Effect of Concentration of Ammonium Poly(acrylate) Dispersant and MgO on Coagulation Characteristics of Aqueous Alumina Direct Coagulation Casting Slurries

Coagulation of aqueous alumina slurries prepared using various concentrations (0.43–1.04 wt% based on alumina) of ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC) of alumina. The slurries prepared at dispersant concentration below 0.84 wt% are not suitable for DCC at room temperature (∼30 ° C) as they undergo premature coagulation. Mixing the slurry with MgO at a low temperature of nearly 5 ° C slows down the reactions leading to coagulation and keeps the slurry viscosity low for a sufficient period of time. Coagulation of slurries prepared at a dispersant concentration of 0.92 wt% and above at room temperature requires MgO concentrations much higher than the equivalent amount required for reaction with the dispersant. This anomalous behavior at higher dispersant concentration is explained such that the Mg-poly(acrylate) formed by the reaction between ammonium poly(acrylate) and MgO formed a sheath over the remaining MgO particles and prevented them from further dissolution at room temperature. Faster coagulation could be achieved by heating the slurries after casting in closed molds. The Mg-poly(acrylate) acts as a binder and stabilizes the coagulated bodies as their strength and stability against oscillatory stresses increase with an increase in dispersant concentration.     

A new direct coagulation casting process for alumina slurries prepared using poly(acrylate) dispersant

Direct coagulation casting (DCC) of concentrated aqueous alumina slurries prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Addition of small amounts of MgO increased the viscosity of the concentrated alumina slurries with time and finally transformed it in to a stiff gel. Sufficient working time for degassing and casting could be achieved by cooling the slurries to a temperature of ∼5 °C after proper homogenization after the addition of MgO. The DCC slip with alumina loading in the range of 50–55 vol% showed relatively low viscosity (0.12–0.36 Pa s at shear rate of 93 s⁻¹) and yield stress (1.96–10.56 Pa) values. The wet coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% had enough compressive strength (45–211 kPa) for handling during mould removal and further drying. The coagulated bodies prepared from slurries of alumina loading in the range of 50–55 vol% showed linear shrinkage in the range of 4.8–2.3 during drying and 17.1–16.2 during sintering respectively. Near-net-shape alumina components with density >98% TD could be prepared by the DCC process.     

Novel Coagulation Method for Direct Coagulation Casting of Aqueous Alumina Slurries Prepared Using a Poly(Acrylate) Dispersant

Coagulation of concentrated aqueous alumina slurries prepared using an ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC). A small amount of MgO (0.2 wt% of alumina) increased the viscosity of the concentrated alumina slurry with time and finally transformed it into a stiff gel. The mechanism of coagulation is proposed such that the time-delayed in situ generation of Mg²⁺ ions from the sparingly soluble MgO forms Mg–poly(acrylate) with the unadsorbed ammonium poly(acrylate) molecules in solution that shift the poly(acrylate) adsorption equilibrium toward the left by depleting the poly(acrylate) molecules adsorbed on the alumina particle surface. This leads to insufficient dispersant coverage on the particle surface and coagulation of the slurry. DCC using MgO is possible only if the slurry is prepared at a dispersant concentration higher than that required for optimum dispersion as the slurries prepared at the optimum dispersant concentration underwent premature coagulation. The gelation time could be tailored within 20 min to a few hours by maintaining the temperature in the range of 70°–30°C. The wet coagulated bodies prepared from 50 vol% alumina slurry showed a compressive strength of nearly 0.05 MPa.     

Effect of fine alumina in improving refractoriness of ceramic shell moulds used for aeronautical grade Ni-base superalloy castings

This paper discusses an improvement in shell refractoriness and dimensional stability of columnar grained (CG) low pressure turbine blade castings made using Ni base superalloy by directional solidification process (DS). Two ceramic shell systems were adopted, namely shell system I and II. Shell moulds were prepared by using ceramic slurries containing zircon flour as a filler material and colloidal silica as a binder. As compared to shell system II (zircon filler with colloidal silica binder and fine alumina), shell system I (zircon filler with colloidal silica binder) showed lower refractoriness. Shell system II showed an increase in the flexural strength both in the green as well as in fired conditions. Shells made from shell system II showed about 13% higher green strength and 55% higher fired strength as compared to shell system I. Shell system II also exhibited superior self sag resistance up to 1625?°C. Moulds prepared from this shell system yielded aeronautical grade casting with high dimensional accuracy even at a metal pouring temperature of 1550?°C. Moulds from shell system I, on the other hand, underwent sagging even at metal pouring temperature of 1500?°C, leading to dimensionally unacceptable castings. The superior performance of shells prepared from shell system II can be ascribed to the presence of fine alumina in the shell.     

Porous mullite blocks with compositions containing kaolin and alumina waste

In the production of alumina by the Bayer process, the calcination step generates a waste containing ~90% aluminum oxide (Al₂O₃). Due to the high content of this oxide, this waste can be used as a source of alumina in porcelain formulations, especially those used in the synthesis of mullite. The purpose of this study was to produce porous mullite blocks using compositions containing kaolin and alumina waste. The compositions were formulated based on a mullite stoichiometry of 3:2. Heat treatments were carried out in a conventional furnace at temperatures of 1450 to 1500 °C, applying a heating rate of 5 °C/min and a 1-h hold time at the firing temperature threshold. The powders were characterized by means of X-ray fluorescence (XRF); X-ray diffraction (XRD); thermal analysis (TGA-DTA); scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The physic mechanical properties of the test specimens: water absorption, apparent porosity, linear shrinkage and flexural strength were also evaluated. The XRD results revealed the formation of mullite as the major phase. The morphological analysis by SEM revealed typical mullite needles originating from clay minerals. The size of the mullite needles was calculated based on the TEM analysis, which indicated diameters smaller than 400 nm, confirming the nanometric dimensions of the needles. The flexural strength test of the specimens indicated that this parameter ​​tends to increase as the temperature is raised.     

Effect of Calcination Conditions on Crystalline Structure and Pore Size Distribution for a Mesoporous Alumina

In this article, a mesoporous commercial alumina was calcined in the temperature range of 600°C–1200°C. The effect of several parameters such as calcination temperature, calcination time, heating rate, and calcination steps on phase transformation and crystal size was experimentally investigated. The characterization of the commercial mesoporous alumina and samples calcined at 1000°C, 1040°C, 1070°C, 1100°C, and 1200°C by single-step and multi-step calcination was performed using XRD and N₂ adsorption/desorption techniques. For the commercial mesoporous alumina, TG/DTA analysis was also performed. Experimental results showed that mostly pure α-Al₂O₃ was obtained at 1100°C.     

Fluorination synthesis of MgF2 nanoparticles synthesized for manufacturing IR windows by hot-pressing

In this study, magnesium fluoride (MgF₂) nanoparticles were synthesized through the fluoridation method, by dissolving magnesium chloride and hydrofluoric acid (HF) in deionized water. The influences of some parameters, including pH, concentration ratio of reactants, F:Mg mole ratio, washing method and calcination temperature were investigated on the particle size, morphology, chemical purity and phase purity of MgF₂ nanoparticles. In order to study the impact of pH on the properties of MgF₂ nanoparticles, different samples at pH values of 1, 5 and 9 were synthesized. The obtained results revealed that as pH value increased from 1 to 9, the morphology of MgF₂ nanoparticles changed from rod to spherical shape. The effect of mole ratio of fluorine ions to magnesium ions on the MgF₂ nanoparticles at three ratios of 2, 6 and 12 also demonstrated that by increment of F:Mg mole ratio, the particle size was decreased from 150 nm to 30 nm. In addition, it was figured out that by the increment of F:Mg mole ratio, the MgO phase was eliminated. Afterwards, by decreasing the HF:MgCl₂ molar ratios from 0.1 to 0.03 the particle size reduced from 300 nm to 30 nm. After determining the optimal synthesis conditions, magnesium fluoride nanoparticles were calcined at 470, 530 and 600 °C. Regarding the results, the powder that was calcined at 600 °C with a particle size of about 30–40 nm was selected as the optimal sample. Ultimately, the resulting powder was sintered using hot-press (HP) at a temperature of 700 °C for 45min in the vacuum pressure of 10^?3 bar. After polishing the sintered piece, its inferred (IR) transparency was over 90% in the wavelength ranges of 3–5 μm.     

MgO活性对新型水化硅酸镁水泥性能的影响

通过对原料轻烧氧化镁粉在不同温度下进行二次恒温煅烧1.5 h制备不同活性MgO,研究了不同活性MgO与硅灰(SF)和磷酸氢二钾(K₂HPO₄)所制备的新型水化硅酸镁水泥胶凝材料(又称水化磷硅酸镁水泥,MSPHC)的凝结时间、流动度、抗压强度、反应溶液pH值。结合X射线衍射(XRD)、热重分析(TG-DTG)和扫描电子显微镜(SEM)测试手段,分析其影响机理。结果表明:随着煅烧温度的升高,MgO衍射峰强度增大,MgO活性降低;活性越高的MgO制备的MSPHC净浆凝结时间越短且流动性越差,而活性适中MgO制备的MSPHC具有较好的力学性能。MSPHC最主要的水化产物是水化硅酸镁(M-S-H)凝胶,另外还有Mg(OH)₂和MgKPO₄·6H₂O(MKP)生成,原料轻烧氧化镁粉中的MgCO₃成分不参与体系反应。活性适中的MgO制备的MSPHC在28 d龄期内的水化产物M-S-H凝胶生成量最多,因此硬化体抗压强度最高。活性越高的MgO在MSPHC反应体系中溶解的速度越快,体系水化反应进程速度也越快。     

Pore structure of aluminas derived from the alkyl derivatives of boehmite

The alkyl derivatives of boehmite (alkoxyalumoxanes; AlO(OH)_1?x (OR) _x ) were synthesized by the reaction of aluminum triisopropoxide in straight-chain primary alcohols at 300 °C for 2 h in an autoclave. In the present work, pore structures of aluminas obtained by calcination of the alkyl derivatives of boehmite were examined. The alumina obtained from the ethyl derivative of boehmite had a broad pore-size distribution, while the pore-size of the alumina obtained from the dodecyl derivative of boehmite distributed in a narrow range in the mesopore region. The mode pore diameter of the latter alumina increased with the increase in calcination temperature (as-syn., 39 Å; 600 °C, 54 Å; 800 °C, 58 Å; 1000 °C, 68 Å), but narrow pore-size distribution was maintained even after calcination at high temperatures.     

Effect of particle size and freezing conditions on freeze-casted scaffold

Freeze casting is one of the emerging and novel manufacturing routes to fabricate porous scaffolds for various applications including orthopedic implants, drug delivery, energy storing devices etc. Thus, it becomes important to understand this process in a deeper sense. Present work was focused to study the effect/influence of basic parameters, particle sizes, and freezing conditions on the mechanical properties and microstructures of porous scaffold fabricated by freeze casting. β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp) powder with particle sizes of 10?μm and 20?nm were used. Prepared slurries were freeze casted at constant freezing temperature (5?°C) and constant freezing rate (1.86?°C/min) to study the effect of freezing conditions on mechanical and microstructural properties of the porous scaffold. It was observed that porous scaffold fabricated by nanoparticles has given better porosity (63.22–76.16%), than scaffold fabricated by microparticles (13–43.05%) at given solid loading of both freezing conditions. Although, the range of pore size of the scaffold fabricated by nanoparticles (CFR: 2.60–0.84?μm; CFT: 1.66–0.46?μm) was lower than that of scaffold fabricated by microparticles (CFR: 9.45–4.83?μm; CFT: 4.72–2.84?μm). The compressive strength of scaffolds prepared by nanoparticles was in the range of trabecular bone. Moreover, the results of present work will pave the way for the fabrication of porous scaffold with desired pore size and porosity for various implants, energy, and drug delivery applications.     

Utilization of granite sludge for production of cordierite ceramics by direct coagulation casting

In this work, an attempt to produce cordierite ceramics from granite sludge waste, talc and alumina was performed by direct coagulation casting process. To optimize the conditions for cordierite formation, three mix-compositions were firstly prepared by processing the starting materials in different conditions. The first mix was prepared by firing the mix of granite sludge, talc and alumina up to 1300 °C while the second and third mixes were fabricated by firing alumina and talc at 1300 °C or 1350 °C, respectively, then the granite sludge was added. Both batches were fired at different temperatures. According to the percentage of formed cordierite, the third mix was selected to be solidified by direct coagulation casting method followed by sintering at different temperatures. The casted cordierite was examined by thermal analysis while the sintered bodies were tested for their physical, mechanical and electrical properties. The results indicated that the pre-heating of alumina and talc at 1350 °C (third mix) enhanced the formation of cordierite and some amounts of spinel. For the casted sintered specimens, the porosity was decreased with increasing the sintering temperature. Also, there was an increase in compressive strength for the samples sintered up to 1250 °C. The dielectric constant values were varied between 4.5 and 5.89 while the dielectric loss was varied between 2 × 10^?3 and 7 × 10^?3, at room temperature.     

Aqueous tape casting of alumina using natural rubber latex binder

Eco-friendly and sustainable tape casting of alumina powder suspensions using concentrated natural rubber (poly isoprene) latex binder has been studied. The high negative zeta potential values of the aqueous alumina slurry (−53 to −72 mV) and rubber latex (−67 to −84 mV) at pH in the range of 9–11.5 enables their co-dispersion to produce tape casting slurries of solids (alumina + rubber) concentration >60 vol% with adequate flow characteristics. Drying of the slurry tape-cast on Mylar substrate is achieved within 15 min at 70 °C due to its high solid concentration. The green tapes containing 14.2 to 18.1 wt% of rubber shows tensile strength and strain at failure in the ranges of 1.85 to 1.61 and 41–254%, respectively. The flexible green tapes turn rigid by annealing at 200 °C due to the self-cross-linking of rubber chains induced by the Lewis acid sites of alumina. Thickness reduction to the extent of 20% by rolling of the green tape before annealing improves the green microstructure which results in an enhancement in sintered density from 93 to 98% of the theoretical value. However, the additional rolling and annealing steps consume extra time and energy compared to the tape casting processes using other reported binders.     

The effect of the calcination temperature of LaFeO3 precursors on the properties and catalytic activity of perovskite in methane oxidation

In the synthesis of perovskite-type LaFeO₃ oxides iron and lanthanum nitrates were used as a precursors. The nitrates were dissolved in water, evaporated, crushed and calcined in temperature range of 650–850?°C. The obtained perovskites were applied as an active layer on monolithic catalysts for the oxidation of methane. The increase in the calcination temperature of the perovskite precursors from 650° to 850°C results in a reduction in the surface area of the powders from 10.1 to 4.2?m²/g. XRD studies revealed that calcination at 800–850?°C caused the formation of an almost homogeneous LaFeO₃ perovskite phase. A decrease in the La/Fe surface ratio from 12 to 5.2 with the rise in calcination temperature from 650° to 800°C was detected by XPS. EDX results confirmed that at 750–850?°C, the La/Fe ratio in the perovskite layer is close to the stoichiometric and amount to 1.01–1.03. The highest activity in methane oxidation was achieved when the LaFeO₃ perovskite was calcined at 700?°C. A further slight increase in the activity was noticed after H₂ treatment. As the calcination temperature of the perovskites is increased, the catalyst activity decreases due to a reduction in the specific surface area, despite the more complete LaFeO₃ perovskite phase formation.     

Mullite-based refractory castable engineering for the petrochemical industry

Refractory castables used in fluid catalytic converter (FCC) risers should present suitable particle erosion and thermal shock resistances at temperatures below 900 °C. Considering that calcium aluminate cement (CAC)-bonded refractories usually start their densification above 1200 °C, the use of sintering additives to induce faster densification is a promising technological alternative. Therefore, this work addresses the evaluation of mullite-based castables containing a boron-based sintering additive and CAC and/or hydratable alumina as the binder sources. Hot elastic modulus, cyclical thermal shock, hot modulus of rupture and cold erosion resistance measurements were carried out to evaluate the compositions. According to the attained results, adding 1.5 wt% of the evaluated sintering additive to the designed castables led to a remarkable increase of the hot modulus of rupture (maximum of 40.4 MPa at 800 °C for the CAC-containing refractory) and high erosion resistance (1.5–2.9 cm³) after pre-firing at 800 °C for 5 h. Moreover, the combination of CAC and hydratable alumina gave rise to an improved refractory (M–2CAC–2HA–S) showing a transient liquid formation at an increased temperature, high thermal shock resistance (no E decay after 8 thermal cycles, ΔT=800 °C) and high mechanical strength at 800 °C and 1000 °C.     

Optimized preparation of alumina based fillers for tuning composite properties

Alumina based particles were prepared from aluminium chloride hydroxide as starting material by sol-gel technique. One series of particles was doped with ferrous oxide. Both series of particles were calcinated at three different temperatures: 700?°C, 800?°C and 900?°C. Poly(methyl methacrylate), PMMA, was used as a matrix and two different types of alumina based particles were added into the matrix to form the composites. All composites consisted of 3?wt% of alumina based particles. The aim of this study was to examine whether and how the temperature of particle calcination affects the microhardness and mechanical properties of the composite. The particles were characterized by the X-ray diffraction (XRD) and physical absorption methods. The morphology of the composites was examined using a field emission scanning electron microscope (FESEM). The microhardness of composites was measured using a traditional Vickers hardness (HV) method. The mechanical characteristics of obtained composites were determined using tensile test and impact testing.     

Alumina nanofibers obtained from poly(vinyl alcohol)/boehmite nanocomposites

Poly(vinyl alcohol) (PVA)/boehmite nanocomposite (precursor) nanofibers were formed by electrospinning using a PVA aqueous solution of dispersed boehmite nanoparticles as the spinning solution. The alumina nanofibers were obtained by calcination of the precursor nanofibers between 500 and 1200°C. The specific surface area of the precursor nanofibers was around 6 m²/g, and that of the γ‐alumina nanofibers calcined at 500°C was around 300 m²/g. The specific surface areas and the fiber diameters were not affected by the alumina contents in the precursors. Also, the diameter of the alumina nanofibers was not affected by the calcination temperature of the precursor nanofibers. The pore characteristics of the alumina nanofibers decreased with increased calcination temperature due to the sintering, and nonporous α‐alumina nanofibers were obtained by calcination of the precursor nanofibers at 1200°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Alumina-zircon filler based ceramic shell moulds for directionally solidified cast shrouded low pressure turbine blades

Ceramic shell moulds for investment casting of shrouded low-pressure turbine (LPT) blades were prepared by using colloidal silica binder and partial substitution of the zircon filler with fine alumina. Among the two ceramic slurry systems designed, the first slurry system comprised of polymer-free colloidal silica binder, and the second slurry system comprised of polymer-containing colloidal silica binder. The samples prepared from the first slurry system showed higher fired residual strength and self-load sag values (lesser sag resistance). The casting of shrouded LPT blades was carried out at 1525 °C and 1550 °C using CM247LC superalloy. Ceramic shell moulds prepared from the second slurry system, containing 30 wt% of fine alumina filler, yielded aeronautical grade casting (at 1550 °C) of blades with required dimensional accuracy and average surface roughness. Microstructural analysis of the cut surfaces of self-load sag tested samples was carried out to understand the effect of fine alumina substitution on shell characteristics.     

Characterization of perovskite LaFeO3 synthesized by microwave plasma method for photocatalytic applications

Perovskite LaFeO₃ nanoparticles were successfully synthesized by microwave plasma method combined with high temperature calcination at 700–1000?°C. The influences of calcination temperature on morphology, crystalline structure, purity and the atomic compositions of samples were studied. The photocatalytic performance of LaFeO₃ was evaluated though the photodegradation of Rhodamine B (RhB) under visible light. In this research, the orthorhombic LaFeO₃ nanoparticles showed band gaps in the range of 2.15–2.30?eV. The particle size increased with increasing in the calcination temperature, leading to the decreasing in the surface area. The LaFeO₃ sample calcined at 900?°C showed the highest photodegradation of 77.8% and the apparent rate constant of 0.0077?min^?1 within 180?min because of the narrower of band gap and the higher crystalline degree and oxygen adsorption.     

Effect of MgO calcination temperature on the reaction products and kinetics of MgO–SiO2–H2O system

MgO-based binders have been widely studied for decades. Recently, the MgO–SiO₂–H₂O system was developed as a novel construction material, however, its reaction mechanism remains unclear. This paper investigated the reaction products and kinetics of MgO/silica fume (SF) pastes with MgO calcinated at different temperatures. The results indicate that MgO presented larger grain size after calcination at higher temperature. Mg(OH)₂ and magnesium silicate hydrate (M–S–H) gel were formed when using MgO calcined at 850, 950, and 1050°C. However, only M–S–H gel was formed when using MgO calcined at 1450°C. The reaction kinetics of MgO could be described using α = 1 − e^−k*t. The reaction rate of MgO increased with decreasing calcination temperature, increasing SF dosage, and the addition of sodium hexametaphosphate. Only M–S–H gel was formed when the reaction rate of MgO was below the demarcation line (about 0.250 × 10⁻⁶ s⁻¹), and the corresponding demarcation area was around 14 days.     

Mineralogical and dielectric properties of mullite and cordierite ceramics produced using wastes

In this work technical ceramics containing industrial inorganic wastes was carried out. Ceramic formulations prepared with clay, magnesium oxide and residues of kaolin and alumina as raw materials, were formed in a disk-shaped specimens using the uniaxial pressing process and sintering at temperatures from 950°C to 1400°C. The mineralogical, physical and dielectric characteristics of the fired samples were investigated. The dielectric properties, the relative dielectric constant (ε_r) and the loss tangent (tan δ) were evaluated at frequencies of 0.1, 1, 10, and 100?kHz at room temperature. Mullite and cordierite were present as major phases at the highest temperatures. Relative dielectric constant values closest to that of mullite (ε_r = ~ 5 to ~ 6) and cordierite (ε_r =?~ 4 to ~ 6) at 1?kHz. On the other hand, the lowest dielectric losses (tan δ ~ 0.06 to ~ 0.04) were observed for the formulations containing the mullite major phase, and tan δ ~ 0.009 to ~ 0.003 for formulations that showed cordierite as main phase. It was verified that an increase in temperature promoted a reduction of porosity, a property that had a direct influence on the dielectric properties of the formulations. The materials obtained from the residues presented low dielectric constants and loss tangents, which make them suitable for use in electrical and electronic systems.