Papers by Keyword: Hydration Resistance

Abstract: Three-dimensional printing has been used as a rapid freeform fabrication process to fabricate a wider range of green ceramic components with complex structures difficult to obtain using traditional ceramic fabrication process. In this study, calcia-based ceramic core composites were fabricated by three dimensional printing and sintering operation. The green bodies were printed using a CaO/TiO₂ powder mixture as a precursor material and ethylene glycol as a binder. They were sintered at 1400-1500 °C for 2 h. The phases and microstructures of these samples were characterized by X-ray diffraction and scanning electron microscopy. The effect of TiO₂ content and the sintering temperature on the density, hydration resistance and bending strength of the sintered bodies was investigated. It was found that increment of TiO₂ content and sintering temperature would result in an increase of density of the sintered bodies and then increase of hydration resistance and bending strength.

Abstract: In order to improve the hydration resistance of MgO-Al-C bricks, Fe₂O₃ was chosen as an additive according to thermodynamic calculations. MgO-Al-C refractories were prepared by using Si, Fe₂O₃ or both as additives. The effects of additives on the hydration resistance of specimens were investigated. XRD was conducted to determine the phase composition. The results showed that the hydration resistance of MgO-Al-C bricks would be slightly bettered with addition of only Si. However, it was helpful when Fe₂O₃ was added along with Si. The mechanism is that Fe₂O₃ could decompose to give off oxygen during heating, which could promote the oxidation of Al powder and inhibit the formation of Al₄C₃ and AlN.

Abstract: To improve the hydration resistance of MgO-Al-C bricks, Na2CO3 was chosen as an additive through thermodynamic calculation. MgO-Al-C refractories were prepared by using Si, Na2CO3 or both as additives. The effects of additives on the hydration resistance of specimens were investigated. XRD was conducted to determine the phase composition. The results showed that the hydration resistance of MgO-Al-C bricks would be improved slightly with addition of only Si due to the densification of the structure. When Na2CO3 and Si were added together, the hydration resistance would be improved remarkably. The mechanism is that Na2CO3 could decompose to give off O2 during heating, which could promote the oxidation of Al metal and inhibit the formation of Al4C3 and AlN.

Abstract: Magnesia-magnesium aluminate (MA) spinel composite refractory was prepared by using fused magnesia, industrial alumina and AlCl₃•6H₂O as the raw materials. The effects of calcining temperature and holding time on phase compositions of MgO-MgAl₂O₄ system samples were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). With the increase of calcining temperature, the hydration mass increasing rate of the sample decreases significantly. The results show that MgO-MgAl₂O₄ composite refractory has quite excellent hydration resistance because of periclase grains with exsolved spinel phase.

Abstract: Effect of different blending amount of LaCrO₃, a composite additive, on the hydration resistance and crystallization properties of magnesia-calcium refractory were studied by measuring hydration rate, using X-ray diffraction and scanning electron microscopy. The results show that the LaCrO₃ at a low addition (≤1%) can significantly decrease the hydration resistance of the MgO-CaO material when the burning temperature is 1550°C, and the hydration rate is only 0.183%. LaCrO₃ can promote the growth of the CaO, while it has little effect on MgO. The Cr₂O₃ and CaO can melt into CaO·Cr₂O₃ in a high temperature. The CaO·Cr₂O₃ can not only promote sintering but also wrap on the CaO, which is negative for the grain to contact with water, and consequently improve the hydration resistance of magnesia-calcium refractory.

Abstract: To overcome the disadvantage of MgO-CaO refractories, as the poor hydration resistance, MgO coating on the MgO-CaO clinker was fabricated by carbothermal reduction MgO with carbon as reduction agent and then the oxidation of Mg vapor. Effect of processing parameters (such as carbon source, reaction temperature and holding time) on the hydration resistance of MgO-CaO clinkers were investigated by hydration resistance test and SEM. The results indicated that the hydration resistance of treated MgO-CaO clinkers was improved when carbon black was used as reduction agent due to easy reaction with MgO than the graphite and coke. The high reaction temperature among 1450~1600°C and prolonged holding time within 4 h resulted in improvement of the hydration resistance of treated MgO-CaO clinker. Deposition mechanism of MgO coating on the MgO-CaO clinker was discussed.

Abstract: For the hydration of magnesia-calcia refractory, this paper uses a new hydration-resistance technology. Namely using chemical methods, through surface and CO₂ occurring chemical reaction which generates CaCO₃ layer. The surface of magnesia-calcia firebricks are modified so that closed or stability, playing a role in hydration-resistance, and significantly improves the hydration-resistance. By SEM energy spectrum analysis and hydration resistance test, we obtained optimal technological parameters: reaction temperature is 700 °C for 4-5 hours, suitable water vapour temperature is 40°C, and the appropriate CO₂ flow is in the 4-4.5L•min^－1.

Abstract: CaO is a promising refractory for melting TiNi shape memory alloys, the carbon contamination can be avoid by using CaO crucible instead of carbon crucible. However, its industrial application is limited by the easy hydration of CaO in the air. In this study, the hydration resistance of CaO is modified by the addition of ZrO2, the powder of CaO adding 6.0 to 15.0mol% ZrO2 and 2.0mol% TiO2 is ball- milled, the mixture is shaped into the chip by the isostatic pressing method (IP), then the samples are sintered at 1750°C. The X-ray diffraction analysis reveals that the sintered samples consist of CaO dissolved with ZrO2 and CaZrO3. The anti-hydration property is described by the weight addition of samples, which are stored at the room temperature and atmosphere. It is shown that the sample with 12% ZrO2 of additive possesses the good hydration resistance, its weight addition stored after 56-days is less than 0.6wt%. Therefore, the powder of CaO with 12.0% ZrO2 additive are used to prepare the crucible by IP, the green crucible is sintered at 1750°C. This home-made crucible is used for melting TiNi alloy by means of the induction method at 1500~1800°C temperature and the vacuum environment, it is found that, the internal and external surface of crucible were almost kept unchanged after melting, this refractory did not obviously react with titanium melts, this result may provide a support for searching a new refractory with the good hydration resistance for melting of titanium alloys.

Abstract: Hydration resistance of three commercially available high-purity MgO-CaO clinkers were investigated and compared in the present paper. The results show that, as well as CaO content, microstructural morphology of MgO–CaO clinker affects its hydration resistance. The microstructure in which CaO is present at the grain boundaries of MgO leads to poorer hydration resistance, whereas the one in which CaO and MgO are distributed alternately confers much improved hydration resistance on the clinker, especially when MgO level is high enough to cover most of the CaO crystals.

Abstract: Zr2Al3C5 has been successfully synthesized via solid state reaction between Al, ZrC and carbon powder at 1600
in vacuum. This complex carbide has very strong bond between metal atoms and carbon atoms. Thus, this material has a potential to be utilized as structural materials. Some properties of Zr2Al3C5 powder from solid-state reaction in vacuum had been tested. It was found that this powder was completely oxidized in air at 900
1 h, and can be hydrated in moist air. These drawbacks might come from the high reactivity of the powder due to synthesis in vacuum. Zr2Al3C5 powder from solid state reaction in vacuum was sintered at various temperatures from 1500
to 2000
under vacuum with pulse electric current sintering (PECS) and pressureless sintering. Zr2Al3C5 started to sinter at 1500
and got partially dense from 1700
. Physical properties and mechanical properties of this material were investigated and discussed.