Advances in Science and Technology Vol. 62

Abstract: Synthesis of dense materials with the compositions of Al2O3/Mo2N=100/0 ~ 40/60 vol% has been attempted directly from Al2O3/Mo mixed raw powder compacts using capsule-free N2 hot isostatic pressing (HIP). During HIPing [1500°C/(16~20)MPa]/1h], solid/gas reaction between Mo and N2 was introduced to form Mo2N. Most sintered composites consisting of only Al2O3 and Mo2N phases reached a higher relative density than 98.0% with closed pores nevertheless capsule-free HIPing. Distribution of Mo2N particles just formed suppressed the grain growth of Al2O3 during sintering. Mechanical properties, such as bending strength (Σb), Vickers hardness (HV), fracture toughness (K1C), and other properties have been evaluated as a function of their compositions. The best mechanical values of Σb (c.a. 573 MPa), HV (c.a. 20.3 GPa) and K1C (c.a. 5.00 MPa･m1/2) were attained at the composition of Al2O3/Mo2N=90/10 vol%, due to a high density (98.6%) and small grain size of Al2O3 matrix (Gs c.a. 4.70 μm). Further addition of Mo2N reduced the sinterability of matrix grains, resulting in low densities of around 90% at the 40/60 vol% composition.

Abstract: The aim of the work is the study of the pyroplasticity in ceramic materials in order to simulate the deformations of complex ceramic component during sintering. A ceramic material undergoing densification can be treated as a linear viscous material. Generally, the viscosity decreases as the temperature increases, however the densification and the consequent grain growth, result in a viscosity increase. A bending creep test is proposed for measuring the change in viscosity of the ceramic material during densification. Equations, based on beam deflection theory, are derived to determine the viscosity during the whole firing cycle by measuring the deflection in the centre of specimens. In addition, dilatometric analyses are performed to measure the sintering shrinkage and the specimen density, which continuously changes during the sintering process. On the basis of an accurate experimental characterization the parameters of Maxwell viscoelastic constitutive law are derived. A numerical-experimental procedure has been adopted in order to calibrate the numerical model that, finally, has been used to predict the pyroplastic deformations of complex ceramic components.

Abstract: This work presents measurements concerning sintering description according to two the: Kuczynski and Frenkel models. Investigation covered selected metal–like carbides: TiC0,8, NbC and WC. Kinetic equations were also developed, according to Kuczynski model to describe sintering of each carbide and it was attempted to determine mechanisms of mass transport mechanism dominating at each sintering stage. Parameters describing sintering process of each of investigated carbides: α and β constants, the initial shrinkage rate α-1, the limit of shrinkage at infinite time β-1 and initial apparent viscosity of system were in turn determined using Frenkel model. Such data allowed to achieve full characterization of carbide sintering.

Abstract: A high throughput combinatorial robot RAMSI (Rapid Automated Materials Synthesis Instrument) has been developed for nanoceramics synthesis. It can rapidly produce nanoparticle libraries that can be used for new materials discovery. The RAMSI robot has 3 motor-driven axes moving in X-Y-Z space and linking the synthesis, cleanup and printing sections. A High Throughput Continuous Hydrothermal (HiTCH) flow synthesis reactor is contained within RAMSI, and reaches significant production capability of 48 tubes of 50mL samples in 8 hours. Supported by the high throughput hardware design, the control system adopts parallel operations and multi-task assignments. Key strategies include intelligent supernatant detection, dot printing protocols and robot arm route schedule etc. The commissioning of RAMSI showed its high efficiency by producing samples of yttrium hydroxide doped with europium. In a 6-hour experiment, 8 unique 100mL samples were synthesized then cleaned and printed into 96 dots (100 minutes only for synthesis time). The ceramic dots were heat-treated at different temperatures up to 1200°C and affixed into a well-plate library. The brightest phosphors could be readily identified by illumination under UV light.

Abstract: Sintering in two-steps has been applied with success for densification of nanometric and submicrometer ceramic powders without grain growth. In this work the applicability of two-step sintering as a means of suppressing the grain growth of submicrometer alumina is verified. Experiments, in which the grain growth and densification were characterized after conventional sintering and two-step sintering, were conducted. Conventional sintering is used as a choice of the steps for two-step sintering. In the first two-step sintering studied, one hypothesis that the maximization of final density with minimum grain growth can be achieved by improving the narrowing of grain size distribution at a pre-densification sintering stage was assumed. And the other two-step sintering is based on works of Chen and Wang, in which the samples are first heated to a higher temperature to achieve an intermediate density, and then cooled down and held at a lower temperature until they are dense. The results showed that the choice of steps does not permit to suppress grain growth, but, the two-step sintering influenced in the development of the final microstructure, taking to microstructures which were finer grained than in alumina sintered conventionally.

Abstract: In tetragonal zirconia, possibility is investigated of densification with finer grain sizes under the combination of doping and sintering in air. The materials used are CIP'ed compacts of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP) doped with a small amount of cations. For a given sintering temperature and initial density of the compacts, while the doped cations enhances densification in the latest stage of sintering, the effect is different in grain growth during densification: a doped cation tended to enhance grain growth, whereas the other cations tended to suppress grain growth. As a result, the doping of the latter cations brings about a grain size finer than that of the undoped 3Y-TZP for a given relative density.

Abstract: Mullite ceramics have been known as the engineering materials due to their special properties such as high refractoriness, good thermal shock resistance, low thermal expansion, good strength at low and high temperatures, good creep resistance and low dielectric constant. In the present work, mullite samples were prepared by reaction sinteing of alumina and silica gel and subsequent sintering in a microwave process. The sintering of samples was carried out at temperatures in the range of 1400-1500°C with no soaking time. Density and porosity measurements showed a high density (89.3% TD) and low porosity (5.8%) for samples sintered at 1400°C under microwave conditions while for samples sintered conventionally at this temperature after 2h heating, density decreased (81.9% TD) and porosity increased (18.5%). By increasing sintering temperature to 1450 and 1500°C under microwave conditions, density increased (93.3 and 93.8% TD) and porosity decreased (0.31 and 0.36%), respectively. Microhardness measurements showed 27.6% increment when sintering temperature increased from 1400 to 1500°C. The mean grain size increased (34.8%) for samples sintered in furnace rather than those sintered in microwave process.

Abstract: Various fluorides (3 - 8 wt%) were added to a La9.33Si6O26 (LSO) powder synthesized by calcining the corresponding oxides mixture at 1100°C for 4 h. The addition of BaF2, AlF3 or Ba3Al2F12 caused an appreciable and substantial increase in bulk density after sintering at 1400º and 1450°C, respectively. These fluorides melt below 1400°C to form liquid phase which could assist the densification at low temperatures. Abnormal grain growth was observed for LSO samples with the addition of AlF3 and Ba3Al2F12, but it was effectively suppressed by stepwise sintering at 1400º and 1450°C. The BaF2 addition brought about the simultaneous promotion of densification and moderate grain growth, leading to the production of a densified LSO sample showing a conductivity of 1.5 x 10–2 Scm–1 at 800°C with an activation energy of 1.23 eV.