ZTA复相陶瓷凝胶注模成形工艺的研究

本文研究了ＺｒＯ２（３Ｙ）－Ａｌ２Ｏ３复相陶瓷的凝胶注模成形工艺,着重研究了低粘度高固相体积分数浓悬浮体的制备。     

Influence of Acidity on the Stability and Rheological Properties of Ionically Stabilized Alumina Suspensions in Ethanol

The ionic stability of alumina particles in moderately concentrated ethanol suspensions is studied. Surface chemistry and interparticle forces are manipulated by controlling the acidity of the suspensions without dispersants. The acidity of ethanol solution is determined using ion transfer functions, wherein the relationships between acidity, alumina particle surface charge, zeta-potential, stability, and suspension rheological behavior are established. Positive isoelectric point (IEP) shift is observed for alumina in ethanol on increasing the solids concentration. However, dilute and concentrated aqueous suspensions of alumina give the same IEP. The viscosity and flow curves for alumina/ethanol suspensions are acidity dependent. The flow curves of the suspensions follow the Casson model, and the Casson yield value is used to evaluate suspension stability.     

Acoustophoretic studies of aqueous suspensions of alumina and 8 mol% yttria stabilised zirconia powders

The iso electric points (IEPS) of four alumina powders and three 8 mol% yttria stabilised zirconia powders were investigated using acoustophoresis to measure the zeta potential as a function of pH. All the powders were characterised by X-ray fluorescence spectroscopy for elemental composition and BET nitrogen adsorption for surface area. Initial experiments revealed that the IEPS of the alumina powders were independent of the volume fraction of the alumina suspensions. Further experiments investigated the effect of adjusting the pH with different bases, i.e. NaOH, KOH, and NH₄OH, on the IEPS of the alumina powders. The IEP of an individual powder was independent of the base used, however, the degree of hysteresis varied depending on the polarisability of the cation. The smallest degree of hysteresis was recorded with additions of NaOH. The differences in IEP between the alumina powders were attributed to the different surface chemistries of the powders. For the zirconia powders the IEP of each suspension was identified using HCl and NaOH to adjust the pH. Again the differences in the IEP were attributed to different surface chemistries of the powders. Two of the powders demonstrated an IEP of approximately 6.5; however the Tosoh powder suspension demonstrated considerable hysteresis and an IEP of 9.3. This was due to its natural pH being an approximate pH unit lower than the other two powders. At this lower natural pH, yttria dissolved out of the Tosoh powder, generating yttrium ions in solution. These precipitated onto the powder surface and altered the surface chemistry with time; i.e increased the zeta potential. Atomic absorption experiments on the supernatant confirmed the dissolution of the yttria from the Tosoh powder.     

Effect of Polyvinylpyrrolidone Additions on the Rheology of Aqueous,Highly Loaded Alumina Suspensions

The control of the rheological behavior of highly loaded ceramic/polymer suspensions affords the development of near‐net shape forming techniques. In this study, suspensions containing sub‐micrometer diameter alumina (up to 56 vol%) were fabricated using an anionic dispersant (≈4 vol%) and water‐soluble polyvinylpyrrolidone (PVP). The amount and ratio of molecular weights of PVP in the suspension were varied to influence flow behavior. The final pH of the system, ≈9.5, was higher than the isoelectric point (IEP) of alumina implying that the alumina powder possesses a negative surface charge. In the case of alumina at this pH, PVP does not adsorb onto the surface of the powder. The flow behavior of the PVP‐containing suspensions displayed yield‐pseudoplastic characteristics that closely agreed with the Herschel–Bulkley fluid model. The addition of PVP significantly changed the rheology of the system, increasing the shear yield stress and altering flow behavior. Interparticle interaction approximations of the suspensions were modeled to correlate with experimental observations.     

Rheological Characterization and Coagulation Casting of Al2O3–Nano Zirconia Suspensions

The rheological behavior of electrosterically dispersed aqueous suspensions composed of submicrometer alumina and nano zirconia particles in different ratios and solids contents (from 34 to 40 vol%) has been studied during in situ coagulation casting. Glucono-δ-lactone was used as the coagulant to achieve destabilization of the ceramic suspensions by acidifying the suspension so that the pH became close to the isoelectric point for both powders. The effects of the lactone content (from 0.5 to 6.7 wt%) and the solids loading on the rheological parameters (dynamic and static measurements) during the coagulation process were studied. Dynamic measurements showed an increasing elastic modulus, G', with time and lactone content (viz. 180 and 340 kPa for 6.7 wt% lactone addition after 40 and 60 min, respectively, and 57.3 kPa for 0.5 wt% lactone after 60 min), suggesting the formation of stronger green bodies at higher additions and longer times. The conditions were optimized to yield homogeneous green samples with a uniform nanozirconia distribution and densities between 58% and 60% of the theoretical. These were subsequently sintered to yield composites with a final density up to 97%.     

Effects of Solids Loading and Dispersion Schedule on the State of Aqueous Alumina/Zirconia Dispersions

The effects of solids loading and dispersion order on the state of aqueous slips of either zirconia or alumina, and binary slips of these two powders, have been examined. Since these powders can acquire surface charge when they are dispersed in aqueous media, changes in the ionic composition of the slip can occur as a result of the addition of solids. At higher solids loading, a substantial fraction of the ionic activity within the slip may be attributed to the solid itself. As the solids loading within unary powder suspensions is increased, it is possible to drive the suspension pH toward the point of zero-charge for the powder, resulting in diminished electrostatic stabilization and possible powder agglomeration. An analytical model was constructed to account for the observed results. This embodied a simple surface site complexation model which was augmented to include material and charge balance constraints on ionized surface sites as well as solvated ions. For binary powder dispersions of alumina and zirconia, it was found that the order of dispersing the two components could have a pronounced effect on the overall suspension behavior. The variability in results is attributed to the solubility of the alumina in the basic processing media.     

Electrokinetic Properties of Colloidal Zirconia Powders in Aqueous Suspension

Pure zirconia, yttria, and three yttria-doped zirconia powders of submicrometer size have been dispersed in various aqueous solutions. The zeta potential (zeta) of the zirconia powders is determined primarily via streaming-current (SC) detection and is confirmed using electrophoretic spectroscopy techniques. The results reveal that the isoelectric point (IEP) of these zirconia powders is in the pH range of 5.6-7.2 and zeta is controlled primarily by the yttrium content of the zirconia powders and the type of electrolyte. In addition, the yttria content strongly affects the potential and SC in zirconia suspensions only at high solids contents (>1 vol%). The electrokinetic data reveal that the surface of the yttria-doped tetragonal zirconia powder (TZP) can be modified via the adsorption of ionic molecules or polymeric species in the suspension. The adsorption of an anionic polymer can stabilize zirconia particles in a solution that is almost neutral or weakly basic (in the IEP range of pure ZrO₂). The interaction of the zirconia and yttria particles with the electrolytes in an aqueous suspension will be discussed to reveal the roles of hydrated oxide formation and zirconia surface interaction with polymeric dispersants.     

Compaction and Sintering Behavior of Bimodal Alumina Powder Suspensions by Pressure Filtration

The compaction behavior of fine alumina powders with different particle sizes or bimodal particle-size distributions that are undergoing pressure filtration was investigated. Three alumina powders—average particle sizes of 0.2—0.86 μm—were compacted to a solids fraction of 62—65 vol% from suspensions at pH 3, which was the pH level at which the suspensions showed their lowest viscosity. When the powders of different average sizes were mixed, the suspensions showed better flowability, and the lowest viscosity was obtained when the fraction of fines was ∼30 vol% and pH = 3. The mixed-sized powder suspensions were compacted to higher density than the suspensions of unmixed fine or coarse powders, and the maximum density was obtained for mixed suspensions that had the lowest viscosity, despite the different particle-size ratio. Maximum densities of 72.5% and 75.0% were attained when the size ratios were 2 and 5, respectively. The compacts that were pressure-filtered from mixed suspensions exhibited a single-peaked pore-size distribution and a homogeneous microstructure, whereas the pore-size distributions of dry-pressed compacts were double-peaked. The sintering behavior of the compacts that were pressure-filtrated from bimodal powders exhibited significantly better sinterability and much-less linear shrinkage than the coarser powders and the dry-pressed powder compacts.     

The Isoelectric Point of Lead Magnesium Niobate

Lead magnesium niobate (PMN) is an important relaxor ferroelectric material commonly used in multilayer capacitor and actuator manufacturing owing to its high dielectric constant and superior electrostrictive properties. However, the isoelectric point of this material in water is not known and there is justification for a detailed investigation. In this work, the isoelectric point (IEP) of aqueous PMN suspensions were determined as a function of solids concentration. Results showed that IEP of the PMN suspensions strongly depended on the solids loading. The IEP was between pH 9 and 10 at particle concentrations between 10 to 20 vol%. The IEP shifted gradually to a lower pH value as the particle concentration decreased. Solubility experiments showed that Pb²⁺ and Mg²⁺ ions dissolved from the PMN surface, especially in the acidic pH range. The study provides a new insight on the aqueous stability of perovskite materials which possess more than one soluble cation in their structure .     

Effect of 2-Phosphonobutane-1,2,4-tricarboxylic Acid Adsorption on the Stability and Rheological Properties of Aqueous Nanosized 3-mol%-Yttria-Stabilized Tetragonal-Zirconia Polycrystal Suspensions

2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) was evaluated as a dispersant for nanosized 3-mol%-Y₂O₃-stabilized tetragonal-ZrO₂ polycrystal (3Y-TZP) suspensions. The adsorption of PBTCA was characterized using the decolorization method of ferric 5-nitrosalicylate complexes. Maximum adsorption of the dispersant on the 3Y-TZP powder was found to occur at pH 3.0. At pH >3.0, the adsorbed amount decreased with increased pH. Semiquantitative analysis using auger electron spectroscopy showed that PBTCA adsorbed irreversibly on the powder. The surface charge of the powder was evaluated by measuring the zeta-potential in dilute powder suspensions. The suspension was most effectively stabilized at high pH by the high charge induced by the adsorption of PBTCA. Rheological properties of the suspension were evaluated as a function of dispersant amount and solids loading. The optimum amount of dispersant increased with increased solids loading for solids loading >20 vol%. A stable suspension of 35 nm 3Y-TZP particles with a solids loading as high as 32 vol% was obtained using PBTCA as dispersant, in contrast to 28 vol% when using ammonium polyacrylate (NH₄PAA). Theoretical calculations of the interaction between 3Y-TZP particles showed that the stabilization of the suspensions was attributed to a combination of the electrostatic repulsion and a steric barrier caused by the adsorbed PBTCA. Induced coupling plasma analysis showed that PBTCA could be completely burned out during sintering, which confirmed its suitability as a dispersant for 3Y-TZP.     

Dissolution and Dispersion Behavior of Barium Carbonate in Aqueous Suspensions

Dissolution of BaCO₃ and its effect on the dispersion behavior of aqueous BaCO₃ suspensions at various pH values have been investigated. The amount of leached Ba²⁺ decreases with increasing pH value, which agrees with thermodynamically calculated results. The dissolution of BaCO₃ also causes an increase in pH value of the suspension, but the change decreases with increasing initial pH value. The isoelectric point (IEP) of leached BaCO₃ powder is at a pH of ∼10–10.5 and remains unchanged with increasing solids loading. The IEP of BaCO₃ shows no significant change with added KCl or K₂CO₃, but shifts to a higher pH with increasing concentration of added BaCl₂.     

Effect of sucrose on rheological properties of aqueous zirconia suspensions with polyacrylate

The role of pH, additive concentration, aging time and solids loading in determining the rheological properties of aqueous zirconia suspensions, which contained polyelectrolyte and sucrose molecules, was investigated. It was found that at alkaline pH, the appropriate sucrose addition (4 wt.%) enhanced the stability of zirconia suspensions with polyacrylate, and the amount of polymer required to obtain the maximum fluidity decreased as sucrose was added. However, the positive effect of sucrose on stability cannot maintain at the higher solids loading (40 vol.%) because of the bridging attraction between the overlapping adlayers. On the other hand, the effectiveness of sucrose would be greatly weakened by hydrolyzation in acidic media as well as biodegradation with aging time.     

Horizontal Dip‐Spin Casting of aqueous alumina‐polyvinylpyrrolidone suspensions with chopped fiber

A novel ceramic processing method, called Horizontal Dip Spin Casting (HDSC), enabled fabrication of tubular ceramic parts with an aligned chopped fiber phase. HDSC was demonstrated using highly loaded aqueous alumina suspensions with >50 vol.% solids loading and ≤5 vol.% water‐soluble polymer employed as a rheological modifier. Chopped carbon fibers were added to the suspensions to attain maximum loadings of 30 vol.%. During forming, cylindrical foam molds were dipped into the suspension while being rotated radially about the long axis. Simultaneously, a doctor blade was placed at a specified distance from the foam surface to facilitate the flow of the suspension to align the fiber and control the thickness of the material that accrued on the mold. Rheological study of alumina‐PVP suspensions with and without chopped carbon fiber showed that the suspensions exhibited a yield‐pseudoplastic flow behavior. The degree of alignment of the carbon fiber phase in the green bodies was characterized for various suspension formulations, carbon fiber contents and forming speeds. Stereological characterization of green body specimens confirmed the effectiveness of HDSC to attain the desired tubular geometry with considerable fiber alignment for a suspension composition containing ≤20 vol.% chopped fibers.     

超细氧化铝水悬浮液分散稳定性研究

阴离子聚电解质分散剂(PAA)使α-Al2O3粉体的等电点降低;超声分散作用改善了分散剂在粒子表面的分布状态,因而使浆料的流变性有明显的改善;离子强度低于0.08mol/L时,浆料的粘度随离子强度提高而增加,稳定性降低.但在0.08～0.12mol/L的范围内,离子强度对浆料粘度的影响不明显;温度对浆料的粘度有显著的影响.分散剂添加量低于饱和吸附添加量时,温度升高,浆料粘度增加,稳定性下降.而在分散剂添加量高于饱和吸附添加量时,温度升高,粘度则降低.     

Aqueous colloidal processing of PLZT ceramics near the morphotropic phase boundary

Abstract

Abstract

Single phase PLZT powder was fabricated using the mixed oxide approach, with a composition (6/60/40) lying on the rhombohedral-tetragonal (morphotropic) phase boundary. Aqueous suspensions showed an IEP of pH?9·7 in the absence of a polyelectrolyte surfactant. Inductively coupled plasma optical emission spectrometry analysis demonstrated considerable leaching of Pb²⁺ and La³⁺ ions under acidic conditions. The addition of an ammonium salt of poly(methacrylate), or PMA-NH₄, decreased the IEP to pH?5-6. Suspensions were readily stabilised at basic pH using low PMA-NH₄ concentration (<0·1?wt-% of solids). Concentrated suspensions exhibited shear thinning behaviour under low shear rates, with a transition to shear thickening at modest rates (i.e. 100?s^?1). Excessive shear thickening imposed a maximum solids loading of 46?vol.-% on concentrated suspensions, which related to the somewhat ‘flake-like’ particle morphology. Slip casting resulted in green and sintered (1250°C for 30?min) densities of 56±1% and 98·3±0·6% of theoretical, respectively.     

The influence of pH on particle packing in YSZ coatings electrophoretically deposited from a non-aqueous suspension

Yttria stabilized zirconia (YSZ) coatings were produced from a YSZ suspension in acetylacetone (ACAC) using electrophoretic deposition (EPD) and then sintered with substrate constraint at 1200 and 1300 °C. Before EPD, the operational pH of the suspension was adjusted by addition of acetic acid or triethanolamine (TEA) base. The effect of suspension pH on the deposition of EPD coatings was studied with respect to the suspension stability, coating density and microstructure. Results showed that the zeta potential had a high positive value on both sides of the iso-electric point (IEP). This probably resulted from the adsorption of TEA, detected by Fourier transform infrared spectroscopy. Three alkalies with different molecular structures were compared and the effect of their molecule length on the interparticle repulsion was discussed. Based on this, particle interactions were estimated for different pH suspensions. The reduced particle coagulation increased the packing density of the EPD coatings from 38% at pH 7.4 to 53% at pH 8.4. Therefore, subsequent sintering of coatings was promoted. The sinterability was evaluated by micro-hardness and microstructure. After sintering at 1200 °C, coatings made in pH 8.4 suspensions obtained a hardness of 786 MPa and had fewer big pores than coatings fabricated in pH 7.4 suspensions that had a hardness of 457 MPa.     

Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions

Stereolithography of UV-curable ceramic suspensions can benefit from the preparation of stable, low viscosity and high solid loading ceramic suspensions without yield stress. Appropriately adding dispersants could optimize the rheological behavior to meet the requirements of stereolithography. In this work, short-chain dicarboxylic acids were utilized to modify the alumina particles and achieve well dispersed ceramic suspensions. The maximum adsorption capacities of dicarboxylic acids were determined by the method of High Performance Liquid Chromatography and the mechanism of surface modification and dispersion was also discussed. Dicarboxylic acids’ influence on the rheology behavior was systematically studied. When doses of dicarboxylic acids reach their maximum adsorption capacities, the alumina suspensions would achieve their lowest viscosities and yield stresses. 45 vol% alumina suspension with a viscosity ˂2 Pa s at shear rate 30 s⁻¹ was successfully formulated. A sintering density of 96.5% can be achieved for the sebacic acid-modified alumina UV-curable suspension.     

Citric Acid—A Dispersant for Aqueous Alumina Suspensions

The interaction between citric acid and alumina in aqueous solution is characterized. Adsorption isotherms of the dispersant on the alumina surface, electrophoretic mobility of the alumina particles as a function of the citric acid concentration, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy of the citratealumina surface complex have been used. The adsorption behavior of citric acid is dependent on the pH of the suspension and the concentration of the citric acid. The maximum amount of citric acid adsorbed on the alumina surface, 2.17 μ.mol/m² at pH 3, decreases to 1.17 μmol/m² at pH 8. The adsorption of citrate causes a highly negatively charged powder surface and a shift of the isoelectric point (IEP) to lower pH values. The IEP of alumina can be fixed at any pH value between 9 and 3 by proper adjustment of the citric acid concentration. In situ ATR-FTIR spectroscopy of the citrate-alumina surface complex gives evidence for a direct interaction between the carboxylate groups of the citrate and the surface aluminum(III) atoms. The rheological properties of alumina suspensions are studied as a function of the citric acid concentration. The data obtained from the viscosity and dynamic electrophoretic measurements correlate well and allow the construction of a stability map of alumina suspensions stabilized with citric acid. The influence of citric acid on the viscosity is discussed using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The interaction potential between the particles is determined by the citrate adsorbed on the surface, leading to a negative particle charge, and the citrate anions remaining in the solution, resulting in an increase of the ionic strength. The adsorption of citric acid also creates a steric barrier that inhibits the complete mutual approach of the individual alumina particles.     

Generation of ceramic–ceramic layered composite microstructures using electrohydrodynamic co-axial flow

In this work different loading volumes (10 and 20 vol.%) of alumina and zirconia were suspended in selected liquid carriers to prepare suspensions. These suspensions were infused through inner and outer capillaries of a co-axial nozzle arrangement, which was then subjected to an electric field. The rate at which these suspensions were perfused (flow rate) and the applied voltage was varied during the experiments. Under optimal conditions, stable cone and jet formation was achieved for selected co-flowing suspensions which subsequently breaks-up into ceramic–ceramic composite droplets. Thus, droplets containing alumina and zirconia were produced and the resulting relics were collected on quartz substrates. These were sintered at 1200 °C and analysed by optical and scanning electron microscopy. The relic size increased as a function of the volume loading and layered encapsulation (shown using SEM and EDX) of alumina by zirconia and vice-versa, using this method, is achievable.     

PAA-NH_4对氧化铝悬浮液沉降性能的研究

采用PAA-NH4作为分散剂制备了超细氧化铝悬浮液,通过沉降实验观察了PAA-NH4的添加量、pH值对α-A12O3低固含量悬浮液的沉降性能的影响,并用Zeta电位仪测定了不同pH值条件下稀固相含量悬浮液的Zeta电位;同时对烧结坯体进行了微观分析.结果表明:分散剂PM-NH4的最佳用量为1.0%左右,最佳pH值为9;对添加PAA-NH4前后的稀悬浮液Zeta电位进行了测定,发现等电势点左移,Zeta电位的绝对值变化显著.添加PAA-Nh4后,pH值的变化对悬浮液的沉降性能和Zeta电位影响很大.