Preparation and properties of AlON powders

Aluminum oxynitride (AlON) powders were synthesized using the raw materials of γ-Al₂O₃ and carbon black through the carbothermal reduction and nitridation process. The carbon content in the γ-Al₂O₃/C mixture and heating temperature were investigated. The AlON powders were synthesized by calcination for 2 h at 1750 °C when the carbon content in the γ-Al₂O₃/C mixture was 5.8 wt%. The particle size of powder is important to the transparency of ceramics, but the size of the synthesized powder was large. Therefore, a few methods, such as freeze-drying, ultrasonic dispersion, and liquid nitrogen ball milling, were used to reduce the particle size of powders. Among the three methods, liquid nitrogen milling had the best results.     

Fabrication of highly transparent AlON ceramics by hot isostatic pressing post-treatment

Highly transparent aluminum oxynitride (AlON) ceramics were fabricated by pressureless sintering with hot isostatic pressing (HIP) post-treatment. The experimental results showed that the optical transparency of AlON ceramics was improved markedly over the visible and near-infrared range by HIP at 1825 °C for 3 h in 200 MPa argon gas, which derived from the elimination of residual pores in the prepared ceramics. For AlON ceramics pre-sintered at 1800 °C, the transmittances of the sample increased from 63.6% to 84.8% at 600 nm and from 75.4% to 86.1% at 2000 nm, respectively. The average grain size of the HIPed sample was about 47.9 μm.     

Pressureless sintering of transparent AlON ceramic with assimilable γ-Al2O3 as sintering promoting additives

Transparent aluminum oxynitride (AlON) ceramic was successfully fabricated without doping sintering additive by the pressureless sintering method. Γ-Al₂O₃ nano-powder that can be assimilated by the AlON matrix was used for promoting the densification of AlON during the sintering. The sintering behavior of AlON and the effects of γ-Al₂O₃ nano-powder on the phase, hardness, and transmittance of AlON have been investigated in detail. The mechanisms of γ-Al₂O₃ nano-powder on the AlON green body modifying and the sintering promoting are revealed. The transmittance of the AlON ceramic is dramatically enhanced by doping γ-Al₂O₃ nano-powder and the 2 mm thick sample doped with 2.5 wt% γ-Al₂O₃ nano-powder shows an inline transmittance above 81% at 1500 nm.     

Fast one-step carbothermal reduction and nitridation method using nano-sized Al2O3 and carbon black to prepare aluminium oxynitride powder for highly transparent ceramics

By fast heating the nano-sized Al₂O₃ and carbon black mixtures at 50°C/min to 1750°C for 30–120 min, single-phase AlON powders were successfully obtained by a fast one-step carbothermal reduction and nitridation (CRN) method. The AlON ceramics pressureless sintered at 1880°C for 150 min by these powders show high transmittances up to 83%–84%, which indicates that the proposed fast one-step CRN method is an effective and efficient way with strong robustness to synthesize single-phase AlON powder for highly transparent AlON ceramics. It was found that α-Al₂O₃ particles do not have enough time to aggregate and coalesce during heating due to the tremendously shortened heating span, which significantly inhibited particle coarsening until the formation of AlON starts. The fast-formed AlON further inhibits the coarsening of α-Al₂O₃ during dwelling. Consequently, single-phase AlON powder of small primary particles can be obtained after 30 min dwelling at 1750°C.     

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).     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Study on robust additive of CaCO3 for fast fabrication of highly transparent AlON ceramics by pressureless sintering

Fabrication of transparent AlON ceramics is extra sensitive to both particle size of starting powder and sintering additive due to shuttling transformation between AlON and Al₂O₃ + AlN during heating. One possible solution is to select robust additive to suppress the shuttling transformation. In this work, three AlON powders with different median particle sizes of 0.6, 0.9, and 1.1 μm were prepared. After studying the effect of CaCO₃ on densification process, AlON ceramics with the maximum transmittance of ≥81.1% were successfully fast prepared by pressureless sintering (PS) at 1880°C for only 2.5 h by using three AlON powders doped with different CaCO₃ amount. Specifically, AlON ceramics prepared from 1.1 μm with 0.5–0.8 wt.% CaCO₃ doping consistently showed the maximum transmittance of ≥85.3%, which indicates that CaCO₃ can serve as a robust additive to enable fast fabrication of highly transparent AlON ceramics even by PS.     

Fabrication of Lu2O3:Eu transparent ceramics using powder consisting of mono-dispersed spheres

Mono-dispersed spherical Lu₂O₃:Eu (5 mol%) powders for transparent ceramics fabrication were synthesized by urea-based homogeneous precipitation technique. The effects of the doped-Eu³⁺ on the synthesis of Lu₂O₃:Eu particles were investigated in detail. The results show that the doping of Eu³⁺ ions into Lu system can significantly decrease the particle size of the resultant precursor spheres. Owing to the sequential precipitation in Lu/Eu system, there are compositional gradients within each of the resultant precursor spheres. Well dispersed, homogeneous and spherical/near spherical Lu₂O₃:Eu powders were obtained after calcination at 600–1000 °C for 4 h. The powder calcined at 600 °C shows better sintering behavior and can be densified into transparent ceramic after vacuum sintering at 1700 °C for 5 h. The luminescence properties of the obtained Lu₂O₃:Eu powder and transparent ceramic were also studied.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Elaboration of CaLa2S4 transparent ceramics from novel precursor powders route

A cost-effective processing route for the production of calcium lanthanum sulfide CaLa₂S₄ (CLS) via a novel fast fabrication of precursor powders is reported. The sinterability of the newly developed powders was investigated by use of Hot-Pressing and pressureless sintering. Complementary techniques (XRD, SEM-EDS, chemical analysis, SSA, FT-IR spectroscopy) were employed to correlate the sintering processes and parameters to the microstructural/compositional developments and optical performances of the densified ceramics. Dense (>99.8% theor.) and homogeneous CLS ceramics were produced by pressureless sintering at 1250 °C for 12 h in H₂S followed by hot-pressing at 1000 °C for 6 h in a powder bed to prevent sulfur loss. Transparency free of impurity absorption has been achieved in the LWIR region (optical transmission of 51% in the 12–14 µm range).     

Sintering kinetics of disperse ultrafine equiaxed α-Al2O3 nanoparticles

The sintering kinetics of ceramic nanoparticles is essential for preparing dense nanocrystalline ceramics with fine grains, but the sintering kinetics of disperse ultrafine α-Al₂O₃ nanoparticles has not been systematically explored so far. In this paper, the sintering kinetics of disperse ultrafine equiaxed α-Al₂O₃ nanoparticles with a mean particle size of 4.5 nm and a narrow size distribution of 2–8 nm without any agglomeration was studied systematically. Finally, α-Al₂O₃ nanocrystalline ceramic with a mean grain size of 36 nm and a relative density of 99.7% was sintered in air by two-step sintering (heated to 1100 °C without hold and then cooled down to 950 °C with a 40 h hold). The sintering temperature is the lowest for pressureless sintering of α-Al₂O₃ and almost fully dense α-Al₂O₃ nanocrystalline ceramic obtained also has the finest grain so far.     

Transparent AlON ceramics by nitriding combustion synthesis precursors and pressureless sintering method

Fine AION powders (D₅₀ = 607 nm) with high sintering activities were synthesized by a high-efficiency solution combustion-based method, and a high-transmittance AlON ceramics can be achieved by a subsequent sintering process. Moreover, the influences of various aluminum resources (Al(NO₃)₃, AlCl₃ and Al₂(SO₄)₃) on the morphology of precursors and nitride-functionalized products have been studied in detail. Using Al(NO₃)₃ as aluminum source resulted in more active precursors and was beneficial for the preparation of ultrafine AlON powders. Then, single-phase AlON powders were obtained by calcining precursors at 1700 °C for 10 min, while residual Al₂O₃ was observed in the calcined products synthesized by other two aluminum sources. As a result, transparent AlON ceramics with high in-line transmittance of 85.9% at 2 μm were obtained. This research provides valuable reference for rapid preparation of AlON powders and transparent ceramics.     

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 heat treatment of powder mixture on the microstructure and optical transmission of Nd:YAG transparent ceramics

Transparent Nd:YAG ceramics were fabricated by solid-state reactive sintering of Y₂O₃, α-Al₂O₃ and Nd₂O₃ powders with TEOS and MgO as sintering aids. The powders were ball-milled, dried, sieved and calcined at different temperatures. Samples sintered at 1745 °C for 10 h were utilized to observe the microstructure and the optical transmission. It is found that heat treatments of the powder mixtures above 600 °C for 1 h are necessary to remove the carbon contamination but below 800 °C for 4 h can avoid strong aggregation of the powder. So there is a room for heat-treatment, between 600 °C and 800 °C that can obtain Nd:YAG ceramics with almost pore-free microstructures and high transparency. Highly transparent Nd:YAG ceramic with 84.3% in-line transmission at 1064 nm was fabricated by sintering the 800 °C-1 h-heat-treated powder mixture at 1745 °C for 50 h. Even at wavelength of 400 nm, the transmittance of the sample reached 82.9% and the optical scattering coefficient was as low as 0.71% cm⁻¹.     

Low temperature sintering of PZT powders coated with Pb5Ge3O11 by sol–Gel method

Low temperature sintering of PZT powders was investigated using Pb₅Ge₃O₁₁(PGO) as a sintering aid. PZT powders with 150 nm particle size were coated with PGO which was prepared from precursor solutions of Ge(OiPr)₄ and Pb(NO₃)₂ by sol–gel method. 1 wt% PGO-added PZT powders were densified at 750°C for 2 h to sintered bodies with the relative density of approximately 95%. An addition of PGO improved the sinterability of PZT powders with a reduction of sintering temperature by about 300°C. Dielectric and piezoelectric properties of PGO-added PZT ceramics sintered at ≦950°C were superior to those without PGO additives. However, a higher sintering temperature above 1000°C deteriorated the dielectric and piezoelectric properties of PGO-added PZT ceramics. This may be attributed to the change of microstructure involving the formation of solid solution between PZT and PGO. The 1 wt% PGO-added PZT bodies sintered at 750°C exhibited an electromechanical coupling factor, Kp, of about 56%.     

Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and α-Al2O3

High purity calcined carbonaceous kaolin and α-Al₂O₃ powders were employed to prepare porous mullite ceramics (Sample A) using graphite as pore former with the reaction sintering method. For the purpose of comparison, porous mullite ceramics (Sample B) was also fabricated from the uncalcined carbonaceous clay incorporated with α-Al₂O₃ powders. Mullitization in the two samples was both nearly complete at 1500 °C, despite the fact that calcination of the clay remarkably depressed mullitization and promoted the formation of glass phase. The Sample A sintered at 1500 °C fractured mainly in an intergranular way, while the Sample B mainly underwent transgranular fracture. The experimental results revealed that densification behavior/open porosity of the Sample A was far more sensitive to sintering temperature. The pore size of the Sample A as well as the Sample B sintered at 1500 °C was in a narrower range of 0.3–5 μm.     

High optical quality Y2O3 transparent ceramics with fine grain size fabricated by low temperature air pre-sintering and post-HIP treatment

High optical quality Y₂O₃ transparent ceramics with fine grain size were successfully fabricated by air pre-sintering at various temperature ranging from 1500 to 1600 °C combined with a post-hot-isostatic pressing (HIP) treatment using co-precipitated powders as the starting material. The fully dense Y₂O₃ transparent ceramic with highest transparency was obtained by pre-sintered at 1550 °C for 4 h in air and post-HIPed at 1600 °C for 3 h (the pressure of HIP 200 MPa), and it had fine microstructure and the average grain size was 0.96 μm. In addition, the in-line transmittance of the ceramic reached 81.7% at 1064 nm (1 mm thickness). By this approach, the transparent Y₂O₃ ceramics with fine grain size (<1.6 μm) were elaborated without any sintering aid.     

Preparation of transparent AlON from powders synthesized by novel CRN method

Aluminum oxynitride (AlON) powders were synthesized by a novel carbothermal reduction and nitridation (CRN) method. Homogenous and fluffy AlOOH/C core-shell nanoparticle precursor was hydrothermally synthesized with aluminum nitrate hydrate, sucrose and urea as starting materials. Then single-phase AlON powders were synthesized by CRN method at 1700 °C for 2 h. The phase transition and growth of Al₂O₃ particles was effectively retarded by the amorphous carbon nano-layers on the surface of precursor, resulting in significantly lower reaction temperature and further smaller particle size. Based on above fine AlON raw material, transparent AlON ceramic was prepared by pressureless sintering at 1880 ℃ with the in-line transmittance above 80 %.     

Annealing effect on dielectric property of AlN ceramics

Effects of slow-cooling at high temperatures and annealing at intermediate temperatures on dielectric loss tangent of AlN ceramics were explored. Y₂O₃ was added as a sintering additive to AlN powders, and the powders were pressureless-sintered at 1900 °C for 2 h in a nitrogen flow atmosphere. In succession to the sintering, AlN samples were slow-cooled at a rate of 1 °C/min from 1900 to 1750 °C and/or annealed at 970 °C for 4 h. Al₅Y₃O₁₂ was detected in the AlN ceramics obtained by the slow-cooling and AlYO₃ was found in the ceramics cooled at a rate of 30 °C/min. AlN ceramics with a relative density of 0.986 were obtained by the slow-cooling method. On the other hand, very low tan δ values between 2.6 and 4.6 × 10⁻⁴ were obtained when the AlN ceramics were annealed at 970 °C for 4 h.     

Alon-based materials prepared by SHS technique

The aim of the presented paper was preparation of the highly reactive in the sintering powders in the Al–O–N system by SHS method. Combustion reactions of metallic aluminium and corundum powder mixtures (from 15% Al–85% Al₂O₃ to 50% Al–50% Al₂O₃) were performed in nitrogen atmosphere. The obtained powders were ground and hot-pressed at 1750, 1850 and 1950 °C for 1 h under 25 MPa in nitrogen flow. In contrast to conventional methods, which require 24 h of the precursor heat treatment at 1200 °C our studies, showed that it is possible to prepare almost pure γ-alon materials using SHS reaction. Sintering of the powders led to obtained dense materials composed of pure γ-alon or γ-alon–AlN composites. The phase composition of the sintered bodies was controlled by the chemical composition of the starting mixture and the sintering temperature.