Fabrication of porous alumina–zirconia ceramics by gel-casting and infiltration methods

Porous alumina–zirconia ceramics were obtained by infiltrating porous alumina ceramics, which were prepared by tert-butyl alcohol (TBA)-based gel-casting method. Back scattering images of the fracture surface and energy dispersive spectroscopy were performed to obtain composition profiles on the fracture surface and across sections of the sintered composites. The porosity, pore size distribution and compressive strength were also investigated. The results show that the content of zirconia can be adjusted effectively by infiltration times and it decreases with increasing distance from the surface of the samples. The porosity and compressive strength can also be controlled by the infiltration times. With increases of the infiltration times from 1 to 3 cycles, the open porosity decreases slightly from 62.43% to 56.62%, while the compressive strength of the porous alumina–zirconia ceramics increases from 13.57 ± 1.21 to 26.87 ± 2.01 MPa, indicating that the porous ceramics with high porosity and high strength can be prepared by TBA-based gel-casting method combined with the infiltration process.     

Thermal shock behaviour of paper‐derived alumina ceramics

Micro‐scaled alumina filled preceramic papers are used to produce paper‐derived alumina ceramics. The thin (0.6–0.7mm) and porous ceramics fabricated by this process can be, besides other possible applications, potentially utilized as setter plates on cordierite kiln furniture to avoid contact between the cordierite and powder metal substrate during sintering. The SiO₂ of the cordierite causes objectionable reactions with the powder metal. For this application it is important to investigate the thermal shock behaviour of the paper‐derived alumina ceramics especially focusing the residual strength, because fast cooling is a common technique in the metal sintering industry to avoid cost and save production time. Two differently processed types of paper were investigated: calendered (additionally roll‐pressed) and uncalendered paper. Their remaining strength has been measured by the B3B‐ test after thermal shocks of a temperature difference ΔT = 100 K up to ΔT = 1000 K to evaluate the critical temperature difference ΔT_C and the type of crack growth according to the method of Hasselman. In order to determine the cyclic thermal shock behaviour, at a temperature difference ΔT of 400 K and 600 K the ceramics have been quenched up to 5 times. The results of these investigations have been compared to the properties of tape casted alumina ceramics, a material already commercially used as setter plates. The initial strength of calendered paper‐derived ceramics was 240 MPa. After quenching at a temperature difference ΔT = 600–700 K, 50% strength decrease was observed. Temperature differences of more than 800 K caused >90% strength reduction. Uncalendered paper‐derived ceramics have 185 MPa strength. Between thermo shocks of temperature differences ΔT = 700–800 K, 50% of the strength reduction was measured. With thermo shocks of a temperature difference ΔT = 800 K only 10% of the initial strength remains. Generally the uncalendered ceramics showed a more stable crack growth than the calendered samples. Cyclic shocking at 400 K causes strength losses for both kinds of paper. But they differ in magnitude. While uncalendered paper only lost 7% strength, calendered paper strength was reduced by 20%, compared to their initial strength. This observation gets even more significant by shocking about 600 K. Calendered ceramics start with a higher initial strength and end up with a lower residual strength than the uncalendered ceramics. Only 30 MPa remain after five cycles of shocking, while the uncalendered paper‐derived ceramics have 90 MPa remaining strength. Uncalendered ceramics show a better thermal stress resistance, which can be correlated to their higher porosity and therefore their increased crack deflection in the microstructure. Tape‐casted alumina ceramics show a lower initial strength and lesser thermal shock resistance against simple and cyclic shocking. SEM pictures of the tape‐casted alumina ceramics show large amounts of small, spherical shape pores, while paper‐derived alumina microstructures show long cylindrical pores left by pyrolized cellulose fibres. Considering these results, paper‐derived ceramics are a serious alternative to tape casted ceramics for an application as setter plates.     

Zirconia toughened alumina ceramic foams for potential bone graft applications: fabrication,bioactivation, and cellular responses

Zirconia toughened alumina (ZTA) has been regarded as the next generation orthopedic graft material due to its excellent mechanical properties and biocompatibility. Porous ZTA ceramics with good interconnectivity can potentially be used as bone grafts for load-bearing applications. In this work, three-dimensional (3D) interconnected porous ZTA ceramics were fabricated using a direct foaming method with egg white protein as binder and foaming agent. The results showed that the porous ZTA ceramics possessed a bimodal pore size distribution. Their mechanical properties were comparable to those of cancellous bone. Due to the bio-inertness of alumina and zirconia ceramics, surface bioactivation of the ZTA foams was carried out in order to improve their bioactivity. A simple NaOH soaking method was employed to change the surface chemistry of ZTA through hydroxylation. Treated samples were tested by conducting osteoblast-like cell culture in vitro. Improvement on cells response was observed and the strength of porous ZTA has not been deteriorated after the NaOH treatment. The porous 'bioactivated' ZTA ceramics produced here could be potentially used as non-degradable bone grafts for load-bearing applications.     

叔丁醇基凝胶注模工艺制备轻质、高强莫来石多孔陶瓷

以叔丁醇为成型溶剂, 莫来石粉为起始原料, 采用凝胶注模成型方法制备出轻质、高强莫来石多孔陶瓷. 莫来石多孔陶瓷中的孔隙形成于干燥过程中叔丁醇的快速挥发, 孔隙分布均匀且相互连通. 随烧结温度升高, 气孔率、开气孔率和比表面积分别由77.8%、76.0%和10.39m²/g下降到67.6%、65.5%和4.26m²/g, 而抗压强度则由3.29MPa显著提高到32.36MPa, 材料孔径大小受烧结温度影响较小, 孔径尺寸呈单峰分布, 且几乎所有的气孔都为开口气孔, 透气度与孔径尺寸具有一致的变化关系. 莫来石多孔陶瓷在高气孔率条件下仍然保持高强度的主要原因是材料中均匀的孔隙结构、孔径尺寸小且相对集中、以及因烧结颈的形成在空间上所表现出的一种颗粒搭接骨架结构.     

A statistical approach to strength evaluation incorporating porosity effect for porous ceramics

ABSTRACT It has not been clear whether the conventional effective volume proposed for dense brittle materials can be applied satisfactorily to the strength evaluation of porous ceramics. In the present study, a modified effective volume was proposed by incorporating the porosity effect in the statistical evaluation of strength properties of porous ceramics. The modified effective volume was derived as the conventional effective volume multiplied by a function of porosity p. In this work, a power function of (1 + p)^a was adopted as the simplest porosity function. To clarify the applicability of the modified effective volume, bending tests were conducted using smooth and notched specimens of 3 wt% MgO partially stabilised zirconia with six different porosities. The porosity dependence appeared in the relation between the conventional effective volume and the mean strength of various zirconia ceramics with different porosities. The exponent a of the porosity function was determined from experimental data obtained by using identically shaped specimens with distinct porosities, and the modified effective volume was calculated for several types of specimens used in the experiments. It was revealed that the mean strength was almost uniquely correlated with the modified effective volume independent of porosity. The experimental correlation verified the applicability of the modified effective volume to strength evaluation of porous ceramics.     

Effect of YSZ fiber addition on microstructure and properties of porous YSZ ceramics

Yttria-stabilized zirconia (YSZ) fiber was introduced as the reinforcement for porous YSZ ceramics fabricated by tert-butyl alcohol-based gel-casting process and pressureless sintering. Effect of YSZ fiber addition on the microstructure and properties of porous YSZ ceramics was studied systematically. Results showed that YSZ fiber obviously obstructed densification during the sintering process and therefore higher porosity could be achieved with the same solid loading of the initial slurry. Mean pore size regularly increased with increasing fiber addition. The reinforcing effect reached its optimum with 10 wt% YSZ fiber addition, yet decreased with increasing porosity. Fiber addition significantly changed the fracture mode of the porous ceramics from brittle to quasi-ductile with increasing fiber additions; fiber pull-out and crack deflection play major roles in the process. Compared with the porous ceramics without fibers, the thermal conductivity decreased a little. With improved mechanical and thermal properties, YSZ fiber-reinforced porous YSZ ceramics are more applicable in thermal insulation materials.     

Effect of starting PMMA content on microstructure and properties of gel casting BN/Si3N4 ceramics with spherical-shaped pore structures

By gel casting with polymethylmethacrylate microbeads (PMMA) as pore-forming agent, porous boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were successfully prepared. The obtained ceramic shows bimodal hierarchical structures that composed of spherical-shaped micro pores depending on PMMA content and irregular sub-micro pores formed by the stacking of ceramic particles. Porosity of the porous BN/Si₃N₄ ceramics can be well controlled from 53.0 to 60.6 % by the PMMA content from 10 to 40 wt%, as well as the mechanical and dielectric properties. Effect of PMMA content on phase composition and the relationship between microstructure and the basic properties of the porous BN/Si₃N₄ ceramics was discussed in detail. Microstructure analysis reveals that the sub-micro pores acted as channels between micro pores. BN particles have a relatively denser distribution on the wall of spherical-shaped micro pores with a window between micro and sub-micro pores, and resulting in a half-closed micro pore structure, which is meaningful for material design with concentration of BN particles on the wall of pore structure.     

Effects of rheological properties on ice-templated porous hydroxyapatite ceramics

Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68–81% and compressive strength of 0.9–2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications.     

Porous ceramics mimicking nature—preparation and properties of microstructures with unidirectionally oriented pores

Abstract

Porous ceramics with unidirectionally oriented pores have been prepared by various methods such as anodic oxidation, templating using wood, unidirectional solidification, extrusion, etc. The templating method directly replicates the porous microstructure of wood to prepare porous ceramics, whereas the extrusion method mimics the microstructures of tracheids and xylems in trees. These two methods are therefore the main focus of this review as they provide good examples of the preparation of functional porous ceramics with properties replicating nature. The well-oriented cylindrical through-hole pores prepared by the extrusion method using fibers as the pore formers provide excellent permeability together with high mechanical strength. Examples of applications of these porous ceramics are given, including their excellent capillary lift of over 1 m height which could be used to counteract urban heat island phenomena, and other interesting properties arising from anisotropic unidirectional porous structures.     

水溶性环氧树脂对注凝成型Al2O3泡沫陶瓷结构和性能的影响

以异丁烯和马来酸酐共聚物(PIBM)为分散剂和凝胶剂, 同时添加两种表面活性剂, 通过机械发泡和注凝成型工艺制备高气孔率Al₂O₃泡沫陶瓷, 并研究了DE211环氧树脂对Al₂O₃泡沫陶瓷结构和性能的影响。结果表明, 该工艺能够制备气孔率达92.4%的Al₂O₃泡沫陶瓷。随着DE211环氧树脂的加入, 泡沫陶瓷的气孔率略有降低, 抗压强度由0.5 MPa提高到3 MPa, 平均气孔尺寸由582 μm下降到331 μm, 孔壁塌陷少。这归因于DE211环氧树脂的环氧基能够与PIBM的酸酐发生反应, 加快凝胶固化速度, 从而有利于稳定泡沫结构。     

从微孔洞起始的热冲击裂纹的数值研究——Ⅱ:能量释放率分析

利用商业的有限元软件ABAQUS,分析了具有微孔洞结构氧化铝陶瓷材料在受到热冲击作用下,从处于材料表面上的开口微孔洞以及临近材料表面的闭口微孔洞的底部起始的裂纹扩展。结果表明:对于从开口微孔洞和闭口微孔洞底部起始的裂纹,其能量释放率随冷却时间的变化规律类似,即随冷却时间t由零开始增加,能量释放率也由零开始快速增大,在冷却时间大约为t=0.1 s时,达到其最大值,随后开始降低。另外对于从开口微孔洞底部起始的裂纹,结果发现裂纹的能量释放率随裂纹长度的增加、微孔洞半径的增加而增加,而对于从闭口微孔洞底部起始的裂纹,其能量释放率随裂纹长度的增加、微孔洞半径的增加、以及微孔洞与材料受热冲击面距离的增加而增加。以开口微孔洞底部起始的裂纹为例,研究了裂纹与临近微孔洞之间的作用,发现临近微孔洞的存在可以明显减小裂纹的能量释放率。这些结果对于从材料的微结构角度设计材料的抗热冲击性能有参考价值。     

Effect of porosity on the crack pattern and residual strength of ceramics after quenching

The effect of porosity on the crack characteristics of ceramics after water-quenching is studied by measuring the cracks in ceramic sheets. The result reveals that the pore volume fraction has a slight effect on the enhancement of thermal shock resistance of ceramics when the porosity ranges from 0 to 20 %, because the length and density of the long crack in porous alumina are always slightly less than that in dense alumina. This result is in agreement with the prediction based on the minimum potential energy principle using experimentally measured data. Moreover, the proportion of the strength reduction in the third regime decreases significantly with increasing porosity, because the strength of unquenched specimens decreases more rapidly than that of quenched specimens with increasing porosity. The results of this study may help to further understand the thermal shock behavior of ceramics.     

含前驱体SiC粉体低压成型低温烧结SiC多孔陶瓷

采用聚碳硅烷和SiC粉体为原料低压成型低温烧结制备SiC多孔陶瓷,研究了聚碳硅烷含量对SiC多孔陶瓷性能的影响。SEM分析表明,聚碳硅烷裂解产物将SiC颗粒粘结起来,多孔陶瓷具有相互连通的开孔结构。烧成SiC多孔陶瓷的孔隙孔径为单峰分布、分布窄,室温至800℃之间多孔陶瓷的平均热膨胀系数为4.2×10-6 K-1。随着聚碳硅烷含量的增大,SiC多孔陶瓷的孔隙率降低、三点弯折强度增大,当聚碳硅烷质量分数为10%时分别为44.3%和31.7MPa。     

Camphene-based freeze-cast ZrO2 foam with high compressive strength

Highly porous zirconia ceramic bodies with interconnected pores were fabricated by freeze-casting technique using camphene-based slurries. The pore volume fraction and pore size are controllable by adjusting the initial solid content in the mixed slurries. The pores are replicas of connected dendrites of frozen camphene, which sublimed during room temperature drying. As the solid content was increased from 10 to 20 vol.%, the compressive strength was significantly increased from 19 ± 2 to 58 ± 3 MPa, and the examined porosity was decreased from 81.5 to 66.5%. This technique is considered potentially useful in producing novel porous ceramics, and introduces a new application field of freeze-casting.     

高强隔热刚玉-镁铝尖晶石-莫来石多孔陶瓷材料的制备

以氧化铝、石英粉和电熔镁砂为主要原料,以纸浆废液为结合剂,通过原位反应烧结制备复相高强隔热陶瓷,研究MgO添加量对所制备多孔陶瓷的显气孔率、抗折强度、耐压强度和抗热震性能的影响。采用X射线衍射(XRD)、扫描电子显微镜(SEM)和电子万能试验机对材料的物相组成、显微结构和力学性能进行表征,并对多孔陶瓷的显气孔率和抗热震性能进行测试。结果表明:5%(质量分数)电熔镁砂与氧化铝、石英粉在1450℃下原位反应烧结3h可制备得到刚玉-镁铝尖晶石-莫来石多孔复相陶瓷,耐压强度达270.25MPa,抗折强度超过45MPa,同时显气孔率达26.46%,常温导热系数为1.469W·m^-1·K^-1,隔热性能良好,且3次热震后的残余抗折强度保持率超过27%,是极具应用前景的工业窑炉内衬材料。其中MgO含量变化会直接影响该多孔陶瓷三相组成、相形态、气孔孔径及分布,使得多孔陶瓷抗折强度、耐压强度和抗热震性能呈现非单调变化的规律。     

Combined Microwave and Laser Heating for Glazing of 8Y–ZrO2 and 8Y–ZrO2/ZrSiO4–Composites

Sintered porous yttria‐stabilized zirconia and zirconia composite ceramics with zirconium silicate are surface glazed by Laser‐Assisted Microwave Plasma Processing (LAMPP). Suitable process parameters for surface glazing are determined for those ceramics. The plasma process is monitored by means of pyrometry and optical emission spectroscopy. In order to prove the quality of the surface glazing and to characterize hot corrosion resistance, tests with molten vanadium pentoxide are performed. After 4 h of exposure, the penetration depth of the molten salt is investigated as a function of ceramic composition and pre‐treatment by glazing. Upon hot corrosion testing of glazed and non‐glazed ceramics, the molten vanadium pentoxide reacts selectively with yttrium oxide, forming yttrium vanadate, and causes crack formation in the zirconia ceramics due to transition to monoclinic zirconia. The results for LAMPP‐glazed ceramics show, that a surficial melting phase is achieved because process temperatures exceed 3000 °C. Hence, a dense, crack‐free and hardness‐enhanced surface layer achieves a better resistance to hot corrosion as compared to non‐glazed ceramics. Due to LAMPP‐glazing, the vanadium ingress is reduced from 33 to 7 μm for yttria‐stabilized zirconia and from 104 to 17 μm for zirconia composite ceramic. Reactions and microstructural changes taking place upon LAMP‐Processing are discussed.

     

Permeability and high temperature strength of porous mullite-alumina ceramics for hot gas filtration

The gas permeability and mechanical properties of mullite-alumina ceramics for potential use as filters in hot gas separation environments are examined. The mullite-alumina ceramics with different levels of induced porosity and pores sizes were fabricated by slip casting and characterised in terms of microstructure and strength properties at ambient and elevated temperatures. Permeability to nitrogen gas flow of the porous structures at ambient temperature was investigated over a range of flow velocities to quantify and assess the permeability. The strength at high temperatures is equivalent to ambient data signifying no discernible degradation. Nitrogen gas permeability tests reveal dramatic reductions in the pressure drop–gas velocity curves with increasing porosity. It is shown that the gas permeability increases with the level of porosity and pore size, with maximum Darcian permeability constant of k = 2.5×10⁻¹⁴ m² for a porosity of 71%.     

粉料形貌对多孔Al2O3支撑体的影响

采用球形和非球形Al2O3粉料，探讨扩出成型中粉料颗粒形貌对多孔支撑体的性能的影响，用压汞仪，扫描电子显微镜（SEM）等于手段分析了1450和1500℃下4h烧成的试样，结果表明，规则球表Al2O3粉料可以按球形密堆积的形成支撑体，使气孔率变小，不利于制备高气孔率的支撑体，并且球表Al2O3粉料颗粒自身的多孔性使制得的试样出现孔径的双峰分布，要在较高烧成温度下可以消除，但该双峰分布结构对透气度没有影响，球形颗粒间的接触面积减少，不利不烧结和支撑体强度，但是，球形粉料形成的孔的形状规则，易于使支撑体整体均匀化。     

Preparation and some properties of porous alumina ceramics obtained by the gelatination of ammonium alginate

This study was carried out to develop a new preparation process of porous alumina ceramics and to examine their porous properties. The gelatination of ammonium alginate by hydrochloric acid was used for the formation of a gel-like precursor composed of alumina particles and alginic polymer. By this simple chemical shaping process of green bodies, porous alumina ceramics with a very sharp pore size distribution and a porosity of 40 to 55% could be obtained after firing at 1000 to 1600°C. The porosity showed an improvement of 15–20% compared with that of porous materials prepared from conventional procedures such as the pressing and slip-casting methods. The optimum concentrations of ammonium alginate and hydrochloric acid were 0.3–2.0 wt% and over 0.1 n, respectively, from the standpoint of the mixing treatment of raw materials and the gelatination.     

莰烯基冷冻浇注成型法制备定向通孔氧化铝陶瓷

由莰烯、氧化铝、Texaphor 963组成的陶瓷料浆,在温度梯度的诱导下,用冷冻浇注成型法制备了氧化铝陶瓷坯体,经干燥、烧结后,制备出了高孔隙率、高强度的定向通孔氧化铝陶瓷.通过扫描电镜观察其孔结构,并对其孔隙率、抗压强度和收缩率进行分析测试.结果表明:料浆的固体含量和烧结温度显著影响试样的气孔率、抗压强度和收缩率;...