Microstructure and thermal expansion behavior of diamond/SiC/(Si) composites fabricated by reactive vapor infiltration

Diamond/SiC/(Si) composites were fabricated by Si vapor vacuum reactive infiltration. The coefficient of thermal expansion (CTE) of composites have been measured from 50 to 400 °C. With the diamond content increasing, CTE of composite decreased, simultaneously, the microstructure of the composites changed from core–shell particles embedded in the Si matrix to an interpenetrating network with the matrix. The CTEs of composites versus temperature matched well with those of Si. The Kerner model was modified according to the structural features of the composites, which exhibited more accurate predictions due to considering the core–shell structure of the composites. The thermal expansion behavior of the matrix was constrained by diamond/SiC network during heating.     

Synthesis and characterization of Si/SiC ceramics prepared using cotton fabric

A Si/SiC ceramic was prepared from cotton fabric by the reactive infiltration of liquid silicon into the carbon template. A large density difference between the samples has been observed. This may be due to the variation in the pore size and its distribution within the sample. Scanning electron microscopy with energy dispersive spectroscopy shows the presence of three distinct phases, i.e., SiC, free Si and free carbon. X-ray diffraction pattern also confirms the presence of SiC and Si phases. However, there is no peak corresponding to carbon. So, it is inferred that the carbon exists in amorphous form. Micro-hardness, fracture toughness and bending strength of the ceramics were also studied. The values are lower than commercially available SiC ceramics. This may be due to the highly porous nature of cotton fabric-based SiC, as compared to commercially available SiC.     

Influence of expansion admixtures on the properties of water‐glass‐based refractory pastes

Water‐glass‐based pastes show good prospect for high‐temperature applications, but they are subject to substantial dry‐shrinkage. Silicon and aluminum powders were added to prevent the dry‐shrinkage. The influence of them on compressive strength, bonding strength and volume of the pastes was discussed. SEM, XRD, and TG/DSC were employed to study the influencing mechanisms. The results show that Si and Al effectively prevented the dry‐shrinkage; Al improved compressive strength and bonding strength, while Si was bad for bonding strength. When Al dosage was 6%, the pastes presented mild expansion of 1.05%, the compressive strength increased by 8.9%, and the bonding strength by 33.4%.     

Microstructural evolution and microwave transmission/absorption transition in polymer-derived SiOC ceramics

Herein, we present the structural evolution of polymer-derived SiOC ceramics with the pyrolysis temperature and the corresponding change in their microwave dielectric properties. The structure of the SiOC ceramics pyrolyzed at a temperature lower than 1200 °C is amorphous, and the corresponding microwave complex permittivity is pretty low; thus, the ceramics exhibit wave transmission properties. The Structural arrangement of free carbon in the SiOC ceramics mainly happens in the temperature range of 1200 °C-1300 °C due to the separation from the Si–O–C network and graphitization, while the structural arrangement of the Si-based matrix mainly occurs in the range of 1300 °C-1400 °C owing to the separation of SiC₄ from the Si–O–C network to form nanocrystalline SiC. In pyrolysis temperature range of 1200 °C-1400 °C, the microwave permittivity of SiOC shows negligible change. At a pyrolysis temperature exceeding 1400 °C, the carbothermal reaction of free carbon and the Si–O backbone becomes significant, leading to the formation of crystalline SiC. The as-formed SiC and residual defective carbon improve the polarization loss of SiOC ceramics. In this case, the SiOC ceramics show significantly increased complex permittivity, exhibiting electromagnetic absorption characteristics. These characteristics promote the application of polymer-derived SiOC ceramics to high-temperature electromagnetic absorption materials.     

Formation and Properties of Ln-Si-O-N Glasses (Ln = Lanthanides or Y)

Homogeneous Y-Si-O-N glasses containing 15 or 20 eq% nitrogen (N) were prepared from compositions with Y/Si ratios in the vicinity of that of the lowest eutectic point on the Y₂O₃–SiO₂ phase diagram. The liquidus on the phase diagram shifted toward lower temperatures by incorporation of N. The density, the elastic moduli, and the glass transition temperature of the Y-Si-O-N glasses increased with incorporation of N. This is due to the closer packing of atoms in the glasses by the substitution of N, which is in three-fold coordination with Si, for O which is in two-fold coordination, and the stronger covalent nature of the Si–N bond compared with the Si–O bond. The coefficient of thermal expansion of the Y-Si-O-N glasses increased with increasing Y content, because the discontinuity of the glass network developed with increasing nonbridging anions by the introduction of Y. In contrast, the glass transition temperature and the elastic moduli increased with Y content due to the high coordination of Y for O, and the relatively high cationic field strength of Y. Furthermore, the effect of cationic field strength on properties of Ln-Si-O-N glasses (Ln = lanthanides or Y) is discussed.     

Understanding thermal expansion of pressurized silica glass using topological pruning of ring structures

We investigate the structural origin of the large thermal expansion coefficient of hot-compressed silica glass upon heating to a threshold temperature using molecular dynamics (MD) simulations. While the simulated thermal expansion at low temperature correlates well with the elongation of the average interatomic separation distance of the Si–O or Si–Si pair, the excess thermal expansion due to hot compression mainly results from change in medium range order, including a decrease in the population of large rings upon heating, without significant modification of the small ring population and without changing the short range ordering, such as the Si-O coordination number. The reduction in the characteristic ring size can be connected to the high thermal expansion through the topological pruning mechanism. Such large thermal expansion is suppressed when the silica glasses are held at their respective quench pressures. The suppression of this excess thermal expansion is consistent with the pressure stabilization of the large ring structures.     

Mechanical properties and densification behavior of β‐spodumene/anorthite composites

Although β‐spodumene/anorthite composites are interesting systems, little research work has been done to study their properties. This study aims at investigating the preparation and properties of β‐spodumene/anorthite composites containing β‐spodumene proportions ranging between 10 and 25 mass %. X‐ray diffraction analysis (XRD), Scanning electron microscopy (SEM), and the coefficient of thermal expansion (CTE) were used to characterize the effect of addition of β‐spodumene on the phase relations, microstructure, and thermal expansion behavior of resultant composites. The results show that the presence of β‐spodumene significantly reduces the porosity and reduces the densification temperature. It reduces thermal expansion and enhances the mechanical properties of anorthite‐containing composites.     

The preparation and properties of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics

Two series (N-9 and N-18 series) of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics were designed. Nominal compositions of the glass samples in equivalent percent (eq%) are xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 91O: 9 N and xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 82O: 18 N (x=0, 2, 4, 6), respectively. The obtained samples were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Densities, Vickers hardness, fracture toughness, glass transition temperature, and thermal expansion coefficient data were established for each sample. Effect of Zr and N content on glass network structure, thermal and mechanical properties was investigated. It was found that the addition of zirconia is effective in preparing Y–Si–Al–O–N oxynitride glasses with lower glass transition temperature and higher hardness.     

Thermal expansion coefficient tailoring of LAS glass-ceramic for anodic bondable low temperature co-fired ceramic application

The Li–Al–Si glass-ceramics were prepared by conventional glass-ceramic fabrication method. The influences of Na₂O content on the sintering property, microstructure, and coefficient of thermal expansion were investigated. The results show that the coefficient of thermal expansion of LAS glass-ceramics can be tailored to match that of silicon by the addition of Na₂O content. Na₂O has a remarkable influence on the crystallinity of Li–Al–Si glass-ceramic. The coefficient of thermal expansion of Li–Al–Si glass-ceramic is thus tunable between that of glass phase and crystal phase. The Si–O bond length change in stretch vibration modes introduced by Na₂O also contributes to the variation of coefficient of thermal expansion of the Li–Al–Si glass-ceramics. The coefficient of thermal expansion of the Li–Al–Si glass-ceramic with 1.5 wt% Na₂O addition is about +3.34 ppm/°C at 350 °C and shows a good compatibility to that of silicon in a wide temperature range, which makes it a promising candidate for anodic bondable low temperature co-fired ceramic substrate applications.     

Thermoelectric properties of Zn2GeO4 nano-crystals grown on ITO and Au coated Si substrates by thermal evaporation

We have reported the comparison of thermoelectric properties of Zn₂GeO₄ samples grown on ITO and Au coated Si substrates. Mixed powder of Zn and Ge with 1:1 ratio was evaporated in the tube furnace on ITO and Si substrates using identical experimental conditions. After growth, as grown samples were annealed at different temperatures in oxygen environment. Seebeck and Hall data suggested that the sample grown on Au coated Si substrate (Au/Si) has higher values of electromotive force, electrical conductivity and power factor as compared to sample grown on ITO substrate, because the sample grown on Au/Si substrate has high density of secondary phases. The presence of secondary phases resulted in the enhancement of Seebeck coefficient and mobility of charge carriers. It is also observed that thermoelectric properties of Zn₂GeO₄ samples were also improved due to increased of annealing temperature due the emergence of cracks and secondary phases in the annealed samples. This argument was further supported by various characterizations tools. XRD data confirmed the formation of Zn₂GeO₄ crystal along with the generation of new phases as the annealing temperature increased up to 900 °C. Furthermore, XRD data demonstrated that the sample grown on Au/Si has prominent Zn₂GeO₄ phases but the sample grown on ITO substrate has weak Zn₂GeO₄ peak along with ZnO phases. SEM images demonstrated the generation of cracks on the surface of Zn₂GeO₄ with annealing temperature due to thermal stress. Raman spectroscopy measurements also supported the XRD and SEM results.     

Effect of nano aluminum nitride filler on mechanical,thermal, and dielectric properties of the glass/mullite composites for low-temperature co-fired ceramic applications

Mullite/glass/nano aluminum nitride (AlN) filler (1–10 wt% AlN) composites were successfully fabricated for the low-temperature co-fired ceramics applications that require densification temperatures lower than 950°C, high thermal conductivity to dissipate heat and thermal expansion coefficient matched to Si for reliability, and low dielectric constant for high signal transmission speed. Densification temperatures were ≤825°C for all composites due to the viscous sintering of the glass matrix. X-ray diffraction proved that AlN neither chemically reacted with other phases nor decomposed with temperature. The number of closed pores increased with the AlN content, which limited the property improvement expected. A dense mullite/glass/AlN (10 wt%) composite had a thermal expansion coefficient of 4.44 ppm/°C between 25 and 300°C, thermal conductivity of 1.76 W/m.K at 25°C, dielectric constant (loss) of 6.42 (0.0017) at 5 MHz, flexural strength of 88 MPa and elastic modulus of 82 GPa, that are comparable to the commercial low temperature co-fired ceramics products.     

Fe doping and magnetic properties of zincblende SiC ceramics

Fe-doped SiC bulk ceramics were fabricated by hot-pressing, and their magnetic and electronic properties were investigated. Si_1−xFe_xC (x ≤ 0.04) samples having a zincblende crystal structure exhibited ferromagnetic hysteresis at room temperature with the saturation magnetization increasing with x. X-ray diffraction measurements revealed the creation of a Fe₃Si phase in the samples with its density increasing with x. The samples were found to be p-type semiconductors with a hole concentration (electrical resistivity) of ∼10¹⁹ cm⁻³ (∼10⁰ Ω cm) at room temperature. The observed magnetic properties of the samples are mainly ascribed to the presence of ferromagnetic Fe₃Si crystallites. The high carrier concentration of the samples likely implies the existence of acceptors due to individual Fe³⁺ occupation of the Si sites in the lattice. The randomly distributed Fe³⁺ ions represent a minor contribution to the magnetization of the samples through the formation of magnetic polarons with the carriers.     

Nonlinear ultrasonic evaluation of load effects on discontinuities in concrete

The presence of discontinuity surfaces in concrete structures, i.e. two or more layers in contact, may be an existing situation with evident relapses on damage formation and progression. Differences occur depending on the type of discontinuity, which could be a thin weaker layer or a pre-existing crack. The behavior of pre-existing interfaces is here studied by means of the Scaling Subtraction Method, a Nonlinear Ultrasonic Non-Destructive Technique, that revealed to be effective in describing the mechanical evolution of concrete samples with discontinuity surfaces under the effects of compressive loads.     

长玻纤增强尼龙66复合材料性能的研究

在自制装置中用硅烷偶联剂KH550对长玻纤(LGF)进行表面处理后,采用熔融共混法制备了尼龙66/长玻纤复合材料。采用微机全自动热膨胀系数测定仪记录了玻纤增强尼龙66复合材料的热膨胀曲线,分析了玻纤含量、温度对复合材料热膨胀系数的影响,结果表明,随着玻纤含量的增加,复合材料的热膨胀系数显著下降,最大降低了74.2%;随着温度的升高,复合材料的热膨胀系数先增大后减小最后趋于平衡,转折温度在37℃左右。测试了复合材料的力学性能,结果显示复合材料的拉伸强度、弯曲强度和缺口冲击强度随玻纤含量的增加而大幅度提高,最大分别增加了173%、186%和283%。通过扫描电镜观察到玻纤嵌入尼龙66基体中,与尼龙66形成了良好的界面黏结。     

High-Temperature Raman Spectra and Thermal Expansion of Wollastonite

Unpolarized single-crystal Raman spectra in the temperature interval of 298–1235 K and powder X-ray diffractometry data from room temperature up to 1295 K have been obtained from a natural triclinic wollastonite of almost-ideal composition. Room-temperature spectra, along with the temperature dependencies of nine major Raman modes, are presented. The temperature shifts of the Raman modes indicate a discontinuity in the temperature range of 950–980 K. No corresponding changes are observed in the temperature dependencies of the lattice parameters. Thermal expansion coefficients for the lattice parameters in the temperature range of 298–1295 K are given.     

Time and strain dependent thermal expansion of carbon-black filled SBR elastomers

Length-temperature measurements were made on unfilled and carbon-black filled vulcanizates of styrene-butadiene rubber. As in earlier experiments on polybutadiene, transients in thermal expansion behavior were observed following abrupt changes in the temperature. The nature of this time dependence was not dramatically affected by either the level of equilibrium strain or the structure and loading of the carbon-black filler. Both the magnitude of the linear coefficient of thermal expansion and the point of thermoelastic inversion were sensitive to the presence of carbon-black. A model based on considerations of occluded rubber and strain amplification in the free rubbery matrix was used successfully to rationalize the observed behavior of the filled compounds.     

Experimental study of the thermo-mechanical behaviour of alumina-silicate refractory materials based on a mixture of Algerian kaolinitic clays

In the framework of a general study carried out on the thermo-mechanical behaviour of refractory products made from Algerian refractory clays, silica-alumina bricks based on a mixture of halloysite from Djebel Debbagh and kaolin from Tamazert were studied. The material's initial chemical composition and physical properties are reported. Uniaxial compression and three-point bending tests were performed under temperature conditions from room temperature to 1200 °C on representative specimens cut from the bricks. The evolution of the material's resistance in tension and compression as of its modulus of elasticity with testing temperature is reported. The thermal expansion of the material was investigated as were optical and SEM micrographs at various temperatures. The evolution of the material rheological behaviour from quasi-brittle, between room and 900 °C, to viscous at higher temperatures was shown by both bending and compression tests. The general behaviour of the material with increasing temperature was analysed through the various microstructure investigations and the presence of micro-cracking induced by the differential expansions of the multiple phases in presence.     

高温下煤灰热膨胀行为研究

为了更好地了解硅铝比在煤炭燃烧和气化过程中对炉体结渣和堵渣产生的影响,选取硅铝比不同的A、B、C三种煤样,在弱还原性气氛下,利用自主研制高温熔融装置,考察三种煤灰样在高温熔融炉中的形变情况,探究煤灰热膨胀行为。针对A煤样,将其灰化后分别添加SiO2和Al2O3,添加量为煤基的1%~5%,研究其高温下XRD物相变化、熔渣流动现象和熔渣表观形貌。结果表明:不同硅铝比的煤灰热膨胀行为存在差异,B、C煤灰产生明显热膨胀现象,A煤灰未出现此现象;添加SiO2的A煤灰产生明显热膨胀现象,而添加Al2O3却未出现膨胀过程。添加SiO2后的煤灰在高温下灰粘度会增加,气体较难透过粘稠液体相,造成煤灰在高温下会出现明显膨胀现象,进而会导致煤炭在燃烧和气化过程中出现炉体内壁的结渣和出渣口堵渣等问题。通过考察弱还原性气氛下,研究不同硅铝比对煤灰高温熔融过程形变的影响,为解决实际生产中气化炉结渣和堵渣问题提供理论依据。     

Electronic phase transition in CrN thin films grown by reactive RF magnetron sputtering

Chromium nitride thin films were prepared by reactive radio frequency magnetron sputtering on glass and Si(001) substrates. Thin films were grown at different nitrogen to argon flow rate ratio and their effect on the film composition, band gap and electronic phase transition was studied by different experimental technique. X-ray diffraction analyses establish that CrN films predominantly grow in [111]_CrN and [002]_CrN directions irrespective of the substrates used for the growths. The band gap was found to vary with the composition of the films. All films are semiconducting at room temperature and show discontinuity in their resistivity versus temperature curve indicating electronic transition. But the low temperature electronic phase depends on the growth conditions and composition of the films.     

Metal recovery from coal ash via chlorination — A thermodynamic study

Coal ash can become an important mineral substitute for the recovery of several metals, especially aluminum and titanium. A plausible route for the recovery could be via high temperature chlorination in the presence of a reducing agent. The thermodynamics of the reaction system for the free energy computations consisted of 135 chemical species containing 12 elements, namely Al, C, Ca, CI, Fe, K, Mg, Na, O, S, Si and Ti. The results indicate that the chlorination route is feasible over a wide temperature range.