Transparent AlON Research Articles

Planetary ball‐milling of AlON powder for highly transparent ceramics

AbstractA detailed investigation of planetary ball‐milling for coarsened AlON powder was carried out. Our results showed that the weight ratios of milling ball‐to‐powder, the revolution rate and the planetary ball‐milling time have significant impacts on the microscopic morphology, particle size distribution and average particle size of powder. The process and mechanism were analyzed, and the outcome of our study can be used to optimize the complicated planetary ball‐milling method by controlling the planetary ball‐milling time or adjusting the revolution rate at the final stage of planetary ball‐milling. Sequentially, using fine and uniform AlON powder by optimized planetary ball‐milling with an average particle size below 300 nm and excellent sintering properties, highly transparent AlON ceramic with an in‐line transmittance of 84% at 2000 nm was successfully prepared through pressureless sintering at 1880°C for 6 hours using the elaborative treated powder synthesized from carbothermal nitridation method.

Transparent AlON ceramic combined with VO2 thin film for infrared and terahertz smart window

Transparent AlON ceramics are intriguing window materials with excellent mechanical strength and superb optical transparency from the near ultraviolet to the mid-infrared range. However, previous studies focused their investigations in the visible range; therefore, the application of transparent AlON ceramics to tunable windows has yet to be reported. In this work, a VO2 thin film with a characteristic semiconductor-metal phase transition (SMT) was fabricated on a transparent AlON ceramic, which exhibited remarkable tunable switching properties in the infrared and terahertz ranges. The transparent AlON ceramic was prepared by a two-step method, which included carbothermal reduction and pressureless sintering. The resulting ceramic exhibited high transparency of over 70% in the visible-infrared range and a notable THz transmission of 64.5–73.9% at 0.1–1.5 THz. The VO2 thin film was prepared on a transparent AlON ceramic using the sol-gel method and showed excellent optical and electric switching performance. The square resistance variance was close to four orders of magnitude, and an infrared switching ratio of over 40% was obtained. Furthermore, the combined structure showed an efficient THz switching ratio of approximately 70.9%. This study proposes a composite material combined with a transparent ceramic and a phase transition oxide and provides great insights into their application in infrared and terahertz smart windows as well as in switching devices.

Strengthening mechanism of twin lamellas in transparent AlON ceramics

Twin lamellas, were observed in transparent AlON ceramics in our previous work. In this work, the influence of the twin lamellas on mechanical strength was evaluated and its mechanism was investigated by SEM, EBSD, and HRTEM. Both the crystallographic direction and the space direction of a crack will deflect when it propagates across a twin lamella. The dynamic mechanism is that the dislocation propagation in front of the crack tips is blocked by the twin boundaries. Because the fraction of twin boundaries increases with grain size in transparent AlON ceramics, the strengthening effect becomes more notable for large-sized samples resulting in inverse Hall-Petch relation. It is a potentially useful way to advance the mechanical properties of transparent AlON ceramics by controlling the content of twin lamellas.

Highly infrared transparent spark plasma sintered AlON ceramics

Abstract

Further experimental investigation on fast densification mechanism of bimodal powder during pressureless sintering of transparent AlON ceramics

In our recent work we demonstrated that a bimodal particle size distribution (PSD) AlON powder could be fast densified by pressureless sintering (Shan et al., Ceram. Int. 41 3992–3998 (2015); Shan et al., J. Eur. Ceram. Soc. 36 67–78 (2016)). To obtain a further insight of its fast densification mechanism, this powder was held for 30min and 60min at 1500–1700°C during heating, respectively. However, the added holdings resulted in a decrease in both relative density and transmittance, compared to that of the sample without holding during heating. Further investigation into phase transformation and microstructure evolution of these samples indicates that high heating rate enables the sintering mixtures to keep a near sphere particle shape in bimodal PSD until phase transformation from Al2O3 to AlON is fully completed. Then mass transport between AlON grains of different size can simultaneously happen at the final fast densification stage, which benefits less formation of pores and fast AlON grains growth. Therefore, high heating rate plays a key role to fast and better consolidation of the bimodal AlON powder.

Fast densification mechanism of bimodal powder during pressureless sintering of transparent AlON ceramics

Both bimodal and unimodal AlON powders were pressureless sintered and their phase assemblages and microstructure evolutions at 1400–1900°C were monitored to reveal the fast densification mechanism of bimodal powder. Phase assemblages analysis of samples sintered at 1400°C suggests that fine AlON particles were transformed into hexagonal α-Al2O3 and AlN, while coarse AlON particles were transformed into cubic η-Al2O3 and AlN. Correspondingly, only a small fraction of the unimodal mixture was η-Al2O3, while the relatively unstable η-Al2O3 kept dominating the bimodal samples and most of them were directly reformed into AlON at a lower temperature of 1600–1700°C for their similarity in crystal structure. As a result, a bimodal grain size distribution was kept in the whole sintering process of the bimodal sample, which led to its fast densification. Using the bimodal AlON powder, highly transparent AlON ceramics were fully pressureless sintered at 1820°C after holding for 2.5h.

Processing of Transparent Polycrystalline AlON:Ce3+ Scintillators

A new polycrystalline ceramic scintillator is reported for potential use in radiation detection and medical imaging applications. The goal was to develop cerium‐activated aluminum oxynitride (AlON:Ce3+) ceramics, which can be produced using ceramic processes in comparison to the high‐cost, low‐yield single‐crystal growth technique. A phase pure AlON:Ce3+ powder with cubic symmetry was successfully synthesized at high temperature under a reducing atmosphere to convert Ce4+ to Ce3+ in the solid solution. Two different activator concentrations (0.5 and 1.0 mol%) were explored. Fully dense and transparent AlON:Ce3+ ceramics were produced by a liquid‐phase‐assisted pressureless sintering. The crystal field splitting around the Ce3+ activator in the AlON was comparable to the splitting induced by Br− and the Cl− ligands, which produced an emission spectrum perfectly matching the maximum quantum efficiency range of the photomultiplier tube for radiation detection. Both optical excitation and radiation ionizations in AlON:Ce3+ were demonstrated. Challenges and mechanisms related to the radioluminescence efficiency are discussed.

Hot isostatic pressing of transparent AlON ceramics with Y2O3/La2O3 additives

Highly transparent AlON ceramics were fabricated by hot isostatic pressing (HIP) of the sintered bodies composed of fine grains (∼20 μm). Sintering additives play an important role on the porosity and pore positions of the sintered AlON bodies, which can determine the final pore elimination during HIP sintering. Compared to single Y2O3 or La2O3 additive, the codoping of Y2O3–La2O3 additive is more effective to prepare sintered AlON body with small intergranular pores and lower porosity. Effects of additives on the phases, microstructures, as well as optical transmittance of the HIPed AlON ceramics were also extensively investigated.

A fast pressureless sintering method for transparent AlON ceramics by using a bimodal particle size distribution powder

Using a pure AlON powder synthesized by carbothermal reduction and nitridation, a bimodal particle size distribution (PSD) powder was prepared by planetary ball mill, and pressurelessly sintered at 1880°C to fabricate transparent AlON ceramics. Relative density of ≥99.72% was achieved for all the obtained AlON ceramics samples held for 90–150min. The maximum infrared transmittance for 3mm thickness sample was up to 81.8% (93.8% of theoretical in-line transmittance). Therefore, the holding time to fabricate transparent AlON ceramics can be shortened to 90min or 150min, compared to 480min holding reported in the literature. The fast sintering of transparent AlON ceramics is a combination of fast heating (40°C/min) and shorter holding time, and should be mainly attributed to fast surface diffusion during heating due to the bimodal PSD of fine powder and fast grain boundary diffusion during both heating and holding due to the transient bimodal grain size distribution.

Highly transparent AlON sintered from powder synthesized by direct nitridation

Abstract Transparent AlON ceramic was pressurelessly sintered from powder synthesized by direct nitridation of Al–Al2O3 starting mixtures. TG, SEM and XRD analyses were conducted to investigate the detail procedure of direct nitridation. The Al spheres were dispersed by submicron Al2O3, then nitrided and reacted with Al2O3 to form AlON. The particle size of AlON is about 10–30 μm with irregular morphology. With the aid of ball milling and sintering additives, 2 mm thick AlON ceramic with in-line transmittance of 80.6% at 1100 nm and above 77.1% at 400 nm was obtained successfully, demonstrating that the direct nitridation method is another promising approach for the preparation of AlON ceramics with high transparency.

Effect of Y2O3 and La2O3 on the sinterability of γ-AlON transparent ceramics

Abstract The paper reports the use of Y2O3 and La2O3 co-doping as a composite sintering additive for the fabrication of γ-AlON transparent ceramics by pressureless sintering. The Y3+ enhanced the mobility of grain boundary and accelerated the grain growth, whereas the La3+ has the opposite effect on grain growth. The sintering additives could cause the formation of liquid phase during sintering, which would greatly promote the densification and eliminate pores. High transparent AlON ceramics with the in-line transmittance of 80.3% at 400 nm wavelength have been prepared when the concentration of sintering additives was 0.12 wt% Y2O3 and 0.09 wt% La2O3.

Preparation of γ-AlON Transparent Ceramics by Pressureless Sintering

In this paper, AlON powders were prepared by carbothermal reduction and nitridation method. By adding a mixture of Y2O3, La2O3 and MgO as sintering agent, transparent AlON ceramics with high strength were fabricated by pressureless sintering at 1950°C for 8h under nitrogen atmosphere. The obtained ceramic had a relative density of 99.9% and average grain size of 150μm. The transmittance of the 2mm thick ceramics was 82.4% at 1100nm and flexural strength was 300 MPa.

Transparent Armor Ceramics as Spacecraft Windows

The slow crack growth parameters of several transparent armor ceramics were measured as part of a program to lighten next generation spacecraft windows. Transparent magnesium aluminate (spinel, MgAl2O4) and AlON exhibit superior slow crack resistance relative to fused silica, which is the historical material of choice. For spinel, slow crack growth, strength, and fracture toughness are significantly influenced by the grain size, and alumina‐rich phases and porosity at the grain boundaries lead to intergranular fracture in coarse grain spinel. Functions describing the required mass for a desired window life imply that transparent ceramics can lighten window panes from a slow crack growth perspective.

Highly Transparent AlON Pressurelessly Sintered from Powder Synthesized by a Novel Carbothermal Nitridation Method

Transparent AlON was pressurelessly sintered from powders synthesized by a two‐step carbothermal nitridation method, using core‐shell structured Al2O3/C mixture derived from the pyrolysis of an Al2O3/ureaformaldehyde‐resin nanocomposite precursor as the starting material. It was found that the amorphous carbon layer on the Al2O3 particle surface could strongly retard the coalescence and growth of Al2O3 particles during the carbothermal nitridation process. As a result, the formation temperature and particle size of the AlON powder were significantly reduced. The AlON powder synthesized showed a bimodal particle size distribution at 0.2 and 0.7 μm and was well dispersed; its maximum particle size was below 0.9 μm. Under the aid of proper sintering additive, transparent AlON ceramics with an average in‐line transmittance above 80% at visible to infrared range were obtained through pressureless sintering.

Fabrication of Transparent AlON Ceramics by Solid-state Reaction Sintering

Transparent AlON ceramic which exhibit excellent optical and mechanical properties is a potential alterna- tive to sapphire for the application of IR windows and armor. In this work, transparent AlON ceramics were success- fully prepared by solid-state reaction sintering. The effects of sintering aids and holding time on densification of AlON ceramics were investigated. The phase composition and microstructure of the ceramics were characterized by XRD and SEM, respectively. In-line transmittance of the ceramics was measured by spectrophotometer. The results show that co-doping of MgO and Y2O3 as sintering aid is more effective to enhance densification of AlON ceramics than the doping of MgO or Y2O3. With doping level of 0.08wt%Y2O3, the transmission of ceramics increases with the increase of MgO content. Co-doped with 0.08 wt% Y2O3 and 1wt% MgO, the ceramics (1mm thick) sintered at 1950℃ for 12h in N2 atmosphere have an in-line transmission of 60% (at 600nm).

Analysis and Evaluation of the Comprehensive Property for Transparent Aluminum Oxynitride (ALON) Ceramics

ALON ceramics with optimal mechanical and optical properties were obtained by cold isostatic pressing the as-prepared powders at 180 MPa and sintering at 1900°C under N2 atmosphere for 8h. Microstructures and IR transmittance of ALON prepared under different sintering conditions were studied systematically to discuss the effect of microstructures on mechanical properties and IR transmittance. In addition, IR spectra of transparent ALON test pieces in different thickness were collected. The results demonstrated that the bending strength decreased while IR transmittance increased with the increase of grain size of ALON ceramics; with the increase of thickness of ALON test piece, IR transmittance decreased and the scope of the decrease reduced with the increase of grain size of ALON ceramics.

Synthesis of Aluminum Oxynitride Powder by Carbothermal Reduction Method

γ-aluminum oxynitride (γ-ALON) powder was prepared by carbothermal reduction method using γ-Al2O3 and activated carbon as raw materials. Effects of activated carbon content, synthesizing temperature, holding time and carbon impurity on ALON powder were studied. It was found a ‘two-step’ procedure, first sintering at about 1550°C-1650°C then at about 1750°C-1850°C, was effective to prepare high-purity ALON powder. The removal of carbon impurity was crucial for the preparation of ALON powder, which effectively improved the purity of ALON powder. The prepared powder was characterized by XRD and SEM, which indicated that pure ALON powder prepared by a ‘two-step’ procedure had well-developed crystalline and homogenous granularity. Transparent ALON ceramics were prepared by hot-press sintering at 1850~1900°C for up to 2~4h using ALON powder and some sintering additive, the transmission of sample with thickness of 1mm was 81% at infrared wave.

Influence of Y<sub>2</sub>O<sub>3</sub> Additive on Transparent of ALON Ceramics

Transparent ALON ceramics with the ultra-fine -ALON spinel powder were prepared by hot-pressing. The powder was prepared by carbothermal reduction method. The optimum condition was hot-pressing between 1850°C to 1920°C for 3 ~ 5 h under 35 MPa. We got transparent samples 70 × 3 mm with in-line transmittance above 70% in the range from the visible to infrared wavelength. The effect of additive contents on transparency and the relation of microstructure and transparency have been discussed in this paper. The transmittance is good when the Y2O3 sintering additive content is 1.5wt%. The sample density is about 99% of its theoretical density. We also measured the flexural strength, elastic modulus and other mechanical properties of ALON.

Preparation of Transparent AlON Ceramics Using Single-Phase Powder Obtained from Carbothermal Reduction

Single-phase AlON powder was prepared by carbothermal reduction in flowing nitrogen atmosphere, using nano-sized alumina and carbon powder as raw materials. Transparent AlON ceramic samples using the single-phase powder were prepared by sintering at the temperature of 1880°C for 10h, 20h and 40h in flowing nitrogen atmosphere, respectively. It showed that the transparency and the density of the ceramic samples increased with increasing temperature holding-time. And the microstructures of the obtained samples were observed.

A New Method for the Preparation of Transparent AlON Ceramic

Transparent AlON ceramics are prepared with single-phase AlON powder synthesized from micron-sized aluminum and nano-sized alumina. The ceramics are prepared through sintering at the temperature of 1880°C for 10, 20 and 40 hours in flowing-nitrogen atmosphere, respectively. The effects of powder and temperature holding-time on transparent ceramic preparation and microstructure and transparency are investigated. The results show that all samples are transparent and of a single-phase AlON and, with increasing holding-time, the average pore size and porosity decreases and consequently, the ceramics are dense and their transparency is improved.