Papers by Keyword: Yttria

Abstract: This research is focused on assessing the feasibility of the new and innovative microwave sintering technology for fabricating iron-chromium composites prepared via powder metallurgy route. Accordingly, the microwave sintered iron-chromium compacts was benchmarked against conventional sintered counterparts fabricated in other researches. We also studied the viability of yttria reinforcement to the iron-chromium composites with varying weight fraction from 5 to 20 %. Comparison on the end properties were also being made on the unreinforced iron-chromium matrix (0 wt. % of yttria). The result revealed that the microwave sintered iron-chromium composites possess improved density and micro hardness value. Process evaluation also revealed that microwave assisted sintering can lead to a reduction of 70 % of sintering time when compared to conventional sintering. The micro hardness property of microwave sintered iron-chromium was slightly improved with 5 wt. % addition of yttria, although the density and compressive strength were reduced with increasing content of the ceramic particulates. Most importantly, the study has established the viability of microwave sintering approach used in place of conventional sintering for iron based powder metallurgy composites.

Abstract: This research is focused on studying the density and mechanical properties of iron-chromium composites consolidated by innovative rapid microwave sintering technology against conventionally sintered counterparts using slow heating crucible furnace. Another aim of this study is to assess the viability of yttria (Y₂O₃) ceramic particulates as reinforcement to the iron-chromium composites. Fabrication of iron-chromium-yttria composites consolidated in microwave furnace and conventional crucible furnace was successfully accomplished. Improvement of density is evident in microwave sintered composites. The Y₂O₃ addition significantly increases the hardness of the composite (118 Hv for microwave specimens as opposed to 110Hv for conventional specimens). The study also successfully established the viability of microwave sintering technique for consolidating iron based powder metallurgy composites by up to 80% reduction of sintering time.

Abstract: Results of morphology and electrical properties evaluation of Y2O3 powders were presented in this article. The tests were made on the two different types of powders. First of them was a powder of the "microsize" type with the particle sizes not crossing10 µm, however the size of particles of the second powder was located in range 30 - 50 nm. Totally different was a morphology of both powders as well. In the first case the form of dense polyhedrons about the smooth surface was observed. However the powder of the type nano was characterized by spongy conglomeration of nano particles type of morphology. The range of investigations included the evaluation of the phase composition of powders (X-ray and electrons diffraction), characteristics of their morphology (SEM, STEM) and evaluation of electric properties by impedance spectroscopy. The obtained results showed that nano powders of oxide Y2O3 is different in the morphology of individual particles, but he is also better ionic conductor in the temperature of 20°C comparing to powder the micro type.

Abstract: The thermal groove technique has been used to measure relative grain boundary energies in two 100 ppm Ca-doped yttria samples. The first has a normal grain size distribution and the boundaries have a bilayer of segregated Ca. In the second sample, there is a combination of large grains and small grains. The boundaries around the large grains are known to have an intergranular film. The results show that the relative energies of boundaries in the sample with normal grain growth and the boundaries around small grains far from larger grains in the second sample are similar. Also, boundaries surrounding the largest grains and small grains immediately adjacent to them have the same and significantly lower energies. The results indicate that grain boundaries with an intergranular film have a lower energy than those with bilayer segregation and that the intergranular film extends beyond the periphery of the largest grains, but not throughout the entire sample.

Abstract: Defect formation in yttria with a small content of acceptor impurities in equilibrium with a hydrogen-containing gas phase is studied theoretically. A statistical-thermodynamic description of the yttriagas equilibrium is based on the approach developed for compounds with a complex electronic structure [Phys. Stat. Sol. B (1991) Vol. 168, p. 233]. The considered model of electronic structure for Y₂O₃ includes, besides valence and conduction bands, acceptor and F-center states. The energy of F-centers was calculated in the framework of the variational quantum-mechanical approach combined with the molecular statics method. It is shown that acceptor states appreciably affect the thermodynamics of defect formation, while the F-centers contribution in a wide range of external parameters is small. The concentrations of defects (protons, oxygen vacancies, electronic defects) and the Fermi level position are determined as functions of temperature and gas phase parameters.

Abstract: Splitting problems at HA-coated implants are generally due to biological reasons. Bond-coatings were used to prevent the splitting problem of zirconia ceramics; this method can be widely seen in industrial applications. Two main groups were used; the first group consisted of spraying a bond layer of titania onto commercially pure titanium. This followed by a spray of HA with 5, 10 and 15 % zirconia (8 % yttria doped) as main layer onto the first bond-coating. For the second group, the samples were coated without bond-coating. Firstly, X-ray diffraction patterns of the starting powders were taken. Then x-ray diffraction patterns of the plasma sprayed samples were taken. In literature, it was seen that 20 % zirconia was sufficient for the transformation into a monoclinic structure for the bond-coated samples. For this study it was found that 10 % zirconia was sufficient to transform to the same structure of the desired crystalline phase transformation. The coating kept its crystal structure and relatively small amount of amorphous transformation was detected. A similar structure was produced using less zirconia. It was thought that the use of titanium-oxide bond-coating layer would play an important role as a third variable in the results. To further investigate these phenomena, more detailed researches must be conducted with using titanium-oxide yittria stabilized zirconia (8 wt %) hydroxyapatite bond-coatings with HA main coatings.

Abstract: The yttria-erbia multilayered coatings were prepared on 316L stainless steel substrates by bipolar pulse magnetron sputtering. Structures of these coatings were examined by X-ray diffraction and scanning electron microscopy. Bonding strength of these coatings on 316L stainless steel substrates was measured by scratch tester. Nano-hardness of these coatings was measured by nano-indenter. Comparative studies of the mechanical properties of the single-layered yttria coatings and multilayered coatings were conducted, and the seven layered coatings presented the highest bonding strength and nano-hardness as well as elastic modulus.

Abstract: The precursor of yttria was prepared by yttrium nitrate and ammonium oxalate in water solution. Pyrolysis behavior of the precursor and the characterization of yttria phase transformation were studied through thermoanalysis. The grain growth kinetic behavior of yttria nanopowders was investigated by the X-ray powder diffraction (XRD) result of the powders obtained through roasting the precursor. Pattern of the yttria powders and agglomeration of its grain were measured by means of SEM and laser particle size analysis methods. The results suggested that cubic crystal phase yttria could be obtained by sample calcination for 1 h under 823 K. When the precursor was calcined under conditions of temperature ranging from 823 K to 1173 K, yttria grain ranging 7 nm ~ 16 nm could be obtained ,with grain growth exponent of n< 0.2 . In addition , an significant increase in grain-growth-rate constant was observed as the temperature went up. The higher temperature, the faster growth. Grain growth activation energy was different in low-temperature and high-temperature fields. The activation energy was 77.42 kJ•mol^-1 when the temperature was lower than 965 K, and became 524.36 kJ•mol^-1 when the temperature was higher than 965 K. Yttria powders is highly dispersed whose agglomerates sized about 400 nm after being dispered in deionezed water.

Abstract: In this study, nanostructured composite coatings of hydroxyapatite (HA)/ 30wt% yttria stabilized zirconia (YSZ) coatings containing 0, 3, 5, and 8 mol% Y2O3 (namely; HA-0YSZ, HA-3YSZ, HA-5YSZ, and HA-8YSZ) were successfully synthesized using the sol-gel method. The crystallite size of the coating was about ~44-58 nm for tetragonal and cubic zirconia grain size and 75-87 nm for hydroxyapatite grain size. Crack-free and homogeneous HA-YSZ composite coatings were obtained with no observable defects. The uniform distribution of zirconia particles in a composite would be highly beneficial for obtaining homogeneous coatings of HA-YSZ film and would hinder grain growth of HA phase during calcinations. In vitro evaluation in 0.9% NaCl showed that Ca2+ dissolution rate of composite coatings was lower than pure HA coatings.

Abstract: The effect of 1mol% calcia additive on sintering of yttria was investigated, and a yttria with elongated grains and micro-pores was developed in present work. The results showed that in calcia added samples heated at 1600°C, calcia contained yttria solid solution grains were formed and distributed among pure yttria grains. With the prolonging soaking time, they were concentrated into rod- or plate-like elongated grains and some of them were embedded into pure coarse yttria grains. Furthermore, in calcia added samples, grain growth was much faster and lots of micro-pores were left in coarse pure yttria grains. Water quench test revealed that thermal shock resistance of calcia added sample was greatly improved by elongated grains and micro-pores.