The Effects of Solid Phase Additives on Sintering Properties of Alumina Bioceramic

In order to reduce the sintering temperature and improve the preparing conditions of alumina bioceramics,the Mg-Zr-Y composite solid phase additives were added into high purity Al 2O 3 micro-powder by chemical coprecipitation method.The powder was shaped under 200MPa cold isostatic pressure,and then the biscuits were sintered at 1600℃ under normal pressure.The sintered alumina materials were tested and the sintering mechanism was discussed.The results show that physical properties of the material were improved comparatively.The Mg-Zr-Y composite solid additives could promote the sintering of alumina bioceramics and the mechanism is solid phase sintering.

Particle Size Distribution,Powder Agglomerates and Their Effects on Sinterability of Ultrafine Alumina Powders

An intensive study of the particle size distribution of four commercial ultrafine alumina powders to obtain information about the powder agglomeration and relate them to the compactibility and the sinterability has been made.

Effect of Temperature on Phase and Alumina Extraction Efficiency of the Product from Sintering Coal Fly Ash with Ammonium Sulfate

A new developed technology for extracting alumina from coal fly ash was studied in this paper. In this technology, coal fly ash is first sintered with ammonium sulfate, forming ammonium aluminum sulfate in the resultant product, where alumina can be easily leached without using any strong acid or alkali. The products obtained under different sintering conditions were characterized by X-ray diffractometry. Alumina extraction efficiency of these products was also investigated. The results show that the sintering temperature and time substantially influence the phase composition and alumina extraction efficiency of sintered products, while the heating rate has little influence. The optimal sintering condition is 400 °C for 3 h in air with a heating rate of 6 °C·min-1.Under the optimal sintering condition, the alumina extraction efficiency from as-sintered coal fly ash can reach 85% or more.

Application Research of Dense Sintered Alumina in Iron Runner Castables

Three different kinds of corundum aggregates-tabular sintered alumina, dense sintered alumina, and fused dense corundum-were introduced into the silica fume .free or silica fume containing Al2O3 -SiC - C iron runner castables to investigate their influences on the flow ability, linear change on heating, bulk density, apparent porosity, cold strength, hot modulus of rupture, therm, al shock resistance, slag resistance, oxidation resistance as well as wear resistance of Al2O3 - SiC - C iron runner castables. The results show that （ 1 ） compared with the specimens with fused dense corundum, the specimens with dense sintered alumina have equivalent installation property, slag resistance and oxidation resistance, equivalent or even higher cold modulus of rupture, cold crushing strength and hot modulus of rupture, exhibiting better thermal shock resistance and cold wear resistance ; （2） adopting bimodal alumina micropowder LISAL22RABL as well as water reducers ZX2 and ZD2 can well reduce the water requirement of silica fume free castables, solving the problem of deteriorated flow ability resulted from the lack of silica fume; since the lack of silica fume avoids the formation of low melting point liquid, the hot modulus of rupture and the thermal shock resistance of the silica fume free castables are both better than those of the silica fume containing castables ; （3） the density of the castable specimens with dense sintered alumina is 4% -6% lower than that of the castable specimens with Jhsed dense corundum so the refractories consumption of one iron runner reduces by 5% by using the tastable with dense sintered alumina, which obviously reduces the cost of refractories.

LOW TEMPERATURE SINTERING OF ALUMINA BIOCERAMIC UNDER NORMAL PRESSURE

Supr fine alumina powder with high pu- rity (mean particle size isless than 0.35 μm) were used as main starting material for sinteringalumina ceramic. A multiple additive MgO-ZrO_2(Y_2O_3) washomogeneously added into the batch by the chemical coprecipitationmethod. Sintering of alumina bioceramic at low tempera- ture(<1600deg C) was achieved resulting in a dense and high strengthalumina ceramic with the bending strength up to 382 MPA and animproved fracture toughness. Mecha- nism that the multiple additivespromote the sintering of a- lumina ceramic is discussed on the baseof XRD and SEM analysis.

Porous Alumina Ceramics Fabricated by Spark Plasma Sintering

Porous alumina of regular spherical particles was fabricated with the spark plasma sintering （SPS） and then compared to those obtained through conventional hot pressing （HP）. The effects of the parameters of the heating process on porosity were also investigated, Microstructural studies suggest that porous ingots including regular pores be made out of regular spherical alumina particles due to the close sphere packages. A comparative study on the relative necks of the specimens produced by SPS and HP indicates an enhancement of neck growth with SPS. Contrasting the theoretical values to the experimental results over the relative necks indicates that a proper relationship between the relative necks and the porosity can be established by a sintering model.

Application Performance of Microporous Sintered Alumina in Alumina Magnesia Castables for Ladles

In this study,a kind of microporous sintered alumina was analyzed and applied.The microporous sintered alumina has a lot of nano and sub-micron closed pores in the crystals,the average pore size of 140 nm,and the thermal conductivity of 4.864 at 800℃,38%lower than that of common sintered alumina.The microporous sintered alumina was used as the aggregates in alumina magnesia castables for ladles to replace the common sintered alumina.The properties of the microporous alumina castable were measured,and the temperature field of the ladle outer wall was simulated and calculated.The results show that the refractoriness under load(T0.6)and the hot modulus of rupture(1400℃,0.5 h)of the microporous castable are the same as the common castable;the thermal conductivity at 800℃decreases by 15%;the retention ratio of modulus of rupture increases by about 5%after five thermal shock cycles(1100℃,water quenching);and the temperature of the ladle outer wall is reduced by 5-41℃.

Rapid Sintering of Alumina in Low Pressure RF Plasma

Three kinds of high-purity A12O3 powders, whose average paticle size is 0. l, 0.3 and 3.0 mp respectively, were used as thestarting power, and their compacts were fired in the radio-frequency plasma generated at a pressure of 80 Pa using N, as the working gas.Experimental results show that the 0. 1 and 0.3 mp powder compacts can be sintered to nearly the theoretical density within 60 s and thissintering is almost finished in the heating period. It is concluded that the mechanism of liquid sintering, the electric charge effect of Al2O3powder, and the effect of tempetrature gradient in the compacts can affect simultaneously on the rapid densification of the compacts inthe plasma sintering.

Mineralogy of Iron Ores of Different Alumina Levels from Singhbhum Belt and Their Implication on Sintering Process

To increase the life of the mines, ores of lower quality are blended with the higher quality ores. In general, the blending of ores at mines site is controlled by the chemical composition of the ores by controlling the Al2O3 and SiO2 levels on the lowest side possible. However, if there is a change in mineralogy, like an increase in hydrous phase the composition may still be the same, but this difference may affect the sinterability and sinter quality. The mineralogical investigation of iron ores and their chemistry are required to avoid the increase in deleterious elements during sintering. By the know-how of the behavior of individual iron ores in terms of their sinterability, in advance, the quality of the iron ores blend can be optimized to produce quality sinter and productivity. Characterisation studies conducted on iron ores of Singhbhum Orissa belt revealed that hematite, goethite, clay, gibbsite and quartz occur in all the iron ore samples but in variable proportions. Traces of alumina, present as solid solution in the iron oxide minerals, has also contributed Al2O3 to the ores.

RESEARCH ON MICROWAVE SINTERING ALUMINA CERAMICS

This paper introduces the characteristic of microwave sintering,introduces and analyses the process of microwave sintering of thealumina ceramics, moreover, compared with the experiments ofconventional sintering method. Based on the research, some resultsare obtained, and experimental basis for microwave sintering ofceramics is put forward.

Alkali desilicated coal fly ash as substitute of bauxite in lime-soda sintering process for aluminum production

By desilication treatment,the Al2O3/SiO2 molar ratio of coal fly ash could be improved to the range of 1.63-2.0.The desilicated coal fly ash(DSCFA)was enriched in alumina extraction.A processing technology was developed for alumina extraction from the DSCFA with the lime sintering process.Ca/(SiO2+TiO2)molar ratio,and NaO/Al2O3 molar ratio,sintering time,and temperature were the most significant parameters impacting on the aluminum extraction efficiency.The optima aluminum extraction efficiency was obtained under conditions of Ca/(SiO2+TiO2)molar ratio of 2.0,NaO/Al2O3 molar ratio of 0.98,and sintering at 1 200 ℃for 60 min.Astandard industrial dissolution method was used under conditions of caustic ratio(αk=n(NaO)/n(Al2O3)of 2.0,Al2O3 concentration of 50 g/L,sodium hydroxide concentration(Nk)of 60.78 g/L,Na2CO3 concentration of 10 g/L,temperature of 85℃, and dissolution duration of 10 min.The final aluminum extraction efficiency was 90%.

Investigation on Synthesis of Sintered Mullite from Low-medium Grade Kyanite

Sintered mullite is prepared by kyanite powder with Al_2O_3 content of 45% and 50%(Yinshan, Nanyang City, China),and the industrial alumina is added to control the chemical composition. The different series of sintered mullite according to the Al_2O_3 content(45%-70%) are designed. The raw materials are mixed and compacted, and then sintered at different temperatures. Physical properties, phase composition, microstructure and refractoriness under load of the synthesized samples are characterized by physical property test, XRD, SEM and RUL test, in order to confirm the quality of sintering and the relationship between temperature and mullitization reaction. The results show that the sintered mullite from low-medium grade kyanite has well developed mullite crystals and a continuous staggered network structure is formed, which has a significant contribution to increasing the high temperature performance of mullite.

Synthesis of Mullite from High-alumina Fly Ash:a Case from the Jungar Power Plant in Inner Mongolia, Northern China

In this paper, an experimental study was conducted in order to test the feasibility of sintering mullite directly from the high-alumina fly ash, without adding any extra material. The results show that the mullite contents in most sintered samples are over 70%. The samples sintered from the beneficiated fly ash have a higher content of mullite than those from the as-received fly ash under the same synthetic conditions. To obtain an equal amount of mullite, a higher sintering temperature is needed for the beneficiated fly ash than for the as-received fly ash. Considering the physical properties of sintered mullite, the favorable sintering temperature is 1400℃ for the as-received fly ash and 1500℃ for the beneficiated fly ash. A higher sintering temperature and a shorter holding time are profitable to sintering mullite. The orthogonal test confirmed that the dominant factor affecting mullite synthesis is sintering temperature, and that the most profitable matching conditions are 200 MPa-1500℃-3 h for the as-received fly ash and 200 MPa-1500 ℃-4 h for the beneficiated fly ash.

Mechanical wet-milling and subsequent consolidation of ultra-fine Al_2O_3-(ZrO_2+3%Y_2O_3) bioceramics by using high-frequency induction heat sintering

Alumina/zirconia composites were synthesized by wet-milling technique and rapid consolidation with high frequency induction heat sintering(HFIHS). The starting materials were a mixture of alumina micro-powder (80%, volume fraction) and 3YSZ nano-powders (20%). The mixtures were optimized for good sintering behaviors and mechanical properties. Nano-crystalline grains are obtained after 24 h milling. The nano-structured powder compacts are then processed to full density at different temperatures by HFIHS. Effects of temperature on the mechanical and microstructure properties were studied. Al2O3-3YSZ composites with higher mechanical properties and small grain size are successfully developed at relatively low temperatures through this technique.

Preparation and Properties of Sintered Corundum at Low Temperatures with Nano-η-Al2O3 Powder

In order to reduce the sintering temperature and improve the properties of sintered corundum,corundum specimens were prepared by granulation and sintering with nano-η-A l203 as the raw material and polyvinyl alcohol as the binder.The effects of different sintering temperatures(1550,1600,1650 and 1700℃)and holding time(2,4 and 6 h)on the properties and microstructures of the specimens were studied,and the transformation mechanism ofη-Al203 was analyzed.The results show that dense sintered corundum with bulk density of 3.74 g/cm^3 and apparent porosity of 1.77%is obtained by calcinating at 1650℃for 6 h;the phase transition fromη-Al203 toα-Al203 occurs first on the surface of alumina particles and then diffuses rapidly to the interior;at lower sintering temperatures there is no abnormal growth of crystals,the bonding between the grains is tight,transgranular fracture is the main fracture mode,there are fewer intergranular pores,and the grain size is in the range of 3.5-7.5μm.

Properties and corrosion behavior of Al based nanocomposite foams produced by the sintering-dissolution process

The properties orAl based nanocomposite metal foams and their corrosion behaviors were investigated in this study. For this, the composite metal foams with different relative densities （porosity） reinforced with alumina nanoparticles were prepared using a powder me- tallurgy-based sintering-dissolution process （SDP） and NaC1 particles were used as space holders. Then, the effect of nanoparticle rein- forcement and different amounts of NaC1 space holders （corresponding porosity） on the microstructure, morphology, density, hardness, and electrochemical specifications of the samples were investigated. It was found that as the relative density increased from 60% to 70%, the wall thickness increased from about 200 to 300 pro, which led to a decrease in pore size. Also, the addition of nanoparticle reinforcement and the increased relative density result in increasing metal foam hardness. Moreover, electrochemical test results indicated that increasing the A1203 content reduced the corrosion rate, but increasing the porosity enhanced it.

Relation Between Sintering Reactivity of Matrix and Thermal Shock Resistance of Ultra-low Cement Bonded Corundum-spinel Castables for Fired Purging Plugs

Purging plugs installed in the bottom of steel ladles are widely used for the secondary refining of high quality steel grades.The dynamic service conditions and temperature gradients caused by the cold inert gas blown through the plug during stirring create a strong thermal shock impact on the materials.This can affect its service life and restrict the safety and efficiency of steel making if the plug fails during use.In this work,the influence of the particle size distribution (PSD) and amount of reactive alumina on the sintering behavior of ultra-low cement bonded corundum-spinel based castables was investigated on lab scale.The relationship between sintering reactivity of matrix and thermal shock resistance of castables was evaluated in detail.Results show that the sintering of castables can be intensified by using finer reactive alumina.However,excessive sintering of the castable through finer reactive alumina is negative for thermal shock resistance.The microstructure characterization reveals that castables with more intense sintering show denser matrix structure,which is less effective in hampering crack propagation and therefore results in decline of their thermal shock resistance.

Low-temperature densification sintering and properties of CaAl_2Si_2O_8 ceramics with MeO·2B_2O_3(Me=Ca,Sr,Ba)

Dense CaAl2Si2O8 ceramics were prepared via a two-step sintering process at temperatures below 1000℃. First, pre-sintered CaAl2Si2O8 powders containing small amounts of other crystal phases were obtained by sintering a mixture of calcium hydroxide and kaolin powders at 950℃ for 6 h. Subsequently, the combination of the pre-sintered ceramic powders with MeO＇2B203 （Me = Ca, Sr, Ba） flux agents enabled the low-temperature densification sintering of the CaAl2Si2O8 ceramics at 950℃. The sintering behavior and phase formation of the CaAl2Si2O8 ceramics were investigated in terms of the addition of the three MeO·2B2O3 flux agents. Furthermore, alumina and quartz were introduced into the three flux agents to investigate the sintering behaviors, phase evolvements, microstructures, and physical properties of the resulting CaA12Si208 ceramics. The results showed that, because of their low-melting characteristics, the MeO·2B2O3 （Me = Ca, Sr, Ba） flux agents facilitated the formation of the CaAl2Si2O8 ceramics with a dense microstructure via liquid-phase sintering. The addition of alumina and quartz to the flux agents also strongly affected the microstructures, phase formation, and physical properties of the CaA12Si208 ceramics.

Preparation and Application of Bauxite-based Sintered Mullite Homogenized Grog in Ladle Castable

Preparation of bauxite-based sintered mullite homogenized grog was introduced and its application in ladle castable was studied.Two kinds of bauxite-based sintered mullite homogenized grogs were used,named as GA and GB,respectively.The results show that bauxitebased sintered homogenized grog with uniform components,stable microstructure and mullite as main crystal phase can be prepared using joint process of three-step homogenization,automatic component batching,wet cogrinding,high-temperature calcinations.It also indicates that carbon steel ladle castables with GA or composite aggregate of GA and GB as main materials have the same excellent general performance indicators,great erosion resistance,anti-strip performance and longer service life in field applications,and compared with castables with single aggregate of GA,the castables with aggregate of GA and GB as main materials had higher cost performance because of lower bulk density of GB.

Reaction Sintering of β-sialon/Al_2O_ 3/SiO_2 Mixtures in Flowing Nitrogen at High Temperatures

β-sialon/alumina/silica mixtures were sintered in flowing nitrogen at 1500℃ and 1600℃ respectively. The sintering properties and morphology of samples were investigated by SEM, EDS and XRD. Results show that sintered properties of the samples are influenced by soaking temperatures and compositions. Bulk densities of samples are increased with the increase of the soaking temperature, whereas the apparent porosities are decreased. Mass loss of the sintered samples at 1500℃ is lower than that at 1600℃, but all the mass loss remains approximately constant with the increase of sialon content. From the point of thermodynamic analysis, Complex chemical reactions take place in samples during sintering process and the volatile of SiO (g) is responsible for the mass loss of samples. Compared to cross section microstructure of β-sialon/Al_ 2O_ 3/SiO_ 2 mixtures sintered in coke powder bed consisting of the dense center with well-developed column of sialon phase, the transitional part with needle like grain nitrogen-containing phase and the loose part with short granular Al_2O_3 phase, those of mixtures sintered in flowing nitrogen is made of well-developed sialon in dense center and slightly loose part with sialon debris.