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Effect of calcium fluoride on sintering behaviour of SiO2-CaO-Na2O-MgO glass-ceramic system  [PDF]
Bahman Mirhadi,Behzad Mehdikhani
Processing and Application of Ceramics , 2012,
Abstract: The crystallization characteristics of glasses based on the SiO2-CaO-Na2O-MgO (SCNM) system containing calcium fluoride (CaF2) have been investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The partial replacement of CaO by CaF2 in the studied glass-ceramics led to the development of different crystalline phase assemblages, including wollastonite and diopside using various heat-treatment processes. With the increase of CaF2 content, the crystallization temperature of the glass and the strength of the crystallization peak temperature decreases. Addition of CaF2 up to 6.0 mol%, as expected, improved the sinterability. This sample reached to maximum density by sintering at 950 °C.
Analysis of isothermal sintering of zinc-titanate doped with MgO  [PDF]
Obradovi? N.,Stevanovi? S.,Mitri? M.,Nikoli? M.V.
Science of Sintering , 2007, DOI: 10.2298/sos0703241o
Abstract: The aim of this work was analysis of isothermal sintering of zinc titanate ceramics doped with MgO obtained by mechanical activation. Mixtures of ZnO, TiO2 and MgO (0, 1.25 and 2.5%) were mechanically activated 15 minutes in a planetary ball mill. The powders obtained were pressed under different pressures and the results were fitted with a phenomenological compacting equation. Isothermal sintering was performed in air for 120 minutes at four different temperatures. Structural characterization of ZnO-TiO2-MgO system after milling was performed at room temperature using XRPD measurements. DTA measurements showed different activation energies for pure and doped ZnO-TiO2 systems. Thus addition of MgO stabilizes the crystal structure of zinc titanate.
Effect of TiO2 on the Sintering Process of Magnesium Oxide from Seawater
Martinac, V.,Labor, M.,Jaki?, J.,Petric, N.
Kemija u Industriji , 2010,
Abstract: This study examines the effect of the addition of TiO2 on the properties of magnesium oxide obtained from seawater by substoichiometrical precipitation with 80 % of the stoichiometrically required quantity of dolomite lime as the precipitation agent. Magnesium oxide obtained from seawater by substoichiometrical precipitation contains boron(III) oxide as an impurity. During the precipitation of magnesium hydroxide, the boron present in seawater (in the form of non-dissociated orthoboric acid H3BO3, and partly as borate ion H2BO3 –) adsorbs onto the magnesium hydroxide precipitate, and therefore has to be reduced as much as possible using the appropriate method of rinsing the magnesium hydroxide precipitate. B2O3 is an undesirable admixture in seawater magnesium oxide. Specifically, the properties of special purpose magnesium oxide, i. e. high strength magnesium oxide, are affected significantly by the content of boron in the product. Good quality sintered magnesium oxide contains a very small amount ofB2O3. The impurity should not exceed w(B2O3) = 0.005 % in refractory magnesium oxide.The combined method of rinsing the magnesium hydroxide precipitate (3+2) was used in sample preparation, i. e. rinsing by decanting was done three times with alkalized distilled water of pH = 12.50 and twice with distilled water of pH = 6.74. Multiple rinsing on filter paper (five times) was done using the same rinsing agent (fresh distilled water of pH = 6.74). The MgO (80 % precipitation) samples were then subjected to isothermal and activated sintering without TiO2addition, as well as with the addition of TiO2, w(TiO2) =5 and 8 %, at the temperature of 1700 oC for 1 hour, with the aim of determining the properties of these samples primarily with regard to the B2O3content in the samples after sintering. The results indicate that the addition of TiO2strongly affects the reduction of the B2O3content during activated sintering of seawater magnesium oxide, and that the samples can be characterized as refractory material with very low B2O3 content. The addition of TiO2, w(TiO2)=5 %, is sufficient to bind the present CaO and thereby lead to greater evaporation of B2O3 in the process of activated sintering of seawater magnesium oxide. The samples thickened significantly during sintering, i. e. all the samples examined had a very smallamount of open pores.
Effects of Sintering Techniques on the Microstructure and Performance of High Purity Beryllium Oxide Ceramics
ZHONG Chao-Wei,LIANG Jian,ZHANG Shu-Ren,ZHANG Shao-Hua,LI B
无机材料学报 , 2010, DOI: 10.3724/sp.j.1077.2010.00415
Abstract: The effects of traditional, multistep and two-step sintering technique on the microstructure and performance of high purity beryllium oxide ceramics were investigated. It was found that the grain size of 99BeO ceramics could be controlled by the multistep sintering method, and the thermal conductivity could be elevated via two-step sintering method. Moreover, the technique parameters of the twostep sintering method, such as the highest sintering temperature (T1), the final sintering temperature (T2) and holding time (t),were studied in detail. The results show that under the condition of T1=1630℃, T2=1550℃ and t=4h, 99 BeO oxide ceramics owing fine grain and compact structure were obtained with the thermal conductivity of 308W/(m·K), and the density of 2.95g/cm3.
Sintering as a process of transport of activated volume  [PDF]
Nikoli? Nata?a S.,Nikoli? M.V.,Radi?-Risti? S.M.,Risti? Mom?ilo M.
Science of Sintering , 2002, DOI: 10.2298/sos0201053n
Abstract: Starting with the fact that sintering is the consequence of the process of transport of activated volume, it has been shown how the kinetics of the sintering process can be defined. The activated volume was in principle defined as a parameter which describes a system’s deffectivity on an atomic level.
Studies on Thermophysical Properties of CaO and MgO by -Ray Attenuation  [PDF]
A. S. Madhusudhan Rao,K. Narender
Journal of Thermodynamics , 2014, DOI: 10.1155/2014/123478
Abstract: The study on temperature dependent γ-ray attenuation and thermophysical properties of CaO and MgO has been carried out in the temperature range 300?K–1250?K using different energies of γ-beam, namely, Am (0.0595?MeV), Cs (0.66?MeV), and Co (1.173?MeV and 1.332?MeV) on γ-ray densitometer fabricated in our laboratory. The linear attenuation coefficients (μl) for the pellets of CaO and MgO as a function of temperature have been determined using γ-beam of different energies. The coefficients of temperature dependence of density have been reported. The variation of density and linear thermal expansion of CaO and MgO in the temperature range of 300?K–1250?K has been studied and compared with the results available in the literature. The temperature dependence of linear attenuation coefficients, density, and thermal expansion has been represented by second degree polynomial. Volume thermal expansion coefficients have been reported. 1. Introduction Density and thermal expansion are fundamental thermophysical properties of solids. The study of temperature dependence of these properties is very important in understanding the temperature variation of other properties like elastic constants, refractive indices, dielectric constants, thermal conductivity, diffusion coefficients, and other heat transfer dimensionless numbers. Thermal expansion of solids is of technical importance as it determines the thermal stability and thermal shock resistance of the material. In general the thermal expansion characteristics decide the choice of material for the construction of metrological instruments and the choice of container material in nuclear fuel technology. A number of methods have evolved for the determination of density and thermal expansion of solids at high temperature like Archimedean method [1–3], pycnometry [4–8], dilatometry [9–12], electromagnetic levitation [13], method of maximal pressure in gas bubble [14–18], method of sessile drop [19], hydrostatic weighing [20, 21], high temperature electrostatic levitation [22], and gamma ray densitometry [23–34]. Using γ-ray attenuation technique Drotning [23] measured thermal expansion of solid materials at high temperatures. He studied thermal expansion of aluminum and type 303 stainless steel at high temperatures and such studies have been extended by him to study the thermal expansion of metals and glasses in the condensed state [24]. The γ-radiation attenuation technique for the determination of thermophysical properties in the condensed state has several advantages over other methods at high temperatures. This is
天然高纯MgO-CaO材料的活化烧结  [PDF]
张治平,黄辉煌,李广平
金属学报 , 1986,
Abstract: 本工作在前文的基础上进一步研究整个MgO-CaO系列材料的活化烧结规律,着重探讨MgO/CaO之比对轻烧-水化试样烧结行为的影响和烧结试样的抗水化性。试验工作证明,天然高纯MgO-CaO系列的两相材料,用轻烧-水化方法,可以在1400℃烧后基本完成致密化过程,在1600℃烧后可得显微结构良好和高度直接结合的MgO-CaO熟料。二端员原料烧结时,晶粒异常长大,密度低,闭气孔率高。轻烧-水化试样的易烧结主要同CaO和MgO对OH-的吸附-脱附作用有关。
Densification Behaviour and Mechanical Properties of Aluminium Oxide and Cerium Oxide-Doped Yttria Tetragonal Zirconia Polycrystal Ceramics Using Two-Step Sintering  [PDF]
M. Golieskardi,M. Satgunam,D. Ragurajan
Journal of Nanoscience , 2014, DOI: 10.1155/2014/303579
Abstract: The densification behaviour, mechanical properties, and microstructure of high-purity Al2O3 and CeO2-doped Y-TZP with different weight percentage varied from 0.3 to 1?wt% were investigated. The samples were pressed uniaxially at 200?MPa into rectangular bars and discs and pressureless-sintered at temperature ranging between 1250°C and 1450°C for 2?h while the microstructure was characterized with a scanning electron microscope (SEM). Two-step sintering process works well for temperature higher than 1400°C and it created most tetragonal phase arrangement for stable structure to delay ageing through phase transformation. The mechanical properties in terms of bulk density, Young’s modulus, Vickers hardness, and fracture toughness were also measured. The results indicate that the addition of dopants accelerated the densification parameters and reinforced and toughened the obtained bodies. The maximum values for the mechanical properties of the Al2O3 and CeO2-doped Y-TZP ceramics were 6.01, 220?GPa, 13.8?GPa, and 7?MPa for density, Young’s modulus, Vickers hardness, and fracture toughness, respectively, which are higher than those of the doped samples. 1. Introduction Yttria-stabilised tetragonal zirconia polycrystalline ceramics (Y-TZP) are becoming popular engineering materials due to their excellent mechanical properties, studied and used in many engineering applications, such as engine parts, valves, cutting tools, and moulds, due to their good fracture toughness, high strength, elastic modulus, and wear resistance [1–3]. In recent years, yttria-tetragonal zirconia polycrystals (Y-TZP), with their superior combination of mechanical properties and chemical inertness, have been employed in the biomedical field as an implant material [4–9]. In general, when a restricted number of ZrO2 particles undergo the transformation during cooling from the sintering temperature, the accompanying volume expansion would cause the development of a fine distribution of microcracks in the ceramic matrix. These microcracks would increase the toughness by interacting with a propagating crack, causing deflection and blunting of the crack. Due to this nature, the engineering application of pure ZrO2 proves to be nonviable as the sintered body would crumble to pieces upon cooling from the sintering temperature [10, 11]. To overcome this undesirable phase transformation, stabilisers such as magnesia (MgO), calcia (CaO), ceria (CeO2), and yttria (Y2O3) have been added in various quantities in zirconia. In general, alloying zirconia with these oxides reduces the change of chemical
Correlation effects in MgO and CaO: Cohesive energies and lattice constants  [PDF]
Klaus Doll,Michael Dolg,Hermann Stoll
Physics , 1996, DOI: 10.1103/PhysRevB.54.13529
Abstract: A recently proposed computational scheme based on local increments has been applied to the calculation of correlation contributions to the cohesive energy of the CaO crystal. Using ab-initio quantum chemical methods for evaluating individual increments, we obtain 80% of the difference between the experimental and Hartree-Fock cohesive energies. Lattice constants corrected for correlation effects deviate by less than 1% from experimental values, in the case of MgO and CaO.
DISSOLUTION BEHAVIOR OF BIOACTIVE GLASS CERAMICS WITH DIFFERENT CaO/MgO RATIOS  [PDF]
MUHAMMAD USMAN HASHMI,SAQLAIN ABBAS SHAH,SHAHZAD ALAM,AHMAD SHAMIM
Ceramics-Silikáty , 2010,
Abstract: In this work, powders of three different compositions, each having 34 SiO2-14.5 P2O5-1 CaF2-0.5 MgF (% wt) and ratio of CaO/MgO varying from 11.5:1 to 1:11.5 were thoroughly mixed and melted under oxy-acetylene flame in a fire clay crucible that made the glass formation cheaper in time and cost. The melt of each composition was quenched in water to form three different glasses. Every glass was sintered at 950°C to form three glass ceramics named G1, G2 and G3 respectively. To study the dissolution behavior, each sample was immersed in a simulated body fluid (SBF) for 2, 5, 10, 20 and 25 days at room temperature. Thin film XRD analysis revealed that the samples with larger CaO/MgO ratio exhibited better bioactivity. pH of SBF increased efficiently in case of G1 whereas in case of G2 and G3, this increase was slower due to greater amount of MgO. The concentrations of Ca, P, Mg and Si ions were measured by Atomic Absorption Spectroscopy. EDS analysis showed the increase in P and Ca ions and presence of C in G1 after 5 days immersion and after 10 days, in case of G2 indicating the higher formation rate of hydroxycarbonate Apatite layer in G1 as compared to G2 due to greater CaO/MgO ratio whereas in G3 Mg-hydroxycarbonate apatite (Ca(Mg)5(CO3)(PO4)3(OH)) (heneuite) layer was recognized after 20 days showing the least bioactivity due to very large amount of Mg and the least CaO/MgO ratio.
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