Study on the subsurface damage mechanism of optical quartz glass during single grain scratching

The single grain scratching SPH simulation model was established to study the subsurface damage of optical quartz glass. Based on the analysis of the stress, strain and scratching force during scratching, the generation and propagation of subsurface cracks were studied by combining with the scratch elastic stress field model. The simulation results show that the cracks generate firstly at the elastic-plastic deformation boundary in front of the grain (φ = 28°) due to the influence of the maximum principal tensile stress. During the scratching process, the median crack closes to form the subsurface damage by extending downward, the lateral crack promotes the brittle removal of the material by extending upward to the free surface, and microcracks remain in the elastic-plastic boundary at the bottom of the scratch after scratching. The depth of subsurface crack and plastic deformation increases with rising scratching depth. The increase of scratching speed leads to the greater dynamic fracture toughness, accompanied by a significant decrease of the maximum depth of subsurface crack and the number of subsurface cracks. The subsurface residual stress is concentrated at the bottom of the scratch, and the residual stress on both sides of the scratch surface would generate and propogate the Hertz crack. When the scratching depth is less than 1.5 μm or the scratching speed is greater than 75 m/s, the residual stress value and the depth of residual stress are relatively small. Finally, the scratching experiment was carried out. The simulation analysis is verified to be correct, as the generation and propagation of the cracks in the scratching experiment are consistent with the simulation analysis and the experimental scratching force indicates the same variation tendency with the simulation scratching force. The research results in this paper could help to restrain the subsurface damage in grinding process.     

Experimental study on the mechanism of strain rate on grinding damage of zirconia ceramics

To study the mechanism of strain rate on the grinding damage of zirconia ceramics (ZrO₂), a ZrO₂ surface grinding experiment was conducted to analyse the effect of strain rate on the grinding force, surface morphology and subsurface cracks. As the strain rate increased, the grinding force decreased significantly; the grinding scratches gradually became continuous and clear, and the grinding surface gradually became smooth. Increasing the strain rate can greatly reduce the length of subsurface cracks; however, the length of subsurface cracks drops slowly and tends to stabilise when the strain rate reaches a certain value. The direction of subsurface cracks is related to the internal defects of the material, and there is a trend of approaching the surrounding cracks and defects. When the internal defects were stimulated by the impact stress wave, the cracks started to grow and expand from the tip, and the expanding direction remained at approximately 125° in the direction of the defect length. The results show that an increase in the grinding strain rate can effectively improve the grinding quality of zirconia ceramics, which has guiding significance for practical production.     

Meldola blue immobilized on a new SiO2/TiO2/graphite composite for electrocatalytic oxidation of NADH

Meldola blue immobilized on a new SiO₂/TiO₂/graphite composite was applied in the electrocatalytic oxidation of NADH. The materials were prepared by the sol-gel processing method and characterized by several techniques including scanning electronic microscopy coupled to energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electronic microscopy (HRTEM). Si and Ti mapping profiles on the surface showed a homogeneous distribution of the components. Ti2p binding energy peaks indicate that the formation of Si-O-Ti linkage is presumably the responsible for the high rigidity of the matrices. The good electrical conductivity presented by the composites (5 and 11 S cm⁻¹) can be related to a homogeneous distribution of graphite particles observed by TEM. After the materials characterization, a SiO₂/TiO₂/graphite electrode was prepared and some chemical modifications were performed on its surface to promote the adsorption of meldola blue. The resulting system presented electrocatalytic properties toward the oxidation of NADH, decreasing the oxidation potential to −120 mV. The proposed sensor showed a wide linear response range from 0.018 to 7.29 mmol l⁻¹ and limit of detection of 0.008 mmol l⁻¹. SiO₂/TiO₂/graphite has shown to be a promising material to be used as a suitable support in the construction of new electrochemical sensors.     

Non-destructive characterisation of alumina/aluminium titanate composites using a micromechanical model and ultrasonic determinations: Part II. Evaluation of microcracking

A method for non-destructive detection of microcracks in ceramic composites is described. The method involves the combination of ultrasound characterisation with the application of a three-phase micromechanical model, which considers cracks and pores as void constituents. Four alumina-aluminium titanate materials with different levels of microcracking, from no cracks (monophase alumina) to severely cracked (alumina + 40 vol.% of aluminium titanate) including an alumina + 10 vol.% aluminium titanate material with incipient microcracking have been developed to test the validity of the method. Specimens have been fabricated by colloidal processing and the longitudinal and transverse ultrasound velocities have been determined by the ultrasonic pulse-echo and transmission ultrasound-immersion techniques, employing a digital signal processing. It has been demonstrated that it is possible to differentiate between pores and microcracks, both modelled as void constituents, in terms of the aspect ratio.     

Carbon coated Li3V2(PO4)3 cathode material prepared by a PVA assisted sol-gel method

Carbon coated Li₃V₂(PO₄)₃ cathode material was prepared by a poly(vinyl alcohol) (PVA) assisted sol-gel method. PVA was used both as the gelating agent and the carbon source. XRD analysis showed that the material was well crystallized. The particle size of the material was ranged between 200 and 500 nm. HRTEM revealed that the material was covered by a uniform surface carbon layer with a thickness of 80 Å. The existence of surface carbon layer was further confirmed by Raman scattering. The electrochemical properties of the material were investigated by charge-discharge cycling, CV and EIS techniques. The material showed good cycling performance, which had a reversible discharge capacity of 100 mAh g⁻¹ when cycled at 1 C rate. The apparent Li⁺ diffusion coefficients of the material ranged between 9.5 × 10⁻¹⁰ and 0.9 × 10⁻¹⁰ cm² s⁻¹, which were larger than those of olivine LiFePO₄. The large lithium diffusion coefficient of Li₃V₂(PO₄)₃ has been attributed to its special NASICON-type structure.     

Finite elements based approaches for the modelling of radial crack formation upon Vickers indentation in silicon nitride ceramics

By having superior properties silicon nitride ceramics can be considered as the state-of-the-art material in the bearing industry. Vickers indentation of this material is typically accompanied by formation of cracks visible on surface. Two Finite Elements models are developed in the current work: the first model is based on fracture mechanics and the second on cleavage stress criterion. Plastic behavior of silicon nitride is included in the modeling, and since little is known on the plasticity of this material, the Drucker-Prager model (used for non-metallic materials) along with the classical J₂-plasticity are explored. The results of the fracture mechanics based model correlate well with experimental results in terms of surface crack length. The numerical results in terms of the morphology of the indented zone (including cracks and plastic zone) are provided by the stress criterion based model, and these results correlate well too, with the experimental data.     

Toughness measurement on ball specimens. Part I: Theoretical analysis

A new toughness test for ball-shaped specimens is presented. In analogy to the “Surface Crack in Flexure”-method the fracture toughness is determined by making a semi-elliptical surface crack with a Knoop indenter into the surface of the specimen. In our case the specimen is a notched ball with an indent opposite to the notch. The recently developed “Notched Ball Test” produces a well defined and almost uniaxial stress field.The stress intensity factor of the crack in the notched ball is determined with FE methods in a parametric study in the practical range of the notch geometries, crack shapes and other parameters. The results correlate well with established calculations based on the Newman-Raju model.The new test is regarded as a component test for bearing balls and offers new possibilities for material selection and characterisation. An experimental evaluation on several ceramic materials will be presented in a consecutive paper.     

Laser-assisted, crack-free surface melting of large eutectic ceramic bodies

The modification of ceramic surfaces by directional laser melting is interesting because it can eliminate surface defects and thus improve the ceramics mechanical performance, as long as one prevents the formation of cracks. The feasibility of surface modification by laser-assisted melting on large t-ZrO₂-Al₂O₃ eutectic ceramic pieces was evaluated in this work. 0.4 mm thick, defect-free, resolidified layers were obtained on plates of 40 mm width by preheating at 1200 °C and processing at 1000 mm/h with a line-shaped CO₂ laser beam and 580 W/cm² irradiance. The surface finish was smooth, free from overlapping-track roughness. The resolidified layer had eutectic microstructure with lamellae-type Al₂O₃ and tetragonal ZrO₂(t-ZrO₂) colonies. The fracture tests of the samples confirmed the absence of crack-type resolidification defects and the removal of surface defects. Although no increase in average flexural strength was observed for surface resolidified samples, they showed significantly lower standard deviation.     

Quantitative analysis of the residual stress and dislocation density distributions around indentations in alumina and zirconia toughened alumina (ZTA) ceramics

Alumina, 10% and 20% ZTA with 1.5 mol% yttria stabiliser were subjected to Vickers indentation testing with loads from 1 to 20 kg. Cr³⁺ fluorescence and Raman spectroscopy were applied to the indent centre and around the indentation in order to investigate the origin of the signal, the effect of indentation loads and zirconia phase transformation on the residual stress and plastic deformation in the plastic zone. The results suggested that with very strong laser scattering, the depth resolution of ZTA materials was very poor, which lead to a very significant amount of the signal being collected from the subsurface regions below the plastic zone. It was also found that zirconia phase transformation reduced the compressive residual stress in the alumina matrix within the plastic zone, except at the indentation centre, due to the tensile residual microstress generated by the zirconia phase transformation. In addition, the dislocation density on the indent surface of the ZTA samples was significantly reduced due to the restriction of crack propagation and energy absorption during the phase transformation process. At the indent centre, the zirconia phase transformation was suppressed by the high compressive stress, therefore, no significant difference between alumina and ZTA in terms of their residual stress and dislocation density were observed. Using TEM observation, it was found that the plastic zone microstructure of pure alumina is different from that of ZTA, which is consistent with the Cr³⁺ fluorescence results.     

A comparison of preparation method on the electrochemical performance of cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2 for lithium ion battery

Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ as a cathode material for Li-ion battery has been successfully prepared by co-precipitation (CP), sol–gel (SG) and sucrose combustion (SC) methods. The prepared materials were characterized by XRD, SEM, BET and electrochemical measurements. The XRD result shows that the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ materials prepared by different methods all form a pure phase with good crystallinity. SEM images and BET data present that the SC-material exhibited the smallest particle size (ca. 0.1 μm) and the highest surface area (7.4635 m² g⁻¹). The tap density of SC-material is lower than that of CP- and SG-materials. The result of rate performance tests indicates that the SC-material showed the best rate capability with the highest discharge capacity of 178 mAh g⁻¹ at 5.0 C, followed by SG-material and then CP-material. However, the cycling stability of SC-material tested at 0.1 and 0.5 C is relatively poor as compared to that of SG-material and CP-material. The result of EIS measurements reveals that large surface area and small particle size of the SC-electrode result in more SEI layer formation because of the increased side reactions with the electrolyte during cycling, which deteriorates the electrode/electrolyte interface and thus leads to the faster capacity fading of the SC-material.     

Nano Structured Metal Oxides as Potential Sorbents for 188W/188Re Generator: A Comparative Study

Nanostructured oxides of Ti, Zr, and Al were synthesized to evaluate their efficacy as sorbent matrices for the preparation of ¹⁸⁸W/¹⁸⁸Re generator. These oxides, in many respects, can improve the efficiency of the generator by providing better selectivity and enhanced capacity for ¹⁸⁸W, which arises from a higher surface area, surface energy, and surface charge, compared to the attributes of conventionally used bulk alumina. The performance of these materials, in the context of preparation of ¹⁸⁸W/¹⁸⁸Re generators, were compared with respect to sorption capacity for ¹⁸⁸W, shelf-life of the generator, purity, and quality of ¹⁸⁸Re for radiopharmaceutical applications.     

Adsorption of Textile Dye on Zinc Stannate Oxide: Equilibrium,Kinetic and Thermodynamics Studies

Zn₂SnO₄ powder was prepared by hydrothermal process at 200°C for 12 h. The material was characterized by X-ray-diffraction and surface area. The synthesized sample presented a pure phase and a surface area of 48.8 m² · g^?1. It was used as adsorbent to remove the Reactive Red 141 that is a azo textile dye. The adsorption kinetics of the textile dye on Zn₂SnO₄ followed the pseudo-second-order model. The adsorption process was found to be controlled by both external mass transfer and intraparticle diffusion. The equilibrium data were in good agreement with both Langmuir and Freundlich isotherms. Thermodynamic parameters were calculated, and the results revealed that the adsorption process is endothermic in nature, with weak forces of the Van der Walls acting.     

Residual Stresses in Machined MgO Crystals

The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.     

Improved electrochemical performance of LiFePO4 by increasing its specific surface area

Cathode material LiFePO₄ with an excellent rate capability has been successfully prepared by a simple solid state reaction method using LiCH₃COO·2H₂O, FeC₂O₄·2H₂O and (NH₄)₂HPO₄ as the starting materials. We have investigated the effects of the sintering temperature and mixing time of the starting materials on the physical properties and electrochemical performance of LiFePO₄. It was found that the rate capability of LiFePO₄ is mainly controlled by its specific surface area and it is an effective way to improve the rate capability of the sample by increasing its specific surface area. In the present study, our prepared LiFePO₄ with a high specific surface area of 24.1 m² g⁻¹ has an excellent rate capability and can deliver 115 mAh g⁻¹ of reversible capacity even at the 5 C rate. Moreover, we have prepared lithium ion batteries based on LiFePO₄ as the cathode material and MCMB as the anode material, which showed an excellent cycling performance.     

Preparation of MnOx/TiO2 ultrafine nanocomposite with large surface area and its enhanced toluene oxidation at low temperature

The TiO₂ support materials were synthesized by a chemical vapor condensation (CVC) method and the subsequent MnO_x/TiO₂ catalysts were prepared by an impregnation method. Catalytic oxidation of toluene on the MnO_x/TiO₂ catalysts was examined with ozone. These catalysts had a smaller particle size (9.1 nm) and a higher surface area (299.5 m² g⁻¹) compared to MnO_x/P25-TiO₂ catalysts. The catalysts show high catalytic activity with the ozone oxidation of toluene even at low temperature. As a result, the synthesized support material by the CVC method gave more active catalyst.     

Experimental study on laser assisted ultrasonic elliptical vibration turning (LA-UEVT) of 70% SiCp/Al composites

SiC_p/Al composites are more and more used in aerospace, military industry and other industries. However, the surface integrity of materials is poor, and the cutting force is large as the anisotropy of materials in the traditional machining (TM) process, which hinders the application of ceramic particle reinforced metal matrix composites. With the requirement of high dimensional accuracy, high efficiency and low damage for materials in these fields, non-traditional machining technology has become a research hotspot. Laser assisted machining (LAM) is a non-contact special machining method. Its advantages in machining SiC_p/Al composites have been proved by experiments, but there are still processing defects such as thermal cracks. Therefore, to further improve the machining quality of 70% SiC_p/Al composites with high volume fraction, a new machining method combining ultrasonic elliptical vibration turning (UEVT) and laser heating assisted turning (LAT) is proposed. High frequency intermittent machining and the adjustment of laser temperature influence on materials can be realized by adjusting the ultrasonic amplitude. Combining the characteristics of the two processing techniques, the feasibility study of the new machining method was studied by turning experiments. In this paper, compared with TM and LAT, the removal mechanism of materials and the effects of different laser heating temperatures and ultrasonic vibration on cutting force, surface quality, subsurface damage and chip morphology are explored. The results show that LA-UEVT can effectively reduce the cutting force and surface roughness, improve the plastic removal ability, and inhibit surface and subsurface damage. And the material removal process is mainly in the form of small particle crushing and particle pressing, which improves the stability of cutting force in the cutting process.     

Synthesis and photocatalytic properties of H2La2Ti3O10/TiO2 intercalated nanomaterial

H₂La₂Ti₃O₁₀/ TiO₂ intercalated nanomaterial was fabricated by successive intercalation reactions of H₂La₂Ti₃O₁₀ with n-C₆H₁₃NH₂/C₂H₅OH mixed solution and acid TiO₂ sol, followed by irradiating with a high-pressure mercury lamp. The intercalated materials possess a gallery height of 0.46 nm and a specific surface area of 31.58 m²·g⁻¹, which indicate the formation of a porous material. H₂La₂Ti₃O₁₀/TiO₂ shows photocatalytic activity for the decomposition of organic dye under irradiation with visible light and the activity of TiO₂ intercalated material was superior to the unsupported one.     

On the development of metallic particles and their initial catalytic properties in the CO + H2 reaction over Co/Al2O3 catalyst

Formation of Co^o phases with different surface structure over 10 wt% Co/Al₂O₃ and their catalytic properties were induced by pretreatments in H₂ at 570 K for 1 h or 20 h. Electronic behaviour of the Co^o phase, which consists of small (after 1 h reduction) or large bulk-like particles (after 20 h reduction), did not change during the CO hydrogenation after 5 h on stream as was determined by XPS. On the basis of the measured C₂₊ hydrocarbon selectivities the CO molecules are suggested to dissociate on small Co particles to a larger extent than on large cobalt particles. The slight decrease in the catalytic activity with increasing time on stream obtained for the long-term reduced sample is explained by the change in the surface Co^o content detected by XPS. The increase in the catalytic activity along with the change in olefin selectivity, measured for the sample reduced for 1 h, is interpreted by the change of a reaction path involving the Co^o-support interface during the initial period of the reaction.     

Elastic stress field model and micro-crack evolution for isotropic brittle materials during single grit scratching

An analytical model for the elastic stress field in isotropic hard and brittle materials during scratching is presented. The model considers the entire elastic stress field and the effect of material densification that was ignored in past studies, and is developed under a cylindrical coordinate system to make the modeling process simpler. Based on the model's predictions, the location and sequence of crack nucleation are estimated and the associated mechanisms are discussed. A single grit scratching experiment with an increasing scratch depth up to 2 µm is conducted for two types of optical glasses representing isotropic brittle materials: fused silica and BK7 glasses. It is found that the model's predictions correlate well with experimental data. Median cracks are found to be formed first during scratching, and the corresponding depth of the scratch sets the basis for determining the critical depth for brittle to ductile machining. Lateral cracks are initiated in the plastic yielding region and deflect to the work surface to cause material removal, while Hertzian cracks interact with lateral cracks to help remove lateral-cracked material. Furthermore, it is found that, owing to its open network molecular structure, fused silica has a much worse ductile machinability than the BK7 glass.     

Low-temperature synthesis of cements from rice hull ash

Rice hull is an agricultural by-product containing about 20% of silica. Usually, this material is burned at the rice fields generating small silica particles, which may cause respiratory and environmental damage. This work describes the use of rice hull ash as a raw material to prepare Ca₂SiO₄-related cements, which is a component of commercial Portland cement. Rice hull was heated at 600 °C rendering silica with a surface area of 21 m² g⁻¹. This material was mixed with CaO and BaCl₂·2H₂O in several proportions, added stoichiometricaly in order to keep a ratio (Ca+Ba)/Si=2. The solids were mixed with water 1:20 (w/w) and sonicated for 60 min. The suspensions were dried and heated at several temperatures (from 500 to 1100 °C). The resulting solids were analyzed by FT-IR spectroscopy and X-ray diffraction. Cements with structure similar to that of β-Ca₂SiO₄ were obtained at temperatures as low as 700 °C, according to the composition.