Elaboration of self-coating alumina-based porous ceramics

Materials used for bone substitution (i.e. hydroxyapatite and calcium phosphate) are highly successful, since when implanted they provide an efficient scaffold that can be colonized by the patient’s bone. However, their poor mechanical properties impede their use for load-bearing applications. In contrast, no material with high mechanical properties also presents a high bioactivity. A possible way of finding a material both strong and bioactive is to use a composite. We propose here a composite deriving its strength from its alumina core and its bioactivity from a calcium phosphate surface. Ceramic scaffolds have been produced by infiltration of polymer open-celled foams. Several compositions of the slurries have been tested, leading to the realization of porous pieces with a biocompatibility gradient at a micrometric scale. The mechanical properties of several new materials are presented and correlated to their microstructure.     

Effect of stress state and microstructural parameters on impact damage of alumina-based ceramics

Alumina discs of two grain sizes (4 and 24m), and three compositions (99.4% purity, 85% purity, alumina + partially stabilized zirconia) were subjected to planar normal impact in a gas gun at a nominal pressure of 4.6GPa. The alumina discs were confined in copper and aluminium capsules, which provided solely compressive and compressive plus tensile pulses in the ceramic, respectively. These experiments were conducted at different pulse durations (controlled by the thickness of the flyer plates). The surface area of cracks per unit volume was measured in order to estimate the impact damage. Compression followed by tension produced significantly more damage than compression alone. The small grain-sized discs exhibited more damage than the large grain-sized discs. The amount of damage increased with the duration of the tensile stress pulse. The addition of partially stabilized zirconia ( 14%) did not enhance the resistance to fragmentation of the discs; X-ray diffraction did not reveal an impact-induced phase transformation. Although the pressures generated were below the Hugoniot elastic limit of alumina, considerable fracturing of the specimens took place. Scanning electron microscopy revealed that the fracture was intercrystalline in regions away from the spall plane. In the spall plane energy was sufficient to comminute the grains, producing considerable grain debris and transgranular fracture. Transmission electron microscopy revealed the onset of damage to the structure, in the form of dislocations (present in only a small fraction of grains), microcracks nucleating at voids, and intergranular microcracks.     

Ti3SiC2陶瓷材料的制备及抗烧蚀行为

以固溶少量Al的Ti3SiC2粉体为原料,采用热压烧结工艺制备出致密度大于99％的Ti3SiC2陶瓷块体材料,其硬度、抗弯强度和断裂韧度分别为775HV,520.46 MPa和7.62 MPa·m1/2.对Ti3SiC2块体在无冷却条件下进行抗氧乙炔焰烧蚀实验,结果表明:烧蚀10 s内Ti3SiC2陶瓷保持表面平整,烧蚀25 s内样品未出现宏观裂纹.SEM和XRD观察分析表明,Ti3SiC2陶瓷在高温乙炔焰和氧气的高热流冲击作用下,表面发生分解和氧化,Si和C被氧化为Si-O化物和C-O化物气体逸出,Ti元素被氧化成高温稳定的TiO2金红石相覆盖在表面;氧化层呈3层结构分布,最外层为结构疏松的TiO2,次表层则为TiO2和Al2TiO5组成的致密复合层,内氧化层为致密Al2O3富集层,Al2O3来源于固溶在原料Ti3SiC2中Al元素的氧化,并在高温下与TiO2反应生成了Al2 TiO5.具有高黏度和高熔点的Al2O3富集层可以有效阻碍O2和热流向基体的扩散,从而降低基体的氧化速率,提高Ti3SiC2材料的抗烧蚀性能.     

Relationship between the ultrasonic velocity and the degree of sintering of alumina-based materials

The aim of this work was to investigate the application of the ultrasonic velocity in the evaluation and characterization of the porosity and density in alumina-based materials. To make this possible, ceramic bodies with 94.3%wt. in Al₂O₃ were uniaxially pressed and sintered at temperatures between 1100 and 1600 °C. In the ultrasonic tests, a piezoelectric transducer with 4 MHz was used. The obtained image was treated via software and an oscilloscope was used for standardization. The ultrasonic velocity was used as a parameter to indicate the correlation between the acoustic properties and the microstructure of the material.     

Sintering and characterization of Bi4Ti3O12 ceramics

Polycrystalline ferroelectric Bi₄Ti₃O₁₂ ceramics have been prepared by the method of reactive liquid phase sintering. The sintering behaviour of the Bi₂O₃-TiO₂ composite was examined by plotting the isothermal densification curves. The results indicate that the starting oxides are involved in the reaction even at temperatures lower than or equal to 800°C, but the reaction advances at a very slow rate. Above solidus, the liquid phase promotes an extended reaction. Saturation observed in two densification curves, at 875 and 1100°C demonstrate that the reaction proceeds by two steps. A completion of the Bi₄Ti₃O₁₂ formation occurs after 60 min of sintering at 1100°C. Optical micrographs of sintered bismuth titanate ceramics show randomly oriented ferroelectric grains separated by a paraelectric intergranular layer. The Bi₄Ti₃O₁₂ crystallites exhibit a platelike morphology, similar in the appearance to mica, as evidenced by scanning electron micrographs. Isothermal annealing (750 to 950°C) does not affect the microstructure and electric properties of sintered bismuth titanate. The considerable value of dielectric permittivity and the appearance of hysteresis have been correlated to the presence of oxygen vacancies within the pseudotetragonal structure of Bi₄Ti₃O₁₂. The oxygen vacancies are preferentially sited in the vicinity of bismuth ions as evidenced by X-ray photoemission data. XPS and AES measurements confirm that the surface concentration of cations comprising the Bi₄Ti₃O₁₂ ceramics does not deviate from the nominal bulk composition.     

The compressive property and brittle-to-ductile transition of Ti3SiC2 ceramics

Compressive tests of polycrystalline Ti₃SiC₂ were performed from room temperature to 1423 K at strain rates of 1×10^–4 s^–1 and 2.5×10^–5 s^–1, respectively. The effect of strain rates on high-temperature compressive property was also investigated. Polycrystalline Ti₃SiC₂ exhibited positive temperature dependence of flow stress (flow stress anomaly) and showed a temperature peak at 1173 K. The brittle-to-ductile transition temperature (BDTT) for polycrystalline Ti₃SiC₂ was strain-rate sensitive, an approximately 100 K decrease in transition temperature was associated with four times of magnitude decrease in strain rate. In addition, the fracture morphology changed from predominately intergranular to mostly transgranular. The mechanism responsible for the brittle-to-ductile transition in Ti₃SiC₂ was involved in the onset of a thermally activated deformation process. Received: 6 July 1999 / Reviewed and accepted: 9 August 1999     

Structural and electrical properties of Bi5Ti3FeO15 ceramics

Bi₅Ti₃FeO₁₅ (BTF) is a multiferroic material of Aurivillius structural family with (perovskite) layered structure. This material has special interest and position in the family because it is a combination of multiferroic BiFeO₃ and ferroelectric Bi₄Ti₃O₁₂, and can be used as new magneto-electric material for devices. The compound (Bi₅Ti₃FeO₁₅) was synthesized by a standard and widely used a high-temperature solid-state reaction method using high purity oxides. Preliminary structural analysis of the compound from the room temperature X-rays diffraction data confirmed the formation and good quality of the material. The nature of the microstructure (i.e., distribution, size and shape of grains, etc.) of sample recorded at room temperature using scanning electron microscopy exhibits formation of high-density sample. Studies of capacitive (permittivity and tangent loss) and resistive (impedance, electrical modulus and electrical conductivity) properties of the material as a function of frequency (1–1,000 kHz) at different temperatures (30–500 °C) using a complex impedance spectroscopy technique have provided many interesting and vital information on contribution of grains, grain boundary and interface in the material.     

Microstructure and mechanism of damage tolerance for Ti3SiC2 bulk ceramics

 Titanium silicon carbide (Ti₃SiC₂) is a damage tolerance material that is expected to be used in a number of high temperature applications. In this work, the microstructure and damage tolerance mechanism of Ti₃SiC₂ was investigated. The result demonstrated that the Ti₃SiC₂ ceramics prepared by the in-situ hot pressing/solid-liquid reaction process had a dual microstructure, i.e., large laminated grains were distributed within small equiaxial grains. This microstructure is analogous to that of platelets reinforced ceramic matrix composites. The bending test using single-edge-notched-beam specimens revealed that Ti₃SiC₂ was a damage tolerance material. The damage tolerance mechanisms for Ti₃SiC₂ are basal plane slip, grain buckling, crack deflection, crack branching, pull-out and delamination of the laminated grains. Received: 7 December 1998/Reviewed and accepted: 15 December 1998     

Dielectric response of Sb-doped CaCu3Ti4O12 ceramics

A series of Sb-doped CaCu₃Ti₄O₁₂ ceramics were fabricated by the conventional solid state method, and their crystalline structures, microstructures and dielectric properties were investigated systematically. All the ceramic samples exhibited perovskite-related structures in space group Im $\bar{3}$ . The grain size decreased slightly as Sb concentration increased; whereas the dielectric permittivity of the ceramics increased slightly. The giant dielectric response was considered to be closely related with a reduction in the potential barrier height at grain boundaries (GBs). The activation energy for the dc conduction process is comparable to that for conduction at GBs, indicating that the dc conduction process is associated with the electrical response of GBs.     

Fabrication and mechanical properties of fine-tungsten-dispersed alumina-based composites

The mechanical properties and microstructure of fine-tungsten-dispersed alumina-based composites, which were fabricated by hot pressing a mixture of fine α-Al2O3 and W powders, have been investigated. Small W particles of approximately 140 nm average size were located within the Al2O3 matrix grains. The mechanical properties were influenced by the metal content and sintering conditions. When the appropriate W content and sintering condition were selected (typically 5–10 vol% W and sintered at 1400°C), the fracture strength was enhanced compared with that of monolithic Al2O3. The metal content dependence of Young's modulus and the Vickers hardness did not obey the rule of mixtures. This may be attributed to the presence of localized residual stress caused by the incorporation of fine W dispersion into Al2O3. On the other hand, high-temperature (1600°C) sintering caused degradation in the properties of the composites due to the grain growth and chemical reaction of W dispersion, which was revealed by X-ray photoelectron spectroscopy analysis. The relations between fabrication condition and mechanical properties are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation and microwave dielectric properties of Bi2Ti4O11 ceramics

Single phase of Bi₂Ti₄O₁₁ ceramics, which belong to meta-stable phase compounds, were synthesized by controlling the reaction time through conventional solid-state method. The effects of annealing time on phase composition of Bi₂Ti₄O₁₁ ceramic powders and sintered ceramics were studied by XRD analysis. Second phase Bi₂Ti₂O₇ appeared when the annealing time shorter than 4 h. However, pure phase of Bi₂Ti₄O₁₁ powders can be formed by prolonging the annealing time to 6 h at 1,000 °C. The sintering temperatures on microstructure and microwave dielectric properties of Bi₂Ti₄O₁₁ ceramics were investigated. The results show that ceramics sintered at 1,075–1,175 °C are single phase of Bi₂Ti₄O₁₁ and present two different sizes of prismatic shape grains. Smaller size crystals grow into larger ones with increasing sintering temperature. The ceramics sintered at 1,125 °C reach a maximum density and have a microwave dielectric properties of ε_r = 51.2, Q × f = 3,050 GHz and τ_f = ?297 ppm/°C.     

Non-Ohmic and dielectric properties of Ba-doped CaCu3Ti4O12 ceramics

The microstructure, dielectric and electrical properties of Ca_1?x Ba _x Cu₃Ti₄O₁₂ (where x = 0, 0.025, and 0.05) ceramics were investigated. Our microstructural analyses revealed that Ba²⁺ doping ions preferentially form in a secondary phase, and are not introduced into the CaCu₃Ti₄O₁₂ lattice. Grain growth rate of CaCu₃Ti₄O₁₂ ceramics was significantly inhibited by the Ba-related secondary phase particles, resulting in a large decrease in their mean grain size. The dielectric permittivity of CaCu₃Ti₄O₁₂ ceramics decreased with increasing Ba content. Their loss tangent decreased after addition of CaCu₃Ti₄O₁₂ with 2.5 mol% of Ba²⁺, and increased with increasing Ba contents to 5.0 mol%. The nonlinear coefficient and breakdown field of the Ca_1?x Ba _x Cu₃Ti₄O₁₂ ceramics were significantly enhanced by adding 2.5 mol% of Ba²⁺, followed by a slight decrease as Ba²⁺ concentration was increased to 5.0 mol%. Using impedance spectroscopy analysis, it was revealed that variations in dielectric and non-Ohmic properties are associated with electrical response of grain boundaries. This supports the internal barrier layer capacitor model.     

Dielectric properties of bismuth doped CaCu3Ti4O12 ceramics

Ca_1?3x/2Bi_xCu₃Ti₄O₁₂ (x = 0.0–0.3) ceramics were prepared by the conventional solid-state reaction method. X-ray powder diffraction analysis confirmed the formation of cubic CCTO phase except for subtle peaks of CuO. SEM micrographs suggested that the morphologies of doped CCTO ceramics had been sheet-like for high Bi-doping amount, and the dominant grain size decreasing could be seen for the small content of Bi-doping CCTO. Dielectric properties of pure and doped CCTO were investigated in a broad temperature range of 20–420 K. The results showed that bismuth doping could decrease the dielectric loss but suppress the dielectric temperature stability at the same time. Bi doped CCTO ceramics presented different relaxation properties. As to pure CCTO and BCCTO (x = 0.3) only one MW relaxation (Relaxation I) could be found, which moves to higher frequency with temperature increasing. However, two relaxation processes (Relaxation I and II) appear for BCCTO (x = 0.1–0.2). By means of complex impedance spectra analysis and Arrhenius fitting, we successfully separated the different conductive segments and explained the mechanisms of the two relaxation processes. Relaxation I appeared at low temperature could be attributed to the V_O doping energies inside CCTO grains which did not showed significant changing of activation energy after bismuth doping. For Relaxation II at higher temperature than Relaxation I, with activation energy obviously depending on the Bi-ion concentration, may be related with the V_O point defects at the grain boundaries.     

CaCu3Ti4O12 ceramics from different methods: microstructure and dielectric

CaCu₃Ti₄O₁₂ (CCTO) ceramics were synthesized by methods of sol–gel, traditional solid-state reaction, and thermal decomposition of organic solution. The results exhibit that the microstructures and electric characteristics are affected by the methods of synthesis. The X-ray diffraction patterns show that all samples have a perovskite-like CCTO phase. Moreover, CCTO ceramic from traditional solid-state reaction have the phases of TiO₂ and CuO. The scanning electron microscopy images show that CCTO ceramics from different methods have different grain sizes, grain boundaries, and densities. Dielectric properties of the CCTO ceramics were characterized in a broad frequency range (10–10⁷ Hz) at room temperature. The CCTO ceramics from thermal decomposition of organic solution have the dielectric constant of more than 5 × 10⁴ at 10 Hz. The nonlinear relationship between the current density and the electric field strength can be observed in all the three samples.     

Elaboration and mechanical characterization of multi-phase alumina-based ultra-fine composites

Al₂O₃-10 vol.% YAG and Al₂O₃-10 vol.% ZrO₂ bi-phase composites as well as Al₂O₃-5 vol.% YAG-5 vol.% ZrO₂ tri-phase composite were developed by controlled surface modification of an alumina powder with inorganic precursors of the second phases. Green bodies were produced by dry pressing and slip casting and then sintered at 1500 °C. In particular, slip casting led to fully dense, defect-free, and highly homogenous samples, made of a fine dispersion of the second phases into the micronic alumina matrix, as observed by SEM. The mechanical characterization proved the predominant role of the final density on the Vickers hardness, while the elastic modulus was affected by the volume fraction of the constituent phases, in fairly good agreement with the rule of mixture prediction. The fracture toughness values of the bi- and tri-phase materials were similar, and their crack paths revealed the importance of the thermal residual stresses at the matrix-reinforcement interfaces, promoting inter-granular propagations.