSi₂涂层耐温性能的影响,并对Al₂O₃/MoSi₂涂层的抗热震性能、发射率进行表征。结果表明,当纳米Al₂O₃与Al₂O₃纤维的质量比小于1∶1时,热考核后Al₂O₃/MoSi₂涂层表面无裂纹产生;当纳米Al₂O₃与Al₂O₃纤维的质量比在1∶2~1∶4之间时,Al₂O₃/MoSi₂涂层中的纤维网络较完整。MoSi₂的含量为20%时,Al₂O₃/MoSi₂涂层抗热震实验循环25次后表面保持完好,热考核后在2.5~25 μm波段的平均发射率在0.85左右,具有较高的发射率。      

Interpretation of ductile fracture toughness temperature dependence of a low strength steel in terms of a local approach

Fracture toughness and notch ductility tests were performed on two heats of A508 steel tested over the temperature range between 100°C and 450°C. Both types of experiments showed that the materials exhibited a ductility trough at temperatures close to 300°C. At this temperature tensile tests showed the existence of strain aging phenomenon. Tests on axisymmetric notched tensile specimens were used to derive the critical value for void growth, R_c/R₀, used in a model for ductile fracture. A good correlation between J_Ic and R_c/R₀ was observed. This was used to predict the variations of J_Ic with temperature. A reasonable agreement between the predicted values and the experimental results is observed.     

Investigation on the cross sensitivity of NO2 sensors based on In2O3 thin films prepared by sol-gel and vacuum thermal evaporation

In₂O₃ thin films have been prepared from commercially available pure In₂O₃ powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In₂O_3−x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In₂O₃ (HVTE) and nearly stoichiometric In₂O_3−x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO₂ gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO₂ the calculated sensitivities (S=R_g/R_a) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In₂O₃ films to CO and CH₄. Negligible H₂O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl₂ and 10 ppm NO. Only 1000 ppm C₂H₅OH has resulted to have a significant cross to the NO₂ response.     

Influence of thickness and growth temperature on the properties of zirconium oxide films grown by atomic layer deposition on silicon

ZrO₂ films of thicknesses varied in the range of 3–30 nm were atomic layer deposited from ZrI₄ and H₂O–H₂O₂ on p-Si(100) substrates. The effects of film thickness and deposition temperature on the structure and dielectric properties of ZrO₂ were investigated. At 272 and 325 °C, the growth of ZrO₂ started with the formation of the cubic polymorph and continued with the formation of the tetragonal polymorph. The ratio between the lattice parameters increased with the film thickness and growth temperature. The effective permittivity, determined from the accumulation capacitance of Hg/ZrO₂/Si capacitors, increased with the film thickness, reaching 15–17 in 25-nm-thick films. The permittivity decreased with the increasing growth temperature. The hysteresis of the capacitance–voltage curves was the narrowest for the films deposited at 325 °C, and increased towards both lower and higher deposition temperatures.