AlON Ceramics Research Articles

Study on one-step fast CRN mechanism for preparing fine AlON powder using carbon coated γ-Al2O3/C precursor

By pre-milling in glucose aqueous solution, γ-Al2O3 particles were uniformly mixed with carbon via hydrothermal treatment technique. Then, directly heating the γ-Al2O3/C precursor at 50 °C/min to 1775 °C for 15–30 min in N2, single phase AlON powder was fast obtained through one-step carbothermal reduction and nitridation (CRN). By approaching the evolution of phase assemblages and morphology during heating and dwelling, the one-step fast CRN mechanism for preparing fine AlON powder was revealed. The carbon around γ-Al2O3 particles delayed the phase transformation of Al2O3 from γ phase to α one. The undesired particle aggregation and growth before formation of AlON was also auxiliarily inhibited by swiftly achieving AlON formation temperature via high heating speed. Further elevating synthesis temperature enabled high reaction kinetics to boost the speed of AlON formation. By these ways, CRN reactions were quickly completed to fast obtain pure AlON powder with small primary particles due to limited particle aggregation and growth. The as-prepared fine AlON powder was easy to be ball-milled to prepare highly transparent AlON ceramics by pressureless sintering.

Fast pressureless sintering of highly transparent AlON ceramics by reducing Y enrichment with nano-sized granular Y2O3 as additive

Employing nano-sized granular Y2O3 (NGY) as additive, AlON ceramics with transmittance ≥80% across wavelengths at 380–4150 nm were successfully fabricated via pressureless sintering at 1880 °C for 2.5 h, the dwelling duration is much shorter than the ≥7 h reported in literature. The distribution of Y2O3 in starting mixtures, along with phase transformation, microstructure evolution and densification process of the samples during heating were thoroughly studied to reveal the fast densification mechanism. Local enrichment of Y2O3 in the AlON/Y2O3 mixtures was significantly reduced when using NGY instead of micro-sized lamellar Y2O3. The reduction of excessive local Y2O3 concentration resulted in less α-Al2O3 was decomposed from AlON during heating, and further suppressed local aggregation/coarsening at ≤1600 °C. The excellent microstructure obtained at early stage of sintering, together with the improved effective Y2O3 concentration in whole sample led by the decrease in local Y2O3 enrichment accelerate the densification process of AlON.

Simultaneously significant transparency enhancement and efficient temperature feedback achieved in AlON ceramic through luminescent rare earth doping

Activator-doped aluminum oxynitride (γ-AlON) could provide an extremely versatile transparent ceramic with photothermal feedback, opening the door to many new applications and devices for use in extreme conditions. However, existing AlON transparent ceramics have failed to achieve temperature-responsive luminescence. To address this, a simple-to-implement luminescent rare-earth doping strategy for fabricating AlON transparent ceramic with efficient temperature feedback is proposed and demonstrated. Luminescent Pr2O3 serves as the illustrative example. Benefiting from the liquid phase formed by Pr2O3 and the thermal coupling between the 3P1 and 3P0 levels of Pr3+, AlON ceramics doped with Pr2O3 exhibit significantly elevated transparency and temperature-responsive luminescence. More importantly, the AlON:Pr ceramic can serve as photothermal feedback windows, which presents attractive performance. This paradigm fills an important gap in the AlON transparent ceramics for temperature feedback windows, and will inspire the controllable design of luminescent rare-earth doped AlON ceramics for use in harsh conditions.

Fabrication of highly transparent AlON ceramics by inhibiting decomposition of AlON

In consolidation of AlON, the extensive decomposition of AlON into α-Al2O3 and AlN during sintering is one of the major reasons that significantly retard its densification process. To accelerate densification of AlON, an ultra-low concentration of La2O3 (0.03 wt%) and Y2O3 (0.02-0.04 wt%) was co-doped as sintering additives to inhibit decomposition of AlON during sintering for fast fabrication of highly transparent AlON ceramics. The phase transformation, microstructure evolution and densification process of the samples undoped and doped additive during heating were studied to investigate the densification mechanism. The results reveal that, compared to the samples undoped additive and single-doped with La2O3, the α-Al2O3 content of the samples co-doped with La2O3 and Y2O3 is significantly deduced at ≤1600 °C, which indicates that the decomposition of AlON is effectively inhibited by co-doping La2O3 and Y2O3. The reduction of α-Al2O3 along with the deceleration effect of La3+ on mass transfer, contributes to more small particles being conserved at high temperatures. Base on this, at later stage of sintering, benefiting from the enhanced mobility of grain boundary by Y3+, AlON ceramics with high relative density (99.79 %) and excellent transparency were obtained via pressureless sintering at 1880 °C for 2.5 h by co-doping 0.03 wt% La2O3 and 0.04 wt% Y2O3. The maximum transmittance of the fabricated AlON ceramics is up to 81.6 % at 3600 nm, and a transmittance of 76.0 % was measured at 400 nm. The ultra-low doping amount of La2O3 and Y2O3 and the high relative density should be the main reasons that lead to the outstanding transparency of the obtained AlON. Additionally, the fabricated AlON ceramics also exhibit a high Vickers hardness of 18.89 ± 0.18 GPa.

Fast densification mechanism of highly transparent AlON ceramics pressureless sintered by co-doping Y2O3 and MgO additives

By co-doping 0.15 wt% Y2O3 and 0.60 wt% MgO, AlON ceramics with high transmittance in visible and infrared range (the maximum transmittance of 81.8%@3600 nm, 78.1%@400 nm) were successfully fast fabricated via pressureless sintering at 1880 °C for 2.5 h. The phase assemblage, microstructure and densification process of samples doped (Y2O3/MgO single- and co-doped) and undoped additives during heating were monitored to explore the densification mechanism. MgO was found to be an exceptional inhibitor for both AlON decomposition and grain growth. By incorporating MgO as one constituent of additives, the content of α-Al2O3 that decomposed from AlON in samples during heating was significantly reduced and Mg2+ illustrated inhibition to grain boundary migration, resulting in particle aggregation/coarsening were effectively suppressed at ≤ 1600 °C. Based on the obtained microstructure with compacted small particles and negligible particle aggregation/coarsening at early stage of sintering, facilitated by the acceleration of Y3+ on mass transfer, rapid densification of AlON was achieved.

Preparation of spherical AlON powder and high transparent ceramics by pressureless sintering

AbstractThe sphericity and fluidity of the powder play a crucial role in the forming and sintering of ceramics. High‐performance AlON spherical powder was prepared for the first time by spray drying of powder fired by solid‐phase reaction, and the preparation of AlON transparent ceramics was realized under pressureless sintering conditions. The effect of AlN dosage on the purity of AlON powder was investigated, and the powder fluidity was improved by spray drying technique. Then the best spray drying process was selected by scanning electron microscopy, AD/TD, green density, MIP and transmittance measurements. The AlON particles obtained under the optimal granulation process have good dispersibility, high green density after pressing, and uniform pore distribution. After dry pressing and sintering at 1900°C for 20 h, the AlON ceramics with linear transmissivity of 84.3% at 2 μm wavelength can be obtained. The excellent pressing and sintering properties of AlON granulated particles make it possible for large‐scale production.

The study on the preparation of AlON transparent ceramic and its ballistic resistance

The AlON transparent ceramic was widely used as a strike-surface layer for transparent armor because of its outstanding mechanical and optical properties. With the same protection capability, the AlON transparent ceramic armor can significantly reduce surface density and thickness compared with bulletproof glass, which can effectively improve the lightweight of armed helicopters and armored vehicles. In this work, AlON transparent ceramics were fabricated by sintering AlON powder sintered at 1960°C, which was synthesized by the method of carbothermic reduction and nitridation. More than 99.8% of the relative density of the samples of AlON ceramics was kept under hot isostatic pressing for 30 hours. The maximum visible light transmission of the AlON transparent ceramic sample amounted to more than 80 %, and the flexural strength and Vickers hardness can reach up to 350 MPa and 15 GPa respectively, exhibiting excellent ballistic performance. When 53-type 7.62 mm API hit AlON transparent ceramic, there are radial and circumferential modes during its crack growth process, which can guide the structural design of the transparent ceramic armor.

Two-step pressureless sintering of ultrafine powders for the preparation of high strength and transmittance AlON ceramics

AlON ceramics with high transmittance and strength are essential for transparent armor application. However, high-temperature sintering for long time would result in large grain sizes associated with low strength. In this paper, a low + high temperature two-step sintering method with ultrafine AlON powders was reported for the preparation of high-strength and high-transmittance AlON ceramics. Firstly, the high-purity AlON powders with high specific surface area of 29.77 m2/g was synthesized by solution combustion synthesis-nitridation method and subsequently ball-milling process. Thereafter a sample with 97% density and small grain size (less than 4 μm) can be obtained after sintered at 1750 °C for 3 h, in which the difference of kinetics between grain-boundary diffusion and grain-boundary migration was used to suppress the final stage of grain growth. Further increasing sintering temperature to 1850 °C resulted in the elimination of the residual porosity, and realizing transparent AlON ceramics with high optical transmission (85.4%) and high flexural strength (375.1 MPa) could be achieved.

Effects of sintering aids on microstructure, mechanical, and optical properties of AlON ceramics synthesized by SPS

AbstractAluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La2O3 + 0.16 wt% Y2O3 + 0.30 wt% MgO. The addition of La3+ and Mg2+ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y3+ facilitates liquid‐phase sintering of AlON ceramics. Moreover, the addition of Mg2+ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La2O3 + 0.16 wt% Y2O3 + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.

Achieving highly flowable AlON spherical powders via spray drying technology

In this work, the mono-dispersed and high-flowable AlON spherical powders were obtained for the first time. Spherical pure AlON particles with a mean size of approximately 51.9 μm were prepared from the C/Al2O3 mixture via spray drying technology followed by a calcined step at 1750 °C for 0.5 h. Compared with the irregular-shaped AlON powders obtained by the conventional route, the as-prepared spherical AlON powders present higher flowability and filling properties. The Carr index and Hausner ratio of these AlON powders are 73.5 and 1.13, respectively. This work provides a paradigm for obtaining highly flowing spherical ceramic powders, which is desirable for the preparation of complex-shaped AlON ceramics.

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

AbstractFabrication 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.

Recent progress and challenges of transparent AlON ceramics

AbstractThe transparent AlON ceramics have attracted a growing investigation and application in the commercial and military fields, such as infrared and visible windows, domes, and transparent armors. Up to now, although great progress has been reported, such as the improved methods, new sintering additives, and fresh explored applications, a systematic and targeted summary is still lacking. In addition, there are some unsolved problems and challenges of the AlON ceramics, which hinder their commercial promotion and application. In this review, we introduce various preparation methods of AlON ceramics, including high temperature solid-state reaction, carbonization based on Al2O3, chemical vapor deposition, and sol−gel method, and thoroughly provide contrastive analysis of their advantages and disadvantages. Furthermore, we also give an insight into the transparency, sintering additives, and sintering technologies of AlON ceramics. At last, the challenges and prospects of AlON ceramics are discussed as well.

Investigation on mechanical properties of AlON ceramics synthesized by spark plasma sintering

AlON ceramics were prepared by spark plasma sintering (SPS) with raw materials (60 mol% Al2O3 and 40 mol% AlN) and sintering aids (0.16 wt% Y2O3 + 0.3 wt% MgO + 0.06 wt% La2O3). The effects of the SPS parameters on the microstructures and mechanical properties of the SPSed AlON ceramics were investigated systematically. This investigation suggested that when the sintering temperature was 1580 ℃, the sintering pressure was 50 MPa, and the holding time was 5 min, the AlON ceramics with excellent mechanical and optical performances were successfully synthesized. They exhibited a high hardness of 18.97 GPa, a good bending strength of 538 MPa, a fracture toughness of 5.88 MPa·m1/2, and particularly, a high transmittance of 76.28% at 750 nm.

Fast fabrication of AlON ceramics with ultra-high transmittance by the assistance of hot isostatic pressing treatment

By employing hot isostatic pressing (HIP) technology, AlON ceramics having ultra-high transmittance up to 88.7% were successfully fast fabricated via dwelling 2.5 h pressureless sintering (PS) and subsequent 2 h in HIP treatment of AlON powder with Y2O3 as additive. Different dwelling durations of PS were conducted to study the mechanism of ultra-high transmittance induced by HIP treatment. The evolution of element distribution, pore size and distribution, relative density and transparency before and after HIP treatment was thoroughly explored. Y concentration in AlON ceramics was observed for the first time. It was found that it was high relative density, pore free (no large pores and much fewer small pores) and no Y concentration that led to the ultra-high transmittance for the fabricated AlON ceramics. Therefore, a rational design of PS and HIP treatment is an effective approach to fabricate AlON ceramics of high transmittance.

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.

Oxidation resistance performance of SiC-AlON ceramic composites at high temperature

The microstructure and oxidation behaviour of the pure AlON ceramics and 8 wt.% SiC-AlON composite were investigated at 700-1500?C. With increasing oxidation temperature, the surface morphology of two ceramics showed a change from acicular to flaky-like and then to granular features. With the addition of nanosized SiC particles, the oxidation resistance of the ceramics was remarkably enhanced above 1100?C. This is attributed to the formation of a dense oxide layer composed of Al2O3, SiO2 and mullite, which could cover the whole matrix and suppress further penetration of oxygen. Due to the dense oxide layer, the oxidation kinetics of the 8 wt.% SiC-AlON composites conformed to a parabolic law, while that of the pure AlON conformed to a linear law. After oxidation at 1500 ?C for 40 h, the weight gain of the 8 wt.% SiC-AlON composites was 3.07mg/cm2, which was only 22.5% of that of the pure AlON.

A comparison of the properties of AlON and LiAlON transparent ceramics prepared by reaction sintering and hot isostatic pressing

The synthesis of LiAlON and AlON ceramics was investigated using reaction sintering of LiAl5O8–Al2O3–AlN and Al2O3–AlN raw powders at 1400–1800 °C. The phase, microstructural evolution, optical transmittance, and lattice structure were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet visible (UV-VIS) and Fourier transform infrared spectra (FTIR), and transmission electron microscopy (TEM), respectively. Transparent LiAlON and AlON ceramics were fabricated by reaction sintering (1750 °C for 20 h) and post-hot isostatic pressing (HIP) in an Ar atmosphere (180 MPa at 1840 °C for 5 h). The results showed that LiAl5O8 promoted the formation of a single oxynitride phase, and single-phase LiAlON and AlON were obtained at 1700 °C and 1750 °C, respectively. LiAl5O8 depressed the volume expansion of the sintered bodies at higher temperatures (≥1600 °C) caused by the chemical reaction between AlN and Al2O3. Volume expansion was evident in the Al2O3–AlN system, but this expansion was eliminated in the LiAl5O8–Al2O3–AlN system. The obtained LiAlON sample had higher in-line transmittance (86.8% at 3.3 μm) and higher flexural strength (approximately 332 MPa). In contrast, the AlON ceramic was less transparent (<4%) and had lower flexural strength (approximately 240 MPa), which was mainly caused by the many residual pores and carbon impurities, as identified by SEM, Ranman spectroscopy, and carbon-sulfur analysis. The Vickers hardness at 0.49–9.8 N loads of oxynitrides exhibited minimal differences.

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

AbstractBy 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.

Investigation of the structural characteristics, dielectric properties, and infrared reflectivity spectra of AlON transparent ceramics

Al(8+x)/3O4−xNx (x = 0.299–0.575) transparent ceramics were fabricated by presintering and subsequent hot isostatic pressing (HIP) technology. The XRD and SEAD analyses verified that a complete solid solution with a cubic spinel structure was achieved in the entire composition range. The SEM characterization confirmed that the x = 0.299−0.510 samples are highly dense with rare residual pores, which was further demonstrated by HRTEM observations. In combination with phonon modes extracted by far-infrared reflectivity spectra, the intrinsic dielectric properties of AlON ceramics were estimated, which were consistent with the experimental results, revealing that the contribution to dielectric properties is dominated by the absorption of structural phonons. Moreover, as the nitrogen content increased, the measured permittivity (εr) of the samples was found to gradually increase from 9.41 to 9.68 at 7 GHz, while the quality factor (Q × f) values fluctuated from 43,750 to 76,923 GHz. Accordingly, the reported AlON transparent ceramics with exceptional dielectric performances are promising candidates for high-frequency 5 G applications.

Hot isostatic pressing of transparent AlON ceramics assisted by dissolution of gas inclusions

In this paper, highly transparent AlON ceramics were fabricated using hot isostatic pressing (HIP) assisted by dissolution of gas inclusions. The influence of additive content, pre-sintering, HIPing and annealing parameters on the sintered AlON ceramics were investigated and optimized. The obtained transparent AlON ceramics kept high transmittance 85.8 % and low loss coefficient 0.005 cm−1 at 2000 nm after annealing. Compared with different sintering additives and pre-sintering atmospheres, densification mechanism of the present material is revealed that SiO was formed from SiO2 during pre-sintering under N2 atmosphere occupying micropores which was then solidified and dissolved into AlON matrix in HIPing processing. This transformation and dissolution of gas into AlON ceramics make it possible to anneal the HIPed ceramics without degradation of microstructure and transmittance.