纳米钛酸钡粉体的分散及水基悬浮体制备

通过测定超重力法制备的纳米钛酸钡悬浮体的Zeta电位和团粒尺寸分布,确定了纳米BaTiO3在水中稳定分散的基本条件,制备了不同体积分数的悬浮体并研究了其流变学特性. 该纳米BaTiO3在水中的等电点约为pH 2.8,加入分散剂PAA-NH4后,颗粒零电点向更低pH值移动;当分散剂用量为干粉量的1.2%(w)时,悬浮体中平均团粒尺寸最小,分散效果最佳. FT-IR分析证明颗粒表面上PAA-NH4的吸附为物理化学吸附. 制备了体积分数高达45%的纳米BaTiO3悬浮体,其流变特性表现为剪切变薄行为,流延成型的BaTiO3陶瓷生坯片具有较高的相对密度(55.4%).     

Effects of dispersant concentration and pH on properties of lead zirconate titanate aqueous suspension

Lead zirconate titanate (PZT) aqueous suspensions were prepared at 60 wt.% solids loading using a commercial ammonium polyacrylate (APA) dispersant. Effects of the dispersant concentration on rheological behavior, dispersion and stability of PZT aqueous suspensions were investigated by means of zeta potential, viscosity and sedimentation height measurements. The results showed that, under suitable conditions, APA dispersant promoted particle dispersion and stabilization in PZT aqueous suspensions. For 60 wt.% solids loading suspensions, the dispersant concentration yielding the lowest viscosity was 0.5 wt.% based on PZT powder dried weight basis. Effects of pH on particle dispersion in the suspensions prepared with APA were studied by laser light scattering technique and scanning electron microscopy. The results showed an improvement in particle dispersion for the alkaline condition, which led to relatively low viscosity and highly stable suspension. Possible particle stabilization mechanisms at various pHs were discussed based on dissociation of the dispersant in water, polymer conformation and adsorption behavior of the dispersant on the particle surface.     

Suspension stability and sintering influence on yttria-stabilized zirconia fabricated by colloidal processing

The stability of nano-zirconia 3YSZ powder in suspension was extensively studied by the colloidal method, and the optimum sintering temperature of the green sample fabricated through slip casting was determined. Zirconia suspensions with 10 vol% powder loading were prepared with distilled water, and HNO₃ was used to adjust the pH of the suspension to pH 1–6. All of the suspensions were subjected to sedimentation test, and the results showed that the suspensions adjusted to pH 2 had the lowest sediment volume. This finding indicates that a suspension with pH 2 produces higher packing density. Viscosity test was carried out for the suspensions added with dispersant ranging from 0.3 wt% to 0.7 wt% polyethyleneimine (PEI) with and without pH adjustment. The suspension containing 0.5 wt% PEI with pH 2 adjustment produced the lowest viscosity because of interparticle bond breakage in the aggregates, thus forming colloidally stable suspensions. The zirconia suspension containing 0.5 wt% PEI and whose pH was adjusted to pH 2 was chosen to be slip casted into cylindrical shape. Green samples were sintered at various sintering temperatures that ranged from 1100 °C to 1500 °C through a two-step sintering method. The sample sintered at 1500 °C was found to be porosite-free, and its highest relative density was 99.6% of the theoretical density. Morphological studies detected pores in the microstructure of the samples sintered at low sintering temperatures (1100 and 1200 °C). By contrast, the samples sintered at 1400 and 1500 °C were fully densified. However, the grain size of the sample sintered at 1500 °C was 230 nm, which indicated excessive grain growth. The Vickers hardness of the sample sintered at 1400 °C was found to be highest (12.9 GPa) and comparable to results found in the literature.     

Preparation of high solids content nano-titania suspensions to obtain spray-dried nanostructured powders for atmospheric plasma spraying

Nanosized TiO₂ powder with an average primary size of ∼20 nm and surface area of ∼50 m²/g (Aeroxide^® P25, Degussa-Evonik, Germany) was used as starting material. A colloidal titania suspension from the same supplier was also used (W740X). The dispersing conditions were studied as a function of pH, dispersant content, and solids loading. Well-dispersed TiO₂ nanosuspensions with solids contents up to 30 vol.% (62 wt%) were obtained by dispersing the powder with 4 wt% PAA. Suspensions with solids contents as high as 35 vol.% were prepared by adding the TiO₂ nanoparticles to the TiO₂ colloidal suspension under optimised dispersing conditions.TiO₂ powder reconstitution was performed by spray drying both types of nanosuspensions to obtain free-flowing micrometre-sized nanostructured granules. The spray-dried nanostructured TiO₂ granules were deposited on austenitic stainless steel coupons using atmospheric plasma spraying. Coating microstructure and phase composition were characterised using scanning electron microscopy and X-ray diffraction techniques.     

Formation of needle-like titanium oxynitride particles through nitridation of hydrated titanates

The process of nitridation of hydrated titanate wires was examined by thermal gravimetric (TG) analysis in an NH₃/Ar (50/50 vol.%) gas mixture, X-ray diffraction (XRD) measurement, field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) observations and X-ray photoelectron spectroscopic (XPS) analysis. The nitridation of the hydrated titanate nanowires was accompanied by a two-stage weight loss. In the first stage, occurring in the temperature range of 50-400 °C, the hydrated titanate wires changed to anatase-type TiO₂ nanoparticles with the releasing of H₂O molecules. In the second stage, occurring in the temperature range of 700-1000 °C, the TiO₂ nanoparticles were converted to rock-salt-type titanium oxynitride (TiN_xO_y) nanoparticles. Subsequently, the TiN_xO_y nanoparticles were sintered each other at around 1000 °C. Under a gas flow of 100% NH₃, the hydrated titanate wires were completely changed to TiN_xO_y particles at a temperature greater than 950 °C, which was maintained for 2 h. It is possible to fabricate needle-like TiN_xO_y particles by selecting thick hydrated titanate wires as the starting materials.     

Direct coagulation casting of YSZ powder suspensions using MgO as coagulating agent

Direct coagulation casting (DCC) of aqueous 8 wt% yttria stabilized zirconia (YSZ) powder suspensions prepared using ammonium poly(acrylate) dispersant has been studied using MgO as coagulating agent. Small amount (<0.1 wt% based on YSZ) of MgO powder dispersed in the YSZ powder suspension at ∼5 °C set the suspension in to stiff wet-coagulated body when exposed to room temperature (30 °C) due to the reaction between ammonium poly(acrylate) and MgO. MgO concentration equivalent to react with dispersant did not coagulate the YSZ powder suspension though it precipitate the whole ammonium poly(acrylate) dispersant as Mg-poly(acrylate). This is because of the ability of the YSZ powder to disperse in water at alkaline pH (∼9.5) without any dispersant by electrostatic mechanism. The YSZ powder suspensions form stiff coagulated bodies at MgO concentration double or more of the equivalent amount required for reacting with the dispersant. Setting of the YSZ powder suspension is due to the heterocoagulation of the YSZ particles and MgO particles added in excess of the equivalent amount to react with the dispersant, having opposite surface charges. The wet-coagulated body showed relatively high compressive yield strength (155 kPa) and Young’s modulus (3.1 MPa). The green bodies prepared by humidity controlled drying of the wet-coagulated bodies sintered to >98% TD at 1550 °C.     

Rheological behaviour of BaZrO3 suspensions in non-aqueous media

The colloidal behaviour of concentrated non-aqueous suspensions of barium zirconate is investigated. Optimum dispersing conditions are investigated for organic suspensions prepared with different solvent systems: methylethylketone (MEK)/ethanol, trichloroethylene (TCE)/ethanol, a methylisobutylketone (MIBK)/methylethylketone/cyclohexanone (CHN) and pure ethanol. Stabilisation of the suspensions is achieved by using two different dispersants: ester phosphate (EPH) and Hypermer KD-6. Rheological studies have been made in order to determine the optimum of each dispersant concentration as a function of the solvent systems. The best stabilisation of 30 vol.% suspensions is obtained by using EtOH with 3 wt.% KD-6 and TCE/EtOH with 1 wt.% EPH. Zeta potential studies show that the use of dispersants increases the positive surface potential on the particles. The zeta potential values increasing from +10 mV in the absence of EPH to +42 mV with 1.0 wt.% EPH and to +33 mV with KD-6. These results suggest that electrostatic repulsion plays an important role on the stability of these organic suspensions. Stabilised suspensions were slip cast in plaster molds and green densities higher than 55% of the theoretical density was obtained.     

Gelcasting of alumina nanopowders based on gelation of sodium alginate

The alumina nanopowder was synthesized via the sol–gel method. θ-Alumina with crystallite size in the range of 25–110 nm was crystallized by calcination of the powder at 900 °C for 1 h. Sodium alginate, a natural innoxious polymer, was applied for in situ forming process of an Al₂O₃ green body, using calcium phosphate as a solidifier agent. Sodium hexa metaphosphate was also utilized as a chelator. Rheological and gelation behaviors of resultant slurry were analyzed. The viscosity of slurries with 15 vol.% alumina and 1.8 vol.% calcium phosphate dispersed by 1 wt.% sodium alginate solution, was less than 800 mPa s. The green bodies from the gelcasting process were dried at room temperature for 36 h and pressureless sintered at 1500 °C for 3 h. A uniform microstructure without huge grain growth was revealed by SEM.     

影响氮化硅浆料流变特性的因素

通过对影响氮化硅浆料流变性因素的研究发现：不同的氮化硅粉体具有不同的等电点（ｉｓｏｅｌｅｃｔｒｉｃ ｐｏｉｎｇ,简称ＩＥＰ）。分散剂的引入明显改善浆料的流动性。对于粉体Ａ,当分散剂用量达１．２ｗｔ％时浆料具有最佳的流动性。颗粒尺寸及其形貌对浆料的流动性均有较大的影响：颗粒尺寸越大,颗粒形貌越不规则,浆料的流动性就越差。     

Preparation of aluminum nitride green sheets by aqueous tape casting

Aluminum nitride green sheets were prepared by aqueous tape casting. The characteristics of a treated AlN were studied in aqueous ball-milling media. The oxygen content picked up with the increase of ball-milling time. It was noted that the oxygen content of AlN powder with the dispersant DP270 was lower than that of AlN powder without the dispersant DP270. The isoelectric points of the treated AlN with and without DP270 were, respectively, at pH 3.35 and pH 3.90. The dispersant DP270 not only efficiently dispersed AlN powder in water to form a stable suspension, but also formed a coat onto AlN surface to limit hydrolysis of the AlN powder. The tape casting slurry exhibited a typical shear-thinning behavior. Aqueous AlN green tape had a smooth surface and a narrow pore size distribution. Its relative density was 52.6%. No other crystalline phase was detected by XRD except for AlN and sintering aid yttria in AlN green sheet.     

Spray-drying of ceramics for plasma-spray coating

The spray-drying process of ceramics which are candidate materials for thermal barrier coatings (TBCs), i.e. 3YSZ+0, 2, 4, 6 wt.% Al₂O₃, is discussed in this paper. The two most important properties of spray-dried powders to determine the coating quality are density and particle size. Polyethyleneimine (PEI) acts as both an organic binder and a dispersant giving low viscosity in the suspension. The optimised suspension composition is: ⩾ 33.6 vol.% powder+1.8 wt.% PEI+ethanol, and operational parameters of the spray-dryer: drying temperature 175°C, feeding rate 55 cm³/min, feeding pressure 1.013×10⁴ Pa.     

Microwave-hydrothermal synthesis of barium titanate under stirring condition

The role of in situ stirring under microwave-hydrothermal (M-H) conditions on the synthesis of barium titanate was investigated for the first time by powder X-ray diffraction and scanning and transmission electron microscopy. Stirring under M-H conditions in the temperature range of 150–200 °C led to enhanced crystallization of Ba titanate as revealed by yields compared to the static condition. In addition, stirring led to smaller and more uniform crystals under M-H conditions compared to those crystallized without stirring. Powder X-ray diffraction revealed the formation of only cubic polymorph of Ba titanate at or below 200 °C in 4 h with or without in situ stirring under M-H conditions. These results show that stirring is an important parameter during M-H synthesis of nanophase Ba titanate.     

Aqueous gelcasting of ZrB2–SiC ultra high temperature ceramics

ZrB₂–SiC ultra high temperature ceramics containing B₄C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB₂–SiC suspension were investigated in detail. A well-dispersed suspension with 50 vol.% solid loading was prepared at pH 10 with 0.4 wt.% PAA after ball milling at 240 rpm for 24 h. Then crack-free green ZrB₂–SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100 °C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405 ± 27 MPa and 4.3 ± 0.3 MPa m^1/2, respectively.     

Effect of SiC content,additives and process parameters on densification and structure–property relations of pressureless sintered ZrB2–SiC composites

The effect of SiC content, additives, and process parameters on densification and structure–property relations of pressureless sintered ZrB₂–(10–40 vol%) SiC particulate composites have been studied. The ZrB₂–SiC composite powders mixed by ball-milling with 1.2 wt% C (added as phenolic resin) and 3 wt% B₄C have been uniaxially cold-compacted and sintered in argon environment at 1950–2050 °C for 2 h, or at 2000 °C for durations between 1/2 and 3 h. The amount of densification is found to increase with sintering duration, and by prior holding at 1250 and 1600 °C for reduction of oxide impurities (ZrO₂, B₂O₃ and SiO₂) on powder particle surfaces by the aforementioned additives. Presence of SiC with average size smaller than that of ZrB₂ appears to aid in densification by enhancing green density, increasing WC content by erosion of milling media, and inhibiting matrix grain growth. Both SiC and WC appear to aid in reduction of oxide impurities. Furthermore, the impurities enriched in W, Fe and Co obtained from milling media are found to be segregated at ZrB₂ grain boundaries, and appear to assist in densification by forming liquid phase, which completely wets the ZrB₂ grains. Hardness increases with SiC content or with sintering duration till 1 h, but decreases for periods ≥2 h due to grain growth. The experimentally measured elastic moduli approaches corresponding theoretically predicted values with increasing SiC content due to reduction in porosity.     

Preparation and characterization of the lead-free piezoelectric ceramic of Bi0.5Na0.5TiO3 doped with CuO

This study investigates the effects of copper oxide (CuO) addition, calcining temperature, and sintering temperature on the microstructure and the electrical properties (such as dielectric constant and loss tangent) of lead-free piezoelectric ceramic of bismuth sodium titanate (Bi_0.5Na_0.5TiO₃), BNT, which was prepared using the mixed oxide method. Three kinds of starting powders (Bi₂O₃, Na₂CO₃ and TiO₂) were mixed and calcined. This calcined BNT powder and a certain weight percentage of CuO were mixed, calcined, and compressed into a green compact of BNT-CuO. This green compact of BNT-CuO was sintered to be a disk doped with CuO, and its characteristics were measured. In this study, the calcining temperature ranged from 700 to 1000 °C, the sintering temperature ranged from 950 to 1050 °C, and the weight percentages of CuO doping included 2, 4, 6, and 8 wt.%. The largest relative density of the BNT-CuO disk obtained in this study was 96.7% at the calcining temperature of 700 °C, the sintering temperature of 950 °C, and 4 wt.% of CuO addition. The corresponding dielectric constant and loss tangent were 494 and 0.181%, respectively. This study shows that adding CuO to the BNT not only improves the relative density and the dielectric constant of the BNT disk, but it also lowers the sintering temperature.     

Studies on slip casting behavior of lanthanum strontium manganite

Dispersion conditions for slip (slurry) formulation of a powder mixture of lanthanum strontium manganite (La_0.84Sr_0.16MnO₃ - LSM) and carbon (pore former) in water was studied through detailed zeta-potential and rheological measurements. The zeta potential variation with pH for the aqueous suspensions of only LSM or carbon exhibited a maximum value in alkaline medium (−40 mV to −50 mV at a pH of 10-11), establishing the pH window for their co-dispersion for slurry formulation. A study of the viscosity variation with shear rate for the slurries with varying solid content (in the range of 45-65 wt.%) exhibited pseudo-plastic flow behavior, indicating presence of flocculates in them. The yield stress values obtained from the Casson equation reduced with decreasing solid content, indicating reduction in the flocculate strength. The slip with solid content of 50 wt.% exhibited optimum flow characteristics to form long tubes with uniform wall thickness (wall thickness 2-4 mm and length of 150-200 mm). The tubular specimens formed after controlled carbon burn out and sintering at 1400 °C for 1 h possessed about 35% open porosity. The porosity remained the same upon further sintering at 1400 °C for 8 h.     

Tape casting of Y-TZP with low binder content

The rheology of Y-TZP slurries with a low content of an acrylic/styrene binder is studied. The optimum amount of dispersant was found to be 0.3 wt.% for the starting slurry. After a 5 wt.% of binder addition the dispersant amount needed to achieve a minimum viscosity increases to 0.5 wt.%. Tapes obtained from slurries with a solid content of 45 vol.% shows the higher green density of 51 th.%. Higher solid contents do not yield to better green densities due to the high viscosity.     

Innovative fabrication of PZT pillar arrays by a colloidal approach

An innovative approach for fabricating pillar arrays for ultrasonic transducer applications is disclosed. It involves the preparation of concentrated piezoelectric lead zirconate titanate (PZT) suspensions in aqueous solutions of epoxy resin and its polymerization upon adding a polyamine based hardener. Zeta potential and rheological measurements revealed that 1 wt.% dispersant, 20 wt.% of epoxy resin and a hardener/epoxy resin ratio of 0.275 mL g⁻¹, were the optimized contents to obtain strong PZT samples with high green strength (35.21 ± 0.39 MPa). Excellent ellipsoidal and semi-circle shaped pillar arrays presenting lateral dimensions lower than 10 μm and 100 μm height were successfully achieved. The organics burning off was conducted at 500 °C for 2 h at a heating rate of 1 °C min⁻¹. Sintering was then carried out in the same heating cycle at 1200 °C for 1 h. The microstructures of the green and sintered ceramics were homogeneous and no large defects could be detected.     

Direct synthesis of barium zirconate titanate (BZT) nanoparticles at room temperature and sintering of their ceramics at low temperature

Using tetrabutyl titanate, zirconium nitrate and barium octahydrate as the raw materials, BZT nanoparticles with a grain size of ~10 nm were directly synthesized at room temperature. With low energy consumption and without any contamination produce, the synthesis process is green, environmental-friendly, convenient and efficient. The graingrowth of the as-prepared nanoparticles annealed at different temperatures was checked, and a rapid graingrowth starting at 600 °C was observed. The sintering characteristics were also studied and it was found that an adapted sintering aid was very important for the sintering of the BZT nanoparticles at low temperature. Here, adding the useful active liquid of Bi₂O₃–Li₂O as the sintering aid results in obtaining the dense BZT ceramics with the relative density of 96% even sintering at the temperature as low as 900 °C for 2 h.     

Investigation on rheological behavior of 8 mol% yttria stabilized zirconia (8YSZ) powder using Tiron

In this research, 8 mol% yttria stabilized zirconia (8YSZ) powder were dispersed in de-ionized water by the use of different amounts of Tiron as dispersant. The results of rheological and sedimentation measurements of each suspension were evaluated and the optimum amount of Tiron was selected (0.8% Tiron). Also, various pH were investigated and the best stabilized suspension is achieved at pH 10. Furthermore, the zeta potential of suspension with and without adding dispersant was obtained. The isoelectric point (IEP) of as-received 8YSZ powder was about 8.5 and shifted obviously to acidic region after adding dispersant to the suspension.