Materials Science Forum Vols. 587-588

Abstract: This paper studies the effects of porosity, sintering atmosphere and composition on the polarisation behaviour of calcium phosphate (CaP) ceramics. Hydroxyapatite (HA) and α- tricalcium phosphate (α-TCP) materials were prepared as both dense discs and as porous samples with interconnecting porosity. Materials were sintered in either air or water vapour at 1300°C and poled at a variety of temperatures, applied electric fields and time periods to optimise the polarisation properties of the materials. Thermally Stimulated Depolarisation Current (TSDC) measurements were used to investigate the degree of polarisation of the HA and α-TCP ceramics. A high depolarisation current was observed for both the porous HA and α-TCP ceramics. Sintering in water vapour also increased the depolarisation current due to reduced dehydration reactions, which result in a greater concentration of hydroxyl ions.

Abstract: The damage imposed on open-cell mullite ceramic foams was evaluated in pre-mixed radiant gas burners. After exposure to the prevailing combustion environment, foams suffered moderate strength degradation as a result of thermal stresses being imposed on the material during service. There was evidence of chemical attack during combustion although thermal shock measurements suggest that damage sustained by the foams results mainly from thermal shock rather than chemical degradation. Indeed, samples from burners subjected to ageing tests did not show additional damage compared to those subjected to short ageing tests indicating that most of damage occurred during start-up. For comparison purposes, a set of ceramic foam samples were subjected to a water quench test so that the extent to which the foams were damage by exposure to the combustion environment, under well controlled conditions, could be established. The strength retained after thermal shock by open-cell mullite foams decreased gradually with increasing quench temperatures. This suggests a cumulative damage mechanism reflecting an increase in damage throughout the material rather than sudden failure owing to propagation of pre-existing cracks along a plane. Damage in mullite foams was mainly localised at the top layer of the burners where higher temperatures and steeper thermal gradients were imposed on the material. Surprisingly, needle-like mullite crystals with a large aspect ratio were also found to have grown at the surface of the burners via a vapour feed gas-liquid catalyst-solid needle-like growth (VLS) mechanism.

Abstract: In this study, the influence of La-rich glass addition and sintering conditions on the densification and mechanical properties of 3 mol.%Y2O3-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO2 powder stabilized with 3 mol.% Y2O3 and La2O3-Rich glass were used as starting powders. Two compositions, ZrO2 containing 5 and 10 wt.% of a La2O3-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90°C for 24 hours, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200, 1300 and 1400°C for 60min, and at 1450°C for 120min, with heating and cooling rates of 10°C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The mechanical properties, hardness and fracture toughness, were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Samples sintered at 1300°C for 60 min presented the optimal mechanical properties with hardness of 1170 kgf.mm-2 and fracture toughness of 8.3 MPam1/2.

Abstract: CoPAN complex has been entrapped in the supercages of Y zeolite and the redox properties of this zeolite-encapsulated complex were investigated by cyclic voltammetry with a new method for the preparation of carbon toray-zeolite-modified electrode. Formation of the CoPAN complex was ascertained by surface analysis (SEM, XRD), chemical analysis (CA), spectroscopy methods (FTIR and UV/vis) and cyclic voltammetry in aqueous medium with zeolite-modified electrodes. The cyclic voltammetry studies obtained with a zeolite-modified electrode shows evidence for electroactivity restricted to boundary associated CoPAN complex.

Abstract: The polymorph phase stability ranges have been studied for the Bi4V2-xMexO11-y solid solutions with Me = Ga and Zr at room temperature. The formation of orthorhombic α- (x = 0.0 and 0.05) and β-phases (x = 0.1, 0.15) and tetragonal phase (0.2 ≤ x ≤ 0.3) has been revealed in BIGAVOX solid solutions. In BIZRVOX solid solutions, α-phase exists at x ≤ 0.05, while β-phase exists at 0.1 ≤ x ≤ 0.3. The second order phase transitions at ~ 308°C (BIGAVOX) and ~ 270°C (BIZRVOX) have been revealed for solid solutions with x = 0.05 using the SHG and DSC methods. In both systems, the β↔γ-phase transition temperatures have been found to decrease with increasing x.

Abstract: A tungsten carbide insert is produced during ductile iron foundry practice to increase the surface hardness of the material. The insert is ensured with the help of a critical quantity of a binder, around 3 weight percent of sodium silicate, previously mixed with the tungsten carbide powder. The binder is verified to form a film around the tungsten carbide particles and bridges amongst these enveloped particles; these films and bridges are replaced by the ductile iron melt during the molding cavity filling. After solidification, a sound inserted layer is formed, composed of tungsten carbide particles in a tungsten enriched cast iron matrix. In the absence of the binder addition to the tungsten carbide powder no inserted layer is produced. The surface hardness of the inserted test pieces is close to 500VHN/4.9N and the respective thickness is around 2.5 mm. The characterization of the effect of the sodium silicate in the mixture has been the main purpose of this work

Abstract: The structure and morphological aspects of highly porous (higher than 90%) cordierite (Mg2Al4Si5O18) foams, prepared by a direct foaming method, have been evaluated by Scanning Electron Microscopy analysis. The resulting ceramic foams consisted of a three-dimensional array of struts forming a well-defined open-cell structure. This type of structure seems very attractive for catalyst support purposes. Attempts have been made in order to control the pore structure since it directly affects the physical properties, namely the mechanical strength. In this respect, the use of a dip coating method to improve the strength of the resulting foams was found to be effective in reducing defects (e.g. pores, flaws) in the struts. Based on image analysis, estimated mean cell sizes were about 550 µm whereas strut thicknesses varied in the range of 60-70 µm. The compressive strength of the developed foams increased by one order of magnitude (0.1 to 1 MPa) by increasing the relative density from 0.06 to 0.18.

Abstract: The aim of the present work is to study the influence of the partial substitution of Si by Ge on the formation of the apatite-type La9.33Si2Ge4O26 phase by mechanical alloying and subsequent annealing. Powders of La2O3, GeO2 and SiO2 were dry milled in a planetary ball milling at increasing rotation speeds of 150, 250 and 350 rpm and milling times up to 50 h. The resulting mixtures were subsequently annealed at increasing temperatures up to 1100 °C. Single phase apatite-like La9.33Si2Ge4O26 was obtained during mechanical alloying at high rotation speed. The higher the rotation speed the lower was the time required for the lanthanum germanosilicate phase formation. For the samples in which complete reaction between initial phases did not occur during milling, La9.33Si2Ge4O26 was always obtained during the annealing process. The more severe was the mechanical alloying process the lower was the annealing temperature required for the apatite phase formation. The formation of apatite phase during mechanical alloying did not provoke significant changes in densification behavior of the milled samples. The addition of GeO2 as raw material promotes a faster formation of the apatite phase as compared to the results obtained using only La2O3 and SiO2.

Abstract: The success of a refractory castable is largely due to the quality of its properties and ease of application. Self-flow refractory castables (SFRC), with high flowability index (>130%), can be easily accommodated in a mould without the application of external energy, being ideal for the manufacture of monolithic linings. SFRC castables without cement require a matrix of very fine particles, which guarantees improved rheological behaviour and performs the role of the binder in the absence of the refractory cement. The presence of the aggregate (coarse particles) hinders the flowability index, but improves the castable mechanical strength and reduces firing shrinkage, and also contributes to the reduction of the castable costs. The control of the maximum paste thickness (MPT) allows the reduction of the coarse particles interference, minimizing the number of contact points among the grains and avoiding the formation of an aggregate skeleton that impairs the flowability of the mixture. In the present work, 100% alumina SFRCs without cement were produced with a fixed matrix of fine particles, whose particle size distribution was optimized using statistical techniques (mixtures design and triangular response surfaces). Different aggregate particle size distributions were used, with several MPT values, with the objective of evaluating which was the mean distance that maximized the flowability index, simultaneously ensuring good mechanical strength for the refractory castable. Ensuring a minimum surface area of 2.22m2/g, the mixtures reach the self-flow turning point with a minimum water content and the maximum flowability is obtained for an aggregate particle size distribution modulus of q=0.22, and consequently an optimized MPT value. SFRC with high mechanical strength (>60MPa) were obtained.

Abstract: This study compares the in vitro behaviour in SBF of glasses from two different systems, TiO2-CaO-P2O5 and SiO2-CaO-P2O5 with the same TiO2 and SiO2 molar content, in order to evaluate the effect of TiO2 and SiO2 on the surface reactivity of those glasses. The glass formation regions in both systems were observed for compositions with less than 40 mol% TiO2 and 40 mol% SiO2, respectively. Four glasses with similar TiO2 and SiO2 molar contents have been selected for comparison. These glasses are equimolar in CaO and P2O5 with TiO2 or SiO2 varying from 4 to 33 mol %. Powder glasses were immersed in Simulated Body Fluid (SBF) and kept at 37°C for different times, up to 14 days. Surfaces were observed by Scanning Electron Microscopy (SEM) and specimen ion leaching to SBF was studied by Inductively Coupled Plasma (ICP) spectroscopy. Preliminary spectroscopic studies by Raman were performed to identify the structure of the glasses. For glasses of the SiO2-CaO-P2O5 system a significant dissolution of all ions was observed together with the formation of phosphoric acid. In opposition, the immersion of TiO2-CaO-P2O5 glasses produced a small Ca consumption and stable Ti and P concentrations, indicating the formation of a Ca-P rich layer on these glasses. The observed differences in the dissolution behaviour are tentatively explained in terms of the glass structures obtained by spectroscopy.