Fused Silica Ceramics Research Articles

High-strength and low-dielectric ZrO2 reinforced fused silica ceramics by gelcasting

High-strength and low-dielectric ZrO2 reinforced fused silica ceramics by gelcasting

Boron and Nitrogen Doping in Fused Silica Ceramics: Structural, High-Temperature Mechanical and Long-Term Ablation Resistance Properties of Si-B-O-N Ceramics

Boron and Nitrogen Doping in Fused Silica Ceramics: Structural, High-Temperature Mechanical and Long-Term Ablation Resistance Properties of Si-B-O-N Ceramics

Effect of solid loading on phase composition, microstructure, mechanical and dielectric properties of fused silica ceramics by gecasting

In this study, the fused silica slurries with an ultra-high solid loading of 85 vol%, low viscosity of 0.34 Pa s at 99.4 s−1, and good suspension stability were successfully prepared using two-gradation fused silica raw powders by optimizing the preparation processes. Owing to the inherent characteristics of gelcasting and good dispersion stability of resulting fused silica slurries, all the fused silica ceramics prepared with various solid loadings exhibited a homogeneous bimodal size distribution microstructure after pressureless sintering at 1250 °C for 4 h. Furthermore, a small amount of nanoscale cristobalite was precipitated from fused silica, and solid loading had no significant effect on the crystallization kinetics of cristobalite. A significant increase in solid loading effectively facilitated the densification of fused silica ceramics, enhanced bulk density, decreased sintering shrinkage, and strengthened mechanical properties. However, the increase in solid loading deteriorated dielectric properties within the range of 2–18 GHz. When the solid loading was 85 vol%, the bulk density, linear shrinkage, flexural strength, dielectric constant, and dielectric loss tangent were 2.12 g/cm3, 1.1 %, 72.3 MPa, 3.91–4.36, 0.0081–0.0171, respectively. These properties were favorable making the materials potential applications in the field of microwave transmission.

Influence of β‐Si3N4 whiskers on sintering and crystallization of fused silica ceramics

AbstractFused silica ceramics are widely applied for radome materials, crucibles, and vanes, but the mechanical properties were deteriorated due to the cristobalite crystallization. The fused silica ceramics added with by β‐Si3N4 whiskers were prepared by a slip‐casting method to retard the cristobalite crystallization. The influences of the sintering environments and the β‐Si3N4 whiskers on the microstructure and phase structure were investigated. The silanol (Si‐(OH)n) and oxygen vacancies (VO) in the fused silica in formed in different conditions were studied by Fourier Transform Infra‐Red (FT‐IR) and X‐ray photoelectron spectroscopy (XPS), and the results indicated that the ball‐milled produced a large amount of the silanol groups onto the surface of the fused silica particles. The fused silica heated in the vacuum created the maximum oxygen vacancies (24.2%) on the surfaces. Silanol groups reacted with the β‐Si3N4 whiskers, and the O atoms in the silanol groups were fixed into the bulk materials. And the crystallization kinetics and the activation energy of Si3N4w/SiO2 ceramics at the temperature ranging from 1200 to 1400°C were calculated based on the JMA(Johnson‐Mehl‐Avrami) model. The activation energy of the fused silica ceramics with the addition of the β‐Si3N4 is 506.2 kJ/mol, increased by 23.6% than that of the pure fused silica ceramic.

Fabrication and characterization of Si 3 N 4 whisker‐reinforced SiO 2 ceramic for radome materials

Fused silica ceramic has become one of the most widely used radome materials in the world since the 1970s. But its poor mechanical properties restricted its application to some extent. To improve the mechanical properties of the fused silica ceramic and keep its characteristic for radome materials, silicon nitride (Si3N4) whisker-reinforced fused silica ceramics were prepared by a slip-casting method in the work. The influence of Si3N4 whisker contents on the properties of the slurry was studied, indicating that the preferable pH values of the slurry were 4–6 and whisker contents were 10 wt.%. The flexural strength of as-prepared Si3N4w/SiO2 ceramic was about 74.35 MPa, exhibiting an increase of 7.75% over that of the pure silica sample. Its dielectric constant in the range from 8 to 12 GHz and tanδ under 10 GHz were, respectively, 3.37 and .0011. It is of great interest to find that Si3N4w/SiO2 has excellent oxidization resistance and its mass maintains even at 1270°C.

Effects of sintering temperature and cristobalite content on the bending strength of spark plasma sintered fused silica ceramics

Fused silica ceramics have been extensively applied in the aerospace, optical, and telecommunication industries. In this study, spark plasma sintering (SPS) at 900–1300 °C was employed to fabricate transparent fused silica ceramics. The particle size of the primary powders was 8 μm and 70 μm according to particle size analyzer (PSA). The X-ray diffraction (XRD) analysis was also used to determine the crystalline phase and evaluate the crystallinity. Field Emission Electron Microscope (FE-SEM) images were taken to investigate the microstructure of the samples. The mechanical strength of the specimens was assessed by the three-point flexural strength technique. Results of XRD analyses showed that, the cristobalite was formed in the sintered samples at a temperature of 1300 °C. Semi-quantitative analyses showed that the sintered sample at 1300 °C with a particle size of 8 μm contained cristobalite at about 15%, while the amount of the cristobalite in the 70-μm sample was evaluated at about 4%. SEM images showed the presence of the microcracks developed as a result of the crystallization of the cristobalite in the sample P70S13. The acquired FTIR spectra indicated the presence of bonds of the cristobalites. Samples P70S11 exhibited bending strength of about 40 MPa, respectively. It was evident that the sample with an initial particle size of 8 μm was of a lower bending strength than the sample with a particle size of 70 μm.

Preparation and properties of fused silica ceramics by Isobam spontaneous coagulation casting

By surface modification with APTMS, spontaneous coagulation casting (SCC) of fused silica based on Isobam was achieved and the possible coagulation and molding mechanism is proposed. Through the interaction between the polar groups on the Isobam molecular chain and the incorporated –NH 2 groups on the surface of the silica particles, Isobam molecular chains were adsorbed on the surface of particles, which initiate the formation of flocs and the solidification of the suspension. The addition of dispersant TMAH results in the hydrolyzation of Isobam, forming more –COO – , which effectively improves the fluidity and stability of the suspension. Then the zeta potential, rheological properties and coagulation behavior of the suspension were systematically investigated and the fused silica suspension with high solid content (up to 52 vol%), low viscosity and good coagulation properties were prepared at 1.8 wt% TMAH and 0.5 wt% Isobam dosage. After sintering at 1260 °C for 4 hours, the fused silica ceramics (50 vol% solid content) shows a high bending strength of 61.59 MPa, the lowest dielectric loss tanδ of 8.46×10 -4 and the dielectric constant of 3.72. Thus, this work provides a simple and effective method for preparing fused silica and other ceramics with negative surface charge by Isobam SCC.

Effect of Cristobalite Content on Physical, Dielectric Constant, and Bending Strength of Fused Silica Ceramics Formed by Slip Casting Method

Fused silica ceramics are widely used in electronics and aerospace industries. In the present study, 70 µm of fused silica powder was milled to 10 µm through fast milling. The appropriate slurry was prepared for slip casting with the powder-to-water ratio of 80:20. After drying the specimens, the samples were sintered at different temperatures of 1100 °C to 1400 °C. The density increased upon increasing the temperature from 1.79 to 1.98 g/cm3. The phase transformation of the samples was investigated using XRD. The structure of the samples was analyzed using FTIR, and their microstructure was examined using a Field Emission Scanning Electron Microscope (FESEM). The bending strength of the samples was measured using the three-point method. According to the results, the cristobalite phase increased upon increasing the sintering temperature. The best flexural strength value (48.7 MPa) was obtained for the sample sintered at 1300 °C. The dielectric constants of the fused silica ceramics were about 3-3.8 in the frequency range of 8 to 12 GHz.

Effect of Carbothermal Reduction Nitriding of Si/SiO2 Composite Green on Properties of Fused Silica Ceramics

Si/SiO2 composite green body with solid content of 50% was prepared by low toxic gel system and sintered in nitrogen at high temperature. Nitrogen compounds were synthesized in situ on fused silica ceramic substrate. The influence of different temperatures on nitriding reaction was discussed: different temperatures were used for nitriding at 1350 °C, 1400 °C, 1450 °C, 1500 °C. The effect of different nitriding times on nitriding reaction was discussed: different holding times were used for nitriding +1 h, 1.5 h, 2 h and 2.5 h. The best nitriding process was obtained by analyzing the composition, microstructure and properties of the nitrided samples.

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO2 composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si2N2O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si2N2O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO2 composite billets can be used to synthesize spike-like and fibrous Si2N2O in situ in nitrogen at 1450 °C. Si2N2O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO2 composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm3 and 114.37 MPa, respectively.

Room temperature electrical properties and excitation performance of Al2O3 doped fused silica ceramics

Al2O3 doped fused silica ceramic samples were synthesized by colloidal processing route. X-ray diffraction technique helped to give insight upon the phase formation of the synthesized samples. The dense growth of grains was depicted from surface micrographs. The frequency-dependent electrical properties of both sintered and quenched samples were examined from 100 Hz till 1 MHz. The frequency-dependent AC conductivity followed Jonscher’s power law. The dielectric relaxation and Nyquist plot confirm the non-Debye nature in both sintered as well as quenched samples. The ferroelectric properties were depicted in the prepared samples. The sintered and quenched Al2O3 doped fused silica samples reveals that the samples get excited on the application of electric field and further can be utilized in filter application.

Porous fused silica ceramics prepared by gelcasting using multigrade fused silica powders

Porous fused silica ceramics were successfully prepared by gelcasting using N, N–dimethylacrylamide (DMAA) as gelling agent and multigrade powders as starting materials. The results suggest that the ratio of fine–grained to coarse–grained mixed powders, solid loading and sintering temperature were highly correlated with microstructure and properties of fused silica ceramics. When the ratio was 0.8 and solid loading was 80 vol%, the resulting slurries had a relatively high viscosity, which ensured the excellent homogeneity and stability of slurries, ultimately endowing fused silica ceramics desirable microstructure and properties. As sintering temperature increased from 1225 °C to 1300 °C, the bulk density (1.78–2.05 g/cm3) and linear shrinkage (1.98–4.23%) were increased, but the flexural strength was only enhanced within 1275 °C with a maximum of 20.69 MPa, since excess fused silica crystallized into cristobalite at 1300 °C, which deteriorated mechanical properties.

Dielectric and impedance spectroscopy of aluminium oxide substituted fused silica samples

This paper shows a comparative study of sintered and thermally quenched Al2O3 substituted fused silica ceramics processed by colloidal suspension route. The structural analysis is performed using X-ray diffraction measurements. The natural surface micrographs present close packing of non-uniform shape of grains. The high permittivity and low loss are illustrated from frequency dependent permittivity. The impedance spectroscopy shows non-Debye type relaxation mechanism. The Nyquist plot shows the presence of only grain effect. The overlapping large Polaron tunnelling model is attributed from the conduction studies of both sintered and quenched samples.

Dynamic modeling of the turning process of slip-cast fused silica ceramics using the discrete element method

Simulation of brittle regime machining of materials (such as ceramics) is often difficult because of the complex material removal mechanisms involved. In this study, the discrete element method is used to simulate the dynamic process for machining of slip-cast fused silica ceramics. Flat-joint contact model is exploited to model contacts between particles in synthetic discrete element method models. This contact model is suitable for modeling of brittle materials with high ratios (higher than 10) of unconfined compressive strength to tensile strength. The discrete element method has the ability to simulate initiation, propagation, and coalescence of cracks leading to chip formation in the brittle regime of cutting. Applying the discrete element method, the influences of operating conditions on the creation of surface/subsurface damages, chip formation, and cutting forces are studied. It is shown that the parameters of the material model determined from conventional calibration tests do not provide quantitatively accurate prediction of cutting forces. As such, model updating is carried out using the forces obtained in the cutting experiments, and the numerical results are verified against experimental cutting forces. The differences between experimental and numerical machining forces are in the range of 10%–30%. Finally, the results of discrete element method simulations reveal that the nature of micro-crack propagation is brittle in the machining process of ceramics.

Erosion Behavior of Gelcast Fused Silica Ceramic Composites

In this paper, the characteristics of solid particle erosion on fused silica ceramics are investigated. Gelcasting, a near net shape forming process, is adopted for the fabrication of ceramics. Three types of ceramics with a combination of pure fused silica, fused silica+5 wt% silicon nitride (Si3N4) + 1 wt% boron nitride (BN) and fused silica+5 wt% silicon nitride (Si3N4) +1 wt% alumina (Al2O3) are prepared at a constant 52 vol% solid loading, 10 wt% monomer content and 10:1 monomer ratio. Different impingement angles (30o, 45o, 60o and 90o) and three impact velocities (86 m/s, 101 m/s and 148 m/s) were chosen to examine the behavior of erosion on gelcasted ceramics using SiO2 particles as erodent. The maximum rate of erosion is obtained at normal impingement angle (90o), which shows the brittle nature of ceramics. The impact velocity and angle of impingement have an appreciable effect on erosion rate. Resistance to erosive wear is found to have improved with the inclusion of reinforcements in the fused silica ceramics. The erosion rates of different ceramics are compared. Ceramic composite with a combination fused silica+5 wt% Si3N4+ 1 wt% BN shows the highest resistance to wear. The surface roughness and morphology of the eroded surfaces have also been studied.

Studies of structural, dielectric and impedance spectroscopy of fused silica ceramics fabricated through colloidal processing

In the present work, the fused silica was fabricated using colloidal processing route. The fabricated samples were quenched (five time thermal cycling) and the results of the structural and electrical properties of the sintered sample and quenched sample have been studied and compared. The structural analysis confirmed the formation of the fabricated ceramics. The morphology of the sample suggested that there is close packing of the grains. The relaxation and conduction mechanism were analyzed using impedance and modulus spectroscopy. The dielectric parameters were inspected on varying temperature and frequency range. The impedance study showed the dependence of electrical properties on both frequencies and temperatures having corresponded with the morphology.

Preparation of high-density fused quartz ceramics based on a J2-optimality mixed orthogonal method

A method for obtaining high-performance fused silica ceramics was investigated in this work. An advanced experimental method was used to prepare green bodies. The optimal experimental scheme was deduced using the hybrid orthogonal experiment method of J2-optimality. The solid-phase content, the amount of dispersant, the pH and the milling time were used as variables. The effects of various factors related to the processing of fused silica ceramics on the performance of the crucible were studied. The structure and properties of the samples were characterized by using a universal testing machine, a scanning electron microscope, an electron density meter and an X-ray diffractometer. The results show that a solid-phase content of 65 wt%, a pH of 5, a dispersant content of 2.5 wt% and a milling time of 15 led to a green body with a flexural strength of 18.5 MPa and a bulk density of 1.91 g/cm3, resulting in ceramics with improved mechanical properties.

Microwave transmission performance of fused silica ceramics in microwave high-temperature heating

Compared with traditional heating methods, microwave heating is widely used in many industrial heating fields because of its advantages, such as material-selective heating, volumetric heating and high-efficiency heating. However, previous studies have shown that the heating mode depends on the transmission performance of refractory insulation materials. In this study, a commonly used refractory material (fused silica ceramics) for microwave heating was studied, according to the law of electromagnetic wave propagation in a single-layer plate, the power transmission coefficient (PTC) of the material at different thickness, temperature and frequency (915 MHz and 2360 MHz) was calculated and analysed, in order to design the refractory structure which has good microwave transmission performance in the whole microwave heating process. The results show that the transmission performance of fused silica ceramics fluctuates regularly with increasing thickness, and there are several microwave transmission peaks in the PTC patterns, the thicknesses corresponding to the transmission peaks at frequency 2360 MHz are in order 0.033 m, 0.065 m and 0.098 m; as the temperature increases, the position of the transmission peaks will shift to a smaller thickness, the transmission performance of fused silica ceramics decreases slightly with increasing temperature. Comparing the transmission performance of mullite, alumina (Al2O3) and fused silica ceramics, it was found that the fused silica ceramics have excellent transmission properties; especially it is at high temperature. The results of this study can provide guidance for the selection of refractory structure materials and sizes as well as the optimization of heating process parameters in high-temperature microwave heating.

Experimental and numerical study of low frequency pulsed Nd:YAG laser heating of slip cast fused silica ceramics for laser assisted turning process considering laser beam overlapping

Machining of ceramics often involves many challenges due to their high hardness, brittleness, and low thermal conductivity. Laser assisted machining (LAM) is a promising technology for improving the machinability of hard-to-cut materials. In this work, the effect of laser heating in the LAM process on slip cast fused silica (SCFS) ceramics is investigated by presenting a numerical thermal analysis of laser effects on material behavior. A transient three-dimensional heat transfer analysis for laser assisted turning (LAT) of SCFS is performed using finite element method. Temperature distributions in SCFS cylindrical specimens are obtained. Moreover, the influence of laser parameters such as power, translational speed, and feed rate on the temperature field are studied. To increase the absorptivity of the ceramic surface, a coating is applied, and the absorptivity of the coated surface is determined by carrying out a series of experiments. Experiments are performed to validate the numerical transient heat transfer finite element model. In addition, the effects of spot overlapping of pulsed laser on temperature distribution and absorptivity of SCFS workpiece are studied. It is for the first time that effect of laser beam overlapping on low frequency pulsed laser heating in LAT is formulated and completely investigated.

High-temperature ablation properties of nano zirconia reinforced fused silica ceramics

The high-temperature ablation properties of nano zirconia reinforced fused silica ceramics were tested using an ultra-high-temperature oxyacetylene ablation equipment. The results show that the linear recession rate and the mass loss rate of the composite ceramics containing 10 wt% zirconia are 0.087 mm/s and 0.016 g/s at 40 s ablation time, respectively, both much lower than that (0.106 mm/s and 0.028 g/s) of the pure fused silica ceramics, indicating superior ablation resistance properties of the composite ceramics. The depth of the melting area along with the flame direction of the composite ceramics is much shallower than that of the pure samples. Moreover, the composite ceramics show excellent heat shielding properties.