Modelling and optimizing the liquid phase sintering of alumina/CaO–SiO2–Al2O3 ceramics using response surface methodology

Alumina is an attractive material for engineering applications due to its unique properties. In this study, CaO–SiO₂–Al₂O₃ eutectic phase was used as an additive phase and liquid phase sintering of the alumina/CaO–SiO₂–Al₂O₃ samples were investigated. The liquid phase sintering was modelled and optimized by Response Surface Methodology (RSM) using Central Composite Design (CCD) to achieve maximum fracture strength and density as responses. Sintering temperature, alumina particles size distribution (PSD), lubricant and eutectic phase content were selected as independent variables. Two cubic models were developed in terms of these variables to describe the responses. The validity and accuracy of the models were checked using Analysis of Variance (ANOVA).Phase identification of the synthesized eutectic phase was evaluated by XRD and fracture strength of the sintered samples was determined by Ring-on-Ring test method. SEM was used to study the fracture surface of the samples. The obtained models for predicting fracture strength and density of the sintered samples showed high conformity with the experimental results. Sintering temperature and alumina PSD were found as the most effective parameters. Therefore, optimized condition based on the defined constraints was obtained for sintering temperature of 1533 °C, alumina PSD of 25%, and lubricant and eutectic phase content of 1.5 wt% and 7.5 wt%, respectively. Results showed that after ball milling of the eutectic phase, the fracture strength of the optimized ceramic sample was improved and it reached to maximum values at smaller amounts of the additive phase.     

Preparation of Ti/Pt/SnO2–Sb2O4 electrodes for anodic oxidation of pharmaceutical drugs

Ti/Pt/SnO₂–Sb₂O₄ electrodes were prepared by alternating Sn and Sb electrodepositions, repeated 4 or 16 times, onto a platinized titanium foil by a thermo-electrochemical method. Chemical, electrochemical, and structural tests have been used for the characterization of Ti/Pt/SnO₂–Sb₂O₄ electrodes. Anodic oxidation of the aqueous solution contaminated by amoxicillin, clofibric acid, diclofenac, and ibuprofen having a concentration of 100 mg L^?1 and 0.035 M of Na₂SO₄ have been applied using Ti/Pt/SnO₂–Sb₂O₄ electrodes at a current density of 10 and 30 mA cm^?2. The chemical oxygen demand removals increased with current density and except for diclofenac, the Ti/Pt/SnO₂–Sb₂O₄ electrode with 4 electrodeposition repetitions gave the best results. The combustion efficiencies for diclofenac and ibuprofen were higher than those obtained with similar electrode material, prepared without platinization, especially in the assay run with Ti/Pt/SnO₂–Sb₂O₄ (16 electrodeposition repetitions).     

Preparation optimization of multilayer-structured SnO2–Sb–Ce/Ti electrode for efficient electrocatalytic oxidation of tetracycline in water

In this study, electrodeposition and thermal decomposition were alternatively used for the fabrication of a series of novel multilayer-structured SnO₂-Sb-Ce/Ti (SSCT) electrodes, and their physiochemical and electrochemical properties were investigated for electrochemical oxidation of tetracycline (TC) in aqueous medium. Experimentally, after the SnO₂-Sb-Ce (SSC) composite was electrodeposited for 120 s on the titanium substrate in aqueous solution, the outer thermal coatings composed of SSC were synthesized by a hydrothermal method. Both influences of electrodeposition time (T_ed) and thermal decomposition time (T_td) were investigated to obtain the optimum preparation. It was found that when increasing T_ed to a certain extent a longer lifetime of electrode can be achieved, which was attributed to a more solid interlayer structure. A notable SSCT_Ted,Ttd electrode, i.e., SSCT_3,10, which was prepared through three times of 120 s' electrodeposition (T_ed=3) and ten times of thermal decomposition (T_td=10) obtained the highest oxygen evolution potential 3.141 V vs. SCE. In this selected electrode, when 10 mg·L^-1 initial TC concentration was added to this wastewater, the highest color removal efficiency and mineralization rate of TC were 72.4% and 41.6%, respectively, with an applied electricity density of 20 mA·cm^-2 and treatment time of 1 h. These results presented here demonstrate that the combined application of electrodeposition and thermal decomposition is effective in realization of enhanced electrocatalytic oxidation activity.     

Preparation and electrochemical performance of SnO2@carbon nanotube core–shell structure composites as anode material for lithium-ion batteries

Carbon nanotube-encapsulated SnO₂ (SnO₂@CNT) core–shell composite anode materials are prepared by chemical activation of carbon nanotubes (CNTs) and wet chemical filling. The results of X-ray diffraction and transmission electron microscopy measurements indicate that SnO₂ is filled into the interior hollow core of CNTs and exists as small nanoparticles with diameter of about 6 nm. The SnO₂@CNT composites exhibit enhanced electrochemical performance at various current densities when used as the anode material for lithium-ion batteries. At 0.2 mA cm^?2 (0.1C), the sample containing wt. 65% of SnO₂ displays a reversible specific capacity of 829.5 mAh g^?1 and maintains 627.8 mAh g^?1 after 50 cycles. When the current density is 1.0, 2.0, and 4.0 mA cm^?2 (about 0.5, 1.0, and 2.0C), the composite electrode still exhibits capacity retention of 563, 507 and 380 mAh g^?1, respectively. The capacity retention of our SnO₂@CNT composites is much higher than previously reported values for a SnO₂/CNT composite with the same filling yield. The excellent lithium storage and rate capacity performance of SnO₂@CNT core–shell composites make it a promising anode material for lithium-ion batteries.     

Metal modified (Ni,Ce, Ta) Ti/SnO2–Sb2O5–RuO2 electrodes for enhanced electrochemical degradation of Orange G

Journal of Applied Electrochemistry - Ni, Ce, and Ta modified Ti/SnO2–Sb2O5–RuO2 anodes were first prepared by thermal decomposition strategy and applied for Orange G degradation to...     

A comparative study on the mineralization of waste butyl acetate by electrochemical oxidation via persulphate/hypochlorite and silver (II) reagents using DSA® anodes: Statistical optimization by response surface methodology

Mediated electrochemical oxidation of butyl acetate (BA) in wastewater was conducted by employing two different techniques based on S₂O₈⁻²/ClO⁻ and Ag (II) reagents using commercial Dimensionally Stable Anode (DSA)®-O₂ or DSA®-Cl₂ anodes. Response surface methodology (RSM) was also engaged to investigate process parameters impacts and their interactions on BA mineralization efficiency and energy consumption. Fourier-transform infrared spectroscopy (FT-IR), UV–visible, and gas chromatography (GC) analyses confirmed BA mineralization and the optimum removal conditions were then attained for both systems. Chemical oxygen demand (COD) analysis showed that electro-oxidation by mixed persulphate/hypochlorite oxidants favours the mineralization of BA (99.7%) compared to the Ag (II) technique (96.56%). The higher removal efficiency obtained by S₂O₈⁻²/ClO⁻ was achieved in a neutral aquatic medium using DSA®-Cl₂ at a lower current density (CD), temperature, and electrolysis time (pH: 6, CD: 0.1 kA/m², and time: 60 min) compared to those obtained by Ag (II) ions (pH: ≤3, CD: 1.69 kA/m², and time: 130 min). The former process occurred under the charge transfer control mechanism at low CDs, whereas at an elevated CD (about 1.0 kA/m²), mass transfer was predominant due to the parasitic oxygen evolution. On the other hand, the latter process was found charge transfer-controlled at low BA concentrations (≤200 ppm), whereas it turned out mass transfer-controlled at higher BA concentrations. Comparing the anode type, energy consumption for BA mineralization by S₂O₈⁻²/ClO⁻ ions using DSA®-Cl₂ at the optimum conditions was 0.009 kW · h/dm³, which was about three times lower than that of the DSA®-O₂ anode.     

Modeling and optimization of operating parameters for abrasive waterjet turning alumina ceramics using response surface methodology combined with Box–Behnken design

Abrasive waterjet turning is a newly emerging non-traditional technology for machining ceramics. In the present study, an attempt has been made to investigate the effect of operating parameters on depth of penetration and surface roughness (Ra) in turning of alumina ceramics using abrasive waterjet. The quadratic regression models were developed to predict the depth of penetration and Ra by experiments using Response Surface Methodology. The influence of each operating factors has been studied through analysis of variance technique. Key parameters and their interactive effects on depth of penetration and surface roughness have also been presented in graphical contours which are useful for choosing operating parameter preciously. The operating parameters for depth of penetration and surface roughness were simultaneously optimized by RSM with desirability function. The absolute average error between the experimental and predicted values at the optimal combination of parameter settings for depth of penetration and surface roughness were calculated as within 5%. Thus the developed model can be effectively used to predict the depth of penetration and surface roughness in the machining of alumina ceramics.     

Electrochemical degradation of the Acid Blue 62 dye on a β-PbO<Subscript>2</Subscript> anode assessed by the response surface methodology

The electrochemical degradation of the anthraquinonic dye Acid Blue 62 in a filter-press reactor on a Ti/Pt/β-PbO₂ anode was investigated using the response surface methodology with the variables: current density, pH, [NaCl], and temperature. The system’s modeling was carried out with the charge required for 90% decolorization (Q ⁹⁰) and the chemical oxygen demand removal percentage after a 30 min electrolysis (COD ³⁰), with good correlations between predicted and observed values. Best conditions for decolorization were attained in acidic solutions (pH = 4) with medium to high [NaCl] (1.0–2.0 g L⁻¹) and lower temperature due to the prevalent oxidant species HOCl and Cl₂. Optimal conditions for COD ³⁰ removal were attained at high current densities in pH > 5 solutions with high [NaCl], when the prevalent oxidant species are HOCl and OCl⁻. The lowest charge per unit volume of the electrolyzed solution necessary for total mineralization was attained at pH 11.     

Optimization of the gas separation performance of polyurethane–zeolite 3A and ZSM-5 mixed matrix membranes using response surface methodology

In the present work, the response surface method software was used with five measurement levels with three factors.These were applied for the optimization of operating parameters that affected gas separation performance of polyurethane–zeolite 3A, ZSM-5 mixed matrix membranes.The basis of the experiments was a rotatable central composite design(CCD).The three independent variables studied were: zeolite content(0–24 wt%), operating temperature(25–45 ℃) and operating pressure(0.2–0.1 MPa).The effects of these three variables on the selectivity and permeability membranes were studied by the analysis of variance(ANOVA).Optimal conditions for the enhancement of gas separation performances of polyurethane–3A zeolite were found to be 18 wt%, 30 ℃ and 0.8 MPa respectively.Under these conditions, the permeabilities of carbon dioxide, methane, oxygen and nitrogen gases were measured at 138.4, 22.9, 15.7 and 6.4 Barrer respectively while the CO_2/CH_4, CO_2/N_2 and O_2/N_2 selectivities were 5.8, 22.5 and 2.5, respectively.Also, the optimal conditions for improvement of the gas separation performance of polyurethane–ZSM 5 were found to be 15.64 wt%, 30 ℃ and 4 bar.The permeabilities of these four gases(i.e.carbon dioxide, methane, oxygen and nitrogen) were 164.7, 21.2, 21.5 and 8.1 Barrer while the CO_2/CH_4, CO_2/N_2 and O_2/N_2 selectivities were 7.8, 20.6 and 2.7 respectively.     

Role of porosity on the stiffness and stability of (001) surface of the nanogranular C–S–H gel

We present here a nanoscale modeling of the bulk and surface states of the nanogranular Calcium Silicate Hydrate (C–S–H) gel. Regarding the mechanical properties as well as porosity, we identify two main phases: a ductile one, for a porosity from 12.4% to 27% and a brittle phase for a porosity lower than 12.4% or higher than 27%. Particularly, our calculations show that 20% of gel porosity gives a better structural stability to C–S–H and a higher stiffness. Besides, we have explored the (001) surface of C–S–H gel at the nanoscale level and have proved that more water on the surface will stabilize thermodynamically the nanostructure. This means that adsorbed H₂O molecules lead to the coexistence of Si–OH and Ca–OH groups on the surface. The calcium-terminated plane is a polar and unstable surface, and will either reconstruct to neutralize the dipole moment or adsorb ions to remove it. Our results also show that the (001) surface of low density C–S–H tends to be more stable than the surface of high density phase since the obtained surface energy decreases with increasing gel porosity.     

Comparison of the effects of FePO4 and FePO4·2H2O as precursors on the electrochemical performances of LiFePO4/C

Carbon-coated LiFePO₄/C composite as cathode materials is synthesized by solid-state method using anhydrous FePO₄ and hydrous FePO₄·2H₂O as precursors.The effects of sintering temperature and carbon content on the properties of LiFePO₄/C composite are compared by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and charging–discharging test. The crystallinity, morphology, and particle size distribution of these two precursors are compared to investigate their effect on the electrochemical performances of LiFePO₄/C composite. Compared with hydrous FePO₄·2H₂O, anhydrous FePO₄ has good crystallinity, uniform particle morphology and symmetrical size distribution, contributing to LiFePO₄/C composite have excellent electrochemical performances. Due to the dehydration of hydrous FePO₄·2H₂O during synthesis, uneven distribution of carbon content and carbon layer is coated on LiFePO₄ surface, deteriorating the electrochemical performance of LiFePO₄/C composite. When anhydrous FePO₄ was used as the precursor, the LiFePO₄/C composite sintered at 700 °C with carbon content of 0.4 by molar ratio show high discharge capacity and stable cycling performance, with discharge capacity of 106.3 mA h g⁻¹ at 10 C, and a capacity retention rate of 99.2% after 200 cycles at 1 C.     

Effects of Precursors for Preparing Intermediate Layer on the Performance of Ti/SnO2+Sb2O3/PbO2 Anode

The Ti/SnO2+Sb2O3/PbO2 anode with SnO2+Sb2O3 intermediate layer obtained by the polymeric precursor method (PPM) and with the conventional route was studied. The morphology and microstructure of SnO2+Sb2O3 intermediate layer derived from different precursors and the top PbO2 active layer were examined by means of ESEM and XRD. The lifetime and electrocatalytic activity of the anode were also assessed by the cyclic voltammetry and accelerated lifetime test in 1.0 mol/L H2SO4 solution. It was found that precursor solvents affected lifetime, microstructure and morphology of the anode, and had little influence on electrocatalysis activity of the electrodes. The accelerated lifetime of Ti/SnO2+Sb2O3/PbO2 anode with intermediate layer prepared by PPM was 29.5 h in 1.0 mol/L H2SO4 solution, which was respectively about four times and twice that of the anode prepared with ethylene glycol and ethanol. After the anode was subjected to thermal corrosion, the lifetime still reached 27 h in contrast to the others.     

Effect of fiber architecture and density on the ablation behavior of carbon/carbon composites modified by Zr–Ti–C

Three carbon/carbon (C/C) composites modified by Zr–Ti–C, with different fiber architecture in preforms and the same density, were prepared using chemical vapor infiltration and reactive melt infiltration methods. Two other samples with the same architecture in preforms and different density were also fabricated by the same methods. Their ablation behaviors were examined by oxy-acetylene flame. The results showed that the samples with chopped web needled perform had better ablation resistance than that of the samples with needle-integrated and fine-weave pierced perform. In the models of ablation behaviors, the sealing time of pores and gaps on the ablated surfaces has been defined to indirectly estimate the ablation property. The analysis of models also indicated that high density of the composites and appropriate small diameter of bundles of carbon fibers led to the short sealing time and good ablation resistance of the C/C–carbide composites.     

Effects of porous C/C density on the densification behavior and ablation property of C/C–ZrC–SiC composites

C/C–ZrC–SiC composites were prepared by precursor infiltration and pyrolysis process using a mixture solution of organic zirconium-containing polymer and polycarbosilane as precursors. Porous carbon/carbon (C/C) composites with density of 0.92, 1.21 and 1.40 g/cm³ were used as preforms, and the effects of porous C/C density on the densification behavior and ablation resistance of C/C–ZrC–SiC composites were investigated. The results show that the C/C preforms with a lower density have a faster weight gain, and the obtained C/C–ZrC–SiC composites own higher bulk density and open porosity. The composites fabricated from the C/C preforms with a density of 1.21 g/cm³ exhibit better ablation resistance with a surface temperature of over 2400 °C during ablation. After ablation for 120 s, the linear and mass ablation rates of the composites are as low as 1.02 × 10⁻³ mm/s and −4.01 × 10⁻⁴ g/s, respectively, and the formation of a dense and continuous coating of molten ZrO₂ solid solution is the reason for their great ablation resistance.     

Preparation of Ni–Mo–C/Ti(C,N) coated powders and its influence on the microstructure and mechanical properties of Ti(C,N)-based cermets

Ni–Mo–C/Ti(C,N) coated powders, namely Ni–Mo alloy and Mo₂C coated Ti(C,N) composite powders, were synthesized by using a heterogeneous precipitation and thermal reduction method, then pressed and vacuum sintered to fabricate cermets. The chemical composition, microstructure and phases of the composite powders and the microstructure and properties of sintered cermets were experimentally investigated. The results show that a fine and uniform microstructure of (Ti,Mo)(C,N)-Ni cermets without the conventional core-rim structure is obtained. The phases formed during the preparation of the coated powders as well as the cermets were analyzed by means of a X-ray diffraction (XRD) technique. The XRD result confirms the formation of the Ni₃Ti phase in the cermets. Due to the formation of the non-magnetic Ni₃Ti and the dissolution of Mo in Ni binder phase, the magnetic properties are strongly retarded. The fracture of the cermets is mainly characterized by inter-granular and dimple fractures. Better mechanical properties can be obtained in comparison with conventionally fabricated ones.     

Fabrication of Pt–Cu/RGO hybrids and their electrochemical performance for the oxidation of methanol and formic acid in acid media

Pt–Cu/reduced graphene oxide (Pt–Cu/RGO) hybrids with different Pt/Cu ratios were prepared by the reduction of H₂PtCl₆ and CuSO₄ by NaBH₄ in the presence of graphene oxide (GO). The Pt–Cu nanoparticles were characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The reduction of GO was verified by ultraviolet–visible absorption spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Compared to Pt/RGO, the Pt–Cu/RGO hybrids have superior electrocatalytic activity and stability for the oxidation of methanol and formic acid. Thus they should have potential applications in direct methanol and formic acid fuel cells.     

Influence of H2, CO and CO2 co-feeding on the catalytic activity of Rh/Ti–SiO2 during the partial oxidation of methane

The influence of the addition of 1, 2 or 5 vol.% of CO, H₂ or CO₂ to the feed during the partial oxidation of methane (POM) was studied over a Rh/Ti–SiO₂ catalyst. The addition of H₂ or CO decreases the conversion and syngas selectivity. This decrease of performance seems to be related to a higher reduction of the catalyst due to the co-feeding of H₂ or CO. The addition of CO₂ also appears unfavourable to the production of hydrogen but increases the CO yield. A combination of the dry reforming and the reverse water–gas shift reactions is suggested to explain the observed modifications in the product yields.     

Crystalline phase of TiO2 nanotube arrays on Ti–35Nb–4Zr alloy: Surface roughness,electrochemical behavior and cellular response

An investigation was made into the electrochemical, structural and biological properties of self-organized amorphous and anatase/rutile titanium dioxide (TiO₂) nanotubes deposited on Ti–35Nb–4Zr alloy through anodization-induced surface modification. The surface of as-anodized and heat-treated TiO₂ nanotubes was analyzed by field emission scanning electron microscopy (FE-SEM), revealing morphological parameters such as tube diameter, wall thickness and cross-sectional length. Glancing angle X-ray diffraction (GAXRD) was employed to identify the structural phases of titanium dioxide, while atomic force microscopy (AFM) was used to measure surface roughness associated with cell interaction properties. The electrochemical stability of TiO₂ was examined by electrochemical impedance spectroscopy (EIS) and the results obtained were correlated with the microstructural characterization. The in vitro bioactivity of as-anodized and crystallized TiO₂ nanotubes was also analyzed as a function of the presence of different TiO₂ polymorphic phases. The results indicated that anatase TiO₂ showed higher surface corrosion resistance and greater cell viability than amorphous TiO₂, confirming that TiO₂ nanotube crystallization plays an important role in the material's electrochemical behavior and biocompatibility.