Papers by Keyword: Fused Silica

Abstract: For achieving high-temperature resistance and a broadband of microwave transmission, ceramic metamaterials consisting of fused silica ceramic substrates and electrically conductive networks/ arrays are desirable. A new strategy of fabricating the fused silica metamaterials is to combine the low temperature co-fired ceramic (LTCC) technique with a method of ceramic joining via green tapes. The important part of the new strategy lies in the preparation of suitable green tapes that are co-firable with a conductive silver-based film/strip and have a strong affinity to the fused silica substrates. Therefore, in this paper, three green tape materials were prepared and intensively characterised using scanning electron microscopy, x-ray diffraction, dilatometry, dielectric measurement, etc. It was found that the tape materials were based on dielectric glasses and crystalline phases of major eulytite and minor cristobalite, leading to rather low levels of dielectric constant (<6) and loss tangent (in the order of 10^-3). The three tape materials also had different levels of thermal expansion coefficients, co-firability with a conductive silver-based paste, and bondability to the fused silica substrates. These findings suggest that one can achieve desirable ceramic matematerials with well-controlled shapes and dimensions of the condutive networks/arrays after properly laminating the green tapes between the fused silica substrates.

Abstract: In order to superpolish fused silica surface, a non-contact polishing method is applied, which is called as computer numerical-controlled (CNC) micro-jet polishing (MJP) technology developed on the base of the hydrodynamic mechanism. In this paper, both the theory and the techniques about this new polishing technology are introduced in detail, and the curved surface of fused silica was polished. The results demonstrate that no new scratches were produced when the nanoparticles removed the atoms away from the surface. The roughness decreased monotonously with the removal of subsurface damage layer. And ideal Ultrasmooth surface without scratches was achieved by MJP with waveness less than 0.2nmrmsRMS while the high-spatial frequency roughness less than 0.1nmrmsRMS (sub-angstrom).

Abstract: Atmospheric Pressure Plasma Processing (APPP) of silicon-based optics and wafers is a form of chemical etching technology developed in recent years. The material removal rate is comparable to those of conventional mechanical processing methods in precision fabrication. Moreover, there is no mechanical contact or physical loading on the substrate surface, hence no surface or sub-surface damages are induced. Inductively coupled plasma is one realization of APPP. In this work, inductively coupled plasma torch is used to generate plasma and excite etchant particles at atmospheric pressure. These active particles then diffused to the workpiece surface, react with its atoms to form volatile products. The activity and number of particles in plasma are influenced by processing parameters such as input power, distance between nozzle and substrate surface, flow rate of plasma gas argon and precursor gas CF₄. These factors have various impacts on material removal rate. Processing experiments are conducted on fused silica to investigate the parameters’ influences on material removal rate. The basic interaction between substrate surface and plasma is illustrated, then the relationships between processing parameters and material removal rate are analyzed. From the experiments some trends are derived. Material removal rate rises with the increase of power and flow rate of CF₄, whereas decreases with the increase of processing distance, etc. The etching footprint is proved to be near Gaussian-shaped and believed to have high potential for deterministic surface processing.

Abstract: History is often marked by the materials and technology that reflect human capability and understanding. Many a times scales begins with the stone age, which led to the bronze, iron, steel, aluminium and alloy ages as improvements in refining, smelting took place. Science made all these possible to move towards finding more advanced materials.Therefore in the present research work, an investigation has been carried out to fabricate and evaluate the microstructure, strength, micro hardness of chilled composites consisting of nickel matrix and fused SiO₂ particles as the reinforcement (size 40-150 μm) in the matrix. The reinforcement being added ranges from 3 to 9 wt. % in steps of 3%. The resulting composites cast in moulds containing metallic chill blocks (MS, SiC & Cu) were tested for their microstructure and mechanical properties. The main objective of the present research is to obtain fine grain Ni/SiO₂ chilled sound composite having very good mechanical properties. A detail of melting and composite preparation is described elsewhere by number of researchers. After melting the matrix material in an induction furnace at around 1600 °C in an inert atmosphere, coated fused SiO₂ particles preheated to 500 °C were introduced evenly into the molten metal alloy by means of special feeding attachments. The moulds for the plate type of castings 150*20*20 mm (American Foundrymen Society standard) were prepared using silica sand with 5% bentonite as binder and 5% moisture and finally they were dried in an air furnace at a temperature of 1580 °C, which was cooled from one end by a chill block set in the mould. After solidification the specimens of chill end were tested for various mechanical and microstructural studies. Keywords: Metal matrix composite, Mechanical properties, Nickel alloy, Fused silica, Chills.

Abstract: Laminated composites with a frequency selective surfaces (FSS) or more complex metamaterials are potential radome materials due to their unique characteristics of electromagnetic wave transmission. For making high-temperature resistant radomes, metamaterials or laminated composites with an FSS should be based on ceramic substrates. However, the processing methods for ceramic metamaterials are very limited and the conventionally used LTCC technique suffers from the shortcoming of large sintering shrinkage rates, which unfortunately impede the production of ceramic-based metamaterials. In this paper, a novel method of a low temperature co-fired ceramic (LTCC) technique combined with a technique of ceramic joining via green tapes was developed to fabricate the fused silica ceramic laminates sandwiched with the FSS. It was found that the newly developed composites with the FSS unit cells of the Ag-Pd strips exhibited near zero shrinkage of the unit cells, showed predictable transmission efficiencies of electromagnetic microwaves, and were able to overcome the poor transmission efficiencies below 11 GHz of the pure fused silica ceramic plates with an identical thickness.

Abstract: The processing methods, strengthening methods, water-proof coating procedures, and some ways to improve the transmission of microwaves have been reviewed in relation to fused silica ceramics and their composites for radome applications. Fused silica ceramics are characterized by a residual porosity (up to 18%), low dielectric constant (3.06-3.32), low loss tangent (0.00053-0.0065), excellent thermal properties, but low mechanical strength (37-65 MPa). To achieve higher mechanical strength and better transmission efficiency, new randome materials would be those engineered composites consisting, for instance, of a dense layer-porous core structure, a continuous fiber reinforcement, a multilayered structure, and/or frequency selective surface (FSS) layer (s)/metamaterials.

Abstract: Surface and subsurface defects of optics are of major concern in improving laser induced damage threshold. SiO₂ single layers were fabricated by physical vapor deposition and sol-gel technique on fused silica substrates. HF acid etching and ultrasonic cleaning process are used to investigate the effect of surface and subsurface defects of substrates on the laser induced damage threshold (1-on-1, 8 ns at 355nm). Experimental data are then fitted with the Gaussian model of threshold distribution, which permits to discriminate different kinds of defects and extract their densities and threshold distribution. The interpretation of these data is further discussed according to their cleaning and fabrication method.

Abstract: To provide a fundamental knowledge for the high efficiency grinding and ultra-precision grinding of fused silica, ductile mode and brittle mode material removal mechanisms were investigated by conducting micro/nanoindentation experiments in the range of 4.9 mN - 1960 mN. Before observing cracks and determining the ductile to brittle transition penetration depth, the samples were etched with hydrofluoric acid to expose cracks. The typical damage morphology of fused silica was discussed by observing the surface and cross-section of indentations, and the depth of SSD was found to be determined by the cone cracks or borderline cracks in the different load range. The ductile to brittle transition penetration depth of fused silica under Vickers indentation was 180 nm.

Abstract: Zirconyl chloride was used as zirconium source and fused silica particles were used as main raw material. First of all, the composite powders were prepared by wet chemical synthesis using ammonia as the precipitator and polyethylene glycol as the dispersant. Then, fused silica nanozirconia composite ceramic containing nanometer particle zirconia with different contents (5%, 15%, 25% and 35%) were fabricated in reduction atmosphere at 1300°C, 1350°C and 1400°C for 1 h. The bulk density and bending strength were measured, microstructure was observed by SEM. The result indicated bulk density and bending strength of composite ceramic increase and microstructure becomes denser with content of zirconia increasing. Bulk density of composite ceramic increases and bending strength which reaches maximum at 1350°C firstly increases then decreases with the increase of sintering temperature. Both high sintering temperature and nanozirconia possessing high energy interface can improve the composite ceramic sintering.

Abstract: In order to get ultra-smooth surface without subsurface damage efficiently for fused silica, the atmospheric pressure plasma processing (APPP) is developed. It is based on chemical reaction between active radicals excited by plasma and workpiece surface atoms, so the subsurface damage caused by contact stress can be avoided and atomic-level precision machining can be achieved. In this paper, the influence on material removal function by the key factors of APPP including the flow rate of reaction gases, input power, and processing distance are discussed. In addition, by the regression model a quantitative mathematical model of the material removal function of the atmospheric pressure plasma processing on fused silica is established. And this model is verified by experimental data.