Highest UV–vis transparency of MgAl2O4 spinel ceramics prepared by hot pressing with LiF

Reaction sintering of MgO and Al₂O₃ with addition of LiF as sintering additive was used to prepare MgAl₂O₄ spinel ceramic by hot pressing. The process parameter (temperature, pressure, dwell time), the stoichiometric ratio of MgO to Al₂O₃ and the selection of the alumina raw powder are equally important for highest transparency of the spinel ceramic. With this optimization highest transparency of 86% in the visible range at λ = 640 nm together with UV transmission of 62% at 200 nm for spinel ceramic with 4 mm thickness was reached.     

Effects of LiF on the microstructure and optical properties of hot-pressed MgAl2O4 ceramics

Transparent magnesium aluminate spinel (MgAl₂O₄) ceramics were fabricated by hot-pressing of the MgO and α-Al₂O₃ powder mixture using LiF as a sintering aid. Effects of the LiF additive on densification, microstructure and optical properties of MgAl₂O₄ ceramics were systematically investigated. It has been found that the addition of LiF can effectively remove the porosity and increase the optical transparency of MgAl₂O₄ ceramics. For the spinel ceramics HP-ed at 1550 °C for 3 h with 1 wt% LiF addition, the average grain size is about 36 µm and the in-line transmittance exceeds 60% at the wavelength of 800 nm.     

Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties

It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry (n = 1) the sintering temperatures may differ by as much as 200 °C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n = 1 and n = 3 with retarded reactive sintering at moderately increased Al₂O₃ concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al₂O₃ mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200 nm wave length up to the IR range.     

Kinetic analysis of magnesium aluminate spinel formation: Effect of MgO:Al2O3 ratio and titania dopant

The mechanistic pathway of MgO-Al₂O₃ reaction in solid state to form MgAl₂O₄ spinel was investigated to correlate the kinetic parameters with ratio of reactants (MgO:Al₂O₃) and with the presence of a doping agent, TiO₂. The time-temperature-expansion data of oxide compacts was analyzed using several model free analyses and model based (linear and non-linear) kinetic algorithms. These indicated that spinel formation process can be best described by single step with n-dimensional Avrami equation for every MgO:Al₂O₃ ratio, irrespective of titania dopant. The activation energy (E_a) of the process was proportional to % spinel formed in each system and validated with quantitative XRD analysis. The higher value of Avrami coefficient (n) in 90 wt% Al₂O₃ compositions has been explained with geometric considerations of powder packing. Incorporations of 1% TiO₂ in the MgO: Al₂O₃ oxide compact did not markedly affect the reaction model, frequency factor and Activation energy.     

Dilatometric sintering study of fine-grained ceramics from ultradispersed admixture composed of Ce0.09Zr0.91O2 and MgO–Al2O3

The experiment was carried out to produce fine-grained ceramics with a grain size of less than 5 μm. Ultradispersed oxide mixture MgO–Al₂O₃ (weight ratio MgO/Al₂O₃ value was 3/97) and solid solution Ce_0.09Zr_0.91O_2?δ were used as initial nanopowders with a crystallite size less than 10 nm. Dilatometric investigation was carried out at the temperature interval 1100–1550 °C using three temperature modes, included various heating and cooling rates and isothermal plots. Initial metal oxide powders were obtained by modified sol–gel technique using N-containing organic compounds for sol stabilization. It was shown that the role of MgO in nanopowdery composition for sintering is to accelerate the sintering due to the formation of the liquid phase with spinel MgAl₆O₁₀. It was determined, that the presence of interim isotherms on the temperature rise curves does not impact the rate and quality of sintering.     

Degradation mechanisms of magnesia-carbon refractories by high-alumina stainless steel slags under vacuum

The corrosion behaviour of a pitch-bonded magnesia-carbon refractory by an Al₂O₃ rich (∼15 wt.%) stainless steelmaking slag was investigated by rotating finger tests in a vacuum induction furnace at high temperature (>1650 °C) and low oxygen partial pressure (1.5–4.3 × 10⁻¹⁰ atm). This study confirms the poor slagline behaviour of MgO-C bricks industrially observed in VOD ladles. Higher temperatures and longer exposure times lead to more severe slag infiltration and direct MgO dissolution. The intrinsic MgO-C reaction is the major decarburisation mechanism, while extrinsic decarburisation by oxygen from the atmosphere and/or reducible slag components (CrO_x, FeO_x) was limited. Three kinds of metallic particles with different size, shape, location, composition and origin were observed in the refractory specimens. Concurrently, the thermodynamic conditions for the formation of a protective Mg(Al,Cr)₂O₄ spinel layer at the slag/refractory interface are investigated. The industrial relevance of this spinel layer formation is discussed with respect to the chosen Al₂O₃ level. Guidelines are proposed to minimise MgO refractory dissolution in VOD slaglines.     

Pressureless sintering of carbon nanotube–Al2O3 composites

Alumina ceramics reinforced with 1, 3, or 5 vol.% multi-walled carbon nanotubes (CNTs) were densified by pressureless sintering. Commercial CNTs were purified by acid treatment and then dispersed in water at pH 12. The dispersed CNTs were mixed with Al₂O₃ powder, which was also dispersed in water at pH 12. The mixture was freeze dried to prevent segregation by differential sedimentation during solvent evaporation. Cylindrical pellets were formed by uniaxial pressing and then densified by heating in flowing argon. The resulting pellets had relative densities as high as ～99% after sintering at 1500 °C for 2 h. Higher temperatures or longer times resulted in lower densities and weight loss due to degradation of the CNTs by reaction with the Al₂O₃. A CNT/Al₂O₃ composite containing 1 vol.% CNT had a higher flexure strength (～540 MPa) than pure Al₂O₃ densified under similar conditions (～400 MPa). Improved fracture toughness of CNT–Al₂O₃ composites was attributed to CNT pullout. This study has shown, for the first time, that CNT/Al₂O₃ composites can be densified by pressureless sintering without damage to the CNTs.     

Sintering behaviour of a ZnO waste powder obtained as by-product from brass smelting

The effect of two sintering methods (conventional sintering and two-step sintering) and of alumina addition on the sintering behaviour of a ZnO-rich waste powder (ZnO > 95 wt%), a by-product from brass smelting industry, was studied aiming to improve the sintered density and grain size. Both conventional sintering and two-step sintering methods did not lead to fully dense powder compacts, as densification was conditioned by abnormal grain growth and the particle size of the ZnO-rich residue. When two-step sintering was used the grain growth was reduced comparatively to conventional sintering method. The highest relative sintered density (about 90%) was achieved when samples of ZnO waste and samples of ZnO waste with 2 wt% added Al₂O₃ were processed by two-step sintering and corresponded to a mean grain size of around 18 µm and 7 µm, respectively. XRD and SEM results indicated that alumina addition helped to inhibit grain growth due to the formation of gahnite spinel (ZnAl₂O₄) precipitates in the grain boundaries of zincite (ZnO) grains.     

Influence of post-HIP temperature on microstructural and optical properties of pure MgAl2O4 spinel: From opaque to transparent ceramics

Transparent MgAl₂O₄ spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500 °C during 2 h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl₂O₄ ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800 °C for 10 h with an in-line transmission of 81% at 400 nm and 86% from 950 to 3000 nm for a thickness of 2 mm (98.8% of the theoretical transmission).     

A comparative study on the effect of different additives on the formation and densification of magnesium aluminate spinel

Commercially available fused magnesia and sintered alumina sources were used to form reaction sintered spinel by solid oxide reaction route in a single firing. The effect of the addition of four different additives, namely MgCl₂, LiF, AlCl_3, and MnO₂, at 2 wt% level was studied. Mixed oxide compositions were compacted under a uniaxial pressure of 150 MPa and then sintered between 1200 and 1600 °C. The dilatometric study and phase analysis was done to observe the spinel formation reaction. Densification study of the sintered product was done to understand the effect of additives. Cold Crushing strength and thermal shock resistance of 1600° sintered pellets were studied. Microstructural study using field emission scattered electron microscopy (FESEM) was also done to understand the grain development on sintering in the compositions and the effect of different additives on sintering. LiF and MgCl₂ were found to strongly enhance the spinel formation reaction. Bulk density values were found to be lower for the additive containing batches at 1200 °C due to enhanced spinel formation but higher at 1600 °C due to greater sintering. Strength values were strongly enhanced by LiF and MnO₂ due to the development of dense, compact microstructure. Also, additives containing compositions showed much higher strength retainment even after 6 cycles of thermal shock.     

Crystallisation of amorphous spray-dried precursors in the Al2O3–SiO2 system

The crystallisation of amorphous precursors has been studied in the whole range of composition in the Al₂O₃–SiO₂ system. The amorphous precursors have been obtained by hydrolysing TEOS directly in a diluted aqueous solution of aluminium nitrate, spray drying the clear solution and heating the resulting powder. Up to 70 mol % Al₂O₃, only mullite crystallises around 980–1000 °C; between 70 and 80 mol % Al₂O₃ mullite and spinel crystallise together; and for more than 80 mol % Al₂O₃ only spinel is formed. In the 70–80 mol % Al₂O₃ range of composition, when both mullite and spinel crystallise, low heating favours the crystallisation of mullite and it is nearly possible to crystallise only mullite from a 75 mol % Al₂O₃ sample. By rapid heating it is also possible to crystallise only spinel from the same 75 mol % Al₂O₃ precursor. The enthalpy and the activation energy for crystallisation are maximum for 60–80 mol % Al₂O₃. Heating the samples up to 1700 °C for 1 h, the phase equilibrium is not reached, particularly when both mullite and spinel crystallise together, and θ-Al₂O₃ is still present.     

On the effects of LiF on the synthesis and reactive sintering of gahnite (ZnAl2O4)

The effects of LiF on the synthesis and reactive sintering of polycrystalline gahnite (zinc aluminate spinel, ZnAl₂O₄) were studied using XRD, high-temperature simultaneous thermal analysis and a spark plasma sintering (SPS) apparatus. It was demonstrated that the LiF reduces the onset of synthesis by about 200 °C and plays an important role in the densification process. SPS consolidation of a LiF-doped ZnO-Al₂O₃ mixture under an applied pressure of 150 MPa and at a sintering temperature of 1100 °C for 20 min generated fully dense gahnite with adequate transparency and mechanical properties.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Al2O3–SiC composites prepared by warm pressing and sintering of an organosilicon polymer-coated alumina powder

Al₂O₃/SiC micro/nano composites were prepared by axial pressing of poly(allyl)carbosilane-coated submicrometre alumina powder at elevated temperature (called also warm pressing, or plastic forming) with subsequent pressureless sintering in the temperature interval between 1700 and 1850 °C. Warm pressing at 350 °C and 50 MPa resulted in green bodies with high mechanical strength and with markedly higher density than in green bodies prepared by cold isostatic pressing of the same powder at 1000 MPa. The sintering of warm pressed specimens moreover yielded the composites with higher final density (less than 4% of residual porosity) with the microstructure composed of micrometer-sized alumina grains (D₅₀ < 2 μm) with inter- and intragranular SiC precipitates. High sintering temperatures (>1800 °C) promoted the formation of intergranular platelets identified by TEM as 6H polytype of α-SiC. The maximum hardness (19.4 ± 0.5 GPa) and fracture toughness (4.8 ± 0.1 MPa m^1/2) were achieved in the composites containing 8 vol.% of SiC, and sintered for 3 h at 1850 °C. These values are within the limits reported for nanocomposites Al₂O₃/SiC by other authors and do not represent any significant improvement in comparison to monolithic alumina.     

Pressureless sinterability of slip cast silicon nitride bodies prepared from coprecipitation-coated powders

The sinterability of compositions from different powder preparation methods (coprecipitation-coating of Si₃N₄ powder or mechanical mixing of Si₃N₄ with Y₂O₃ and Al₂O₃) and compaction routes (dry pressing or slip casting) was compared. Both the coating method and the slip casting process improved silicon nitride sinterability over the mechanical mixing method and dry pressing route. However, the minimisation of powder agglomeration in the green bodies achieved by slip casting is more determinant to the sintering behaviour than the homogeneous distribution of the sintering additives around the Si₃N₄ offered by the coated powder. The coating powder method in combination with the slip casting process is the most favourable processing route, leading to a homogeneous and fully dense microstructure by pressureless sintering at a relatively low temperature of 1750°C. This technique produced materials with hardness of 15·2 GPa, fracture toughness of 7 MPa  m^1/2 and flexural bending strength of 910 MPa.     

Reclamation of used SiC grinding powder and its sintering characteristics

A process of recycling used abrasive SiC powder after grinding Si wafer was proposed to raw powder for sintering. The used SiC powder could be successfully converted to composite powders consisting of SiC particle and Si₃N₄ whisker via a heat treatment in N₂ atmosphere, in which iron oxide acted as a catalyst in the vapor–liquid–solid (VLS) formation of Si₃N₄. With the addition of 3 mass% Al₂O₃ and 1 mass% Y₂O₃, the composite powders sintered at 1900 °C for 2 h exhibited a 3-point bending strength of 626 ± 48 MPa and a fracture toughness of 3.9 ± 0.1 MPa m^1/2, which were significantly enhanced as compared with those of using recovered powder merely composed of SiC particle. The strength and fracture toughness of the sintered material could be improved by optimization of chemical and heat treatment parameters and controlling the amount of sintering additives and hot pressing conditions.     

New pathway for the preparation of pyrochlore Nd2Zr2O7 nanoparticles

Pyrochlore Nd₂Zr₂O₇ nanoparticles were prepared by complex-precipitation in aqueous media, followed by calcination in MgO matrix and subsequent dissolution processing. A suite of characterization techniques, including X-ray diffraction, Raman, TEM, SEM, dynamic light scattering, and nitrogen sorption, was employed to investigate the structure and particle size of the synthesized nano materials. Results show that calcination at 1200 °C for 20 h forms Nd₂Zr₂O₇ with pyrochlore structure. The matrix phase (MgO) had no effect on the formation of pyrochlore phase. The MgO phase was readily removed by dissolution at 0.5 M HNO₃ aqueous solution; and the remaining pyrochlore Nd₂Zr₂O₇ nanoparticles had a diameter of approximately 200 nm estimated by TEM and approximately 550 nm determined by light scattering due to slight aggregation. The bulk density of the pelletized powder reached approximately 99% of theoretical value, after uniaxial pressing at 2.0–2.5 MPa and sintering at 1400 °C for 48 h.     

Evolution of elastic properties and microstructural changes versus temperature in bonding phases of alumina and alumina–magnesia refractory castables

The microstructural evolutions of high alumina refractory concretes, based on the systems CaO–Al₂O₃ and CaO–Al₂O₃–MgO, have been studied by the way of ultrasonic high temperature measurements. Since such a refractory concrete can be considered as a composite material with two constituents, a continuous matrix (so called bonding phase) and aggregates, investigations of matrices made of mixtures containing cement, reactive alumina and/or magnesia, constitute a preliminary study which is presented in this paper. The elastic behaviour of these matrices has been followed from room temperature to 1550 °C via a specific ultrasonic method. During the first thermal treatment, different changes of slope are observed in the curve E = f(T). Between 200 °C and 400 °C, dehydration mechanisms involve a microstructural reorganisation correlated with a strong decrease of the elastic properties. At high temperature, the Young's modulus evolutions are associated with the expansive formations of CA₂^b and/or in-situ spinel at 1100 °C and then CA₆ (see endnote b) at 1450 °C, which directly depend on the CaO/Al₂O₃ and MgO/Al₂O₃ ratios in the mix. The forming of bond linkage between CA₆ and in-situ spinel grains in the matrix is believed to enhance the elastic properties at high temperature.     

Mechanical Properties and Microstructure of Nano-SiC–Al2O3 Composites Densified by Spark Plasma Sintering

Heterogeneous precipitation method has been used to produce 5 vol% SiC–Al₂O₃ powder, from aqueous suspension of nano-SiC, aqueous solution of aluminium chloride and ammonia. The resulting gel was calcined at 700°C. Nano-SiC–Al₂O₃ composites were densified using spark plasma sintering (SPS) process by heating to a sintering temperature at 1350, 1400, 1450, 1500 and 1550°C, at a heating rate of 600 °/min, with no holding time, and then fast cooling to 600°C within 2–3 min. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 vol% SiC–Al₂O₃ densified by SPS at 1450°C reached as high as 1000 MPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al₂O₃ grains and the fracture mode of the nanocomposites was mainly transgranular fracture.     

High transparency Cr,Nd:LuAG ceramics prepared with MgO additive

Cr (0.2 at.%) and Nd (0.8 at.%) co-doped Lu₃Al₅O₁₂ ceramics were fabricated with MgO as the sintering additive. The addition of a small amount of MgO can affect the grain boundary mobility and influence the number and location of micropores in ceramics during the sintering process. The results show that when the MgO content is 0.02 wt.%, high-transparency Cr,Nd:LuAG ceramics can be obtained by vacuum sintering at 1670 °C for 5 h followed by hot isostatic pressing (HIP) post-sintering at 1750 °C in an argon atmosphere (P = 200 MPa) for 5 h. The optimum in-line transmittance of the HIPed Cr,Nd:LuAG ceramics (3 mm thick) is 83.5% at a wavelength of 840 nm and 84.0% at 710 nm.