Highly transparent cubic γ-Al2O3 ceramic prepared by high-pressure sintering of home-made nanopowders

Highly transparent gamma-aluminum oxide (γ-Al₂O₃) ceramics were fabricated for the first time, by combining homogeneous precipitation and high-pressure sintering in the absence of exogenous dopants. The resulting cubic γ-Al₂O₃ transparent ceramic material exhibits a promising replacement for single-crystal sapphire. The optimum optical properties are achieved in response to sintering at 5 GPa and a temperature of 300 °C and include maximum transmittance of 86% in the range of 0.6–1.2 µm which are properties that are comparable to those of single-crystal sapphire (∼86%). Vickers hardness (16 GPa) and compressive strength (350 MPa) in response to high-pressure sintering are also similar to those of a conventional sapphire single crystal. Meanwhile, the dielectric constant (9.46) is comparable to that of sapphire in the C-axis direction. These findings will facilitate further development of transparent Al₂O₃ ceramics for use in a wider range of optical applications.     

One-step carbothermal reduction and nitridation (CRN) for fast fabrication of fine and single phase AlON powder by using hydrothermal synthesized α-Al2O3/C mixture

Glucose and ball-milled α-Al₂O₃ mixed powder was hydrothermal treated to synthesize Al₂O₃/C mixture, which was directly heated to 1700–1750 °C for 10 min holding in N₂. Fine and single phase AlON powders were successfully fast prepared by one-step carbothermal reduction and nitridation (CRN). With aggregation of as-purchased α-Al₂O₃ being effectively disconnected by ball milling, α-Al₂O₃ particles were perfectly coated with carbon by hydrothermal treatment, which inhibited coalescence of Al₂O₃ during heating and enabled fast formation of AlON to further shorten holding duration. As a result, one-step fast fabricated pure AlON powder has fine primary particles with weak interconnections. At 170 rpm, powders were ball-milled to D₅₀ ≤ 1.00 μm for 4 h, and D₅₀ = 0.33 μm for 24 h. Moreover, primary particle size is controllable by adjusting holding time, synthesis temperature and heating speed. One-step CRN method provides a practical way to prepare pure AlON powder with the required primary particles.     

Effect of processing parameters on the development of anisotropic α-Al2O3 platelets during molten salt synthesis

It is of utmost importance to control the size and morphology of anisotropic α-Al₂O₃ platelets owing to its extensive usage in various applications, including most recent synthetic biomimetic abalone nacre-like composites. In the present article, a wide range of well-developed and well-dispersed α-Al₂O₃ platelets were produced with diameter and thickness ranging from 4 - 8 μm and 0.6–1 μm respectively, by taking γ-Al₂O₃ precursor and sodium sulfate as low melting flux. The effects of various processing parameters like calcination temperatures, molar ratios of Al/Na and soaking time on the phase formation and morphology development of the final platelets were investigated in rigor. The results indicated that high calcination temperature of 1200 °C and moderate soaking time (60 min) helped to develop α-Al₂O₃ platelets with hexagonal morphology and uniform sizes. The sulfate salts created favorable conditions for growth of anisotropic platy α-Al₂O₃ and Al/Na molar ratio has significant effect in controlling the aspect ratio of the platelets. The impact of deagglomeration treatment by the ultrasound irradiation on the calcined aggregates of platelets was also examined. The breakage of platelets clusters by fracture/erosion due to the interaction with imploding cavitation bubbles under ultrasound energy, reached maximum after 90 min of deagglomeration treatment. The detail phase transformation sequences from γ-Al₂O₃ to α-Al₂O₃ was explained by correlating thermal (DT-TGA) and X-ray analysis. The growth mechanism of the anisotropic α-Al₂O₃ particles in the presence of molten Na₂SO₄ liquid was also unraveled.     

Combustion of Al–Al2O3 mixtures in air

An experimental study on the aluminum oxynitride and aluminum nitride formation by combustion of mixtures of micron-sized aluminum powder (average particle diameter a_s ∼9.0 μm) and alumina nanopowder (a_s ∼0.05 μm) of the fixed mass (∼10 g) and different mass ratios (Al/γ-Al₂O₃ = 0.1–19.0) in air is reported. The formation of aluminum oxynitride (Al₃O₃N) and aluminum nitride during the combustion of powdery aluminum-based mixtures in air is discussed in this study. The combustion synthesis of Al₃O₃N and AlN was carried out in self-sustaining way. XPS-FESEM, XRD and chemical analysis were executed on final products of synthesis. The combustion process was also recorded by a video-camera. It was found that powdery mixtures, ignited by local heating, burned in one- or two-stage self-propagating regime. The combustion regime is different for different initial mass ratios Al/γ-Al₂O₃ and mainly depends on the content of fuel (aluminum powder) in mixture.     

High temperature creep and tensile properties of alumina formed on Fecralloy foils doped with yttrium

The three batches of Fecralloy foils, which differ from each other in contents of yttrium, that is, 10, 280 and 560 ppm, respectively, were chosen as the thermally grown oxide (TGO), alumina (α-Al₂O₃) forming substrate. The creep tests were performed with the Fecralloy foils, which have the α-Al₂O₃ TGO of 0–4 μm thickness, on the surfaces. The creep rates decreased as the TGO thickness increased. The yttrium content above 280 ppm delayed the creep rate of the Fecralloy substrate. The higher creep rate than that of the stand-alone polycrystalline alumina and the dependency of the creep rate on TGO thickness agrees with the hypothesis that the high temperature creep of the TGO is a consequence of the inter-grain growth of the TGO. The yttrium content of lower than 560 ppm did not affect on the creep rate of TGO. Tensile tests were performed with the same alloys, which have the α-Al₂O₃ TGO of 0–3 μm thickness. The tensile strength of the substrate itself increased with Y content by ～20%. The tensile stress of the α-Al₂O₃ TGO decreased with Y-content but it is almost constant, regardless of the TGO thickness. The peak stresses were found at the strain range of ? = 0.7–1.5%, regardless of the TGO thickness, and the batches, thereafter, the parallel cracks perpendicular to the loading direction, formed on the surface. The obtained stress–strain curves of TGO fluctuated. It showed a common feature, that is, a sharp stress drop after the initial yield point at σ_Y = 40–85 MPa, but the stress increased again until the peak points, σ_UTS = 50–110 MPa.     

Effects of electron-beam irradiation,a thin-Ti layer,and a BeO additive on the diffusion of titanium in synthetic sapphire

Titanium was allowed to diffuse into synthetic sapphire (α-Al₂O₃) at 1773–1923 K for 200 h in air. Specimens were prepared by four different methods. Samples were irradiated with a 10 MeV electron beam to fluencies of 2×10¹⁷ cm⁻² for 1 h to induce vacancy formation. A 1-μm layer of titanium was sputtered onto sapphire samples to provide intimate contact with the diffusing elements. Ti diffusion was performed using TiO₂ powder or a mixture of TiO₂ and BeO powders in a ratio of 95:5 to take advantage of the beryllium activity. Ti diffusion was profiled using scanning electron microscope-energy dispersive X-ray spectrometry (SEM–EDX). The diffusion coefficients of Ti were as follows:     

Advanced spinel and sub-μm Al2O3 for transparent armour applications

Hardness is important for a high ballistic strength, and with HV10 = 20–22 GPa sintered sub-μm Al₂O₃ is the hardest of all transparent materials for compact windows. However, light transmission through polycrystalline Al₂O₃ is limited by birefringent scattering losses: high transmissions are known at larger IR wavelengths for grain sizes of about 0.5 μm but the visible real in-line transmission RIT is only 70–75% of the theoretical maximum at 0.8–1 mm thickness. These losses will be the higher for thicker components whereas a safe ballistic performance requires 1.5–2 mm thickness at least. New technologies bring the transmission closer to the limit associating grain sizes of 0.3 μm with an RIT of 84–93% of the theoretical maximum (thickness 0.8 mm). However, even these extreme results give again rise to doubt that it will ever be possible to manufacture larger and thicker Al₂O₃ windows with a sufficiently high transparency.On the other hand, new results are presented for fine-grained spinel with RIT close to the theoretical maximum and with a hardness that approaches sapphire. In first ballistic tests this spinel outperformed sapphire of different orientations. It is, therefore, suggested that sub-μm Al₂O₃ may be a good choice for IR windows or as armour for low threat applications where thinner tiles can be used. Most threats, however, require thicker windows where the new spinel appears as one of the most favourable candidates.     

Effects of Cr content and morphology on the luminescence properties of the Cr-doped alpha-Al2O3 powders

Al₂O₃:Cr³⁺ samples were synthesized via hydrothermal and microwave solvothermal methods and thermal decomposition of Cr³⁺ doped precursors. The sample characterizations were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD results indicated that Cr³⁺ doped samples were pure α-Al₂O₃ phase after being calcined at 1573 K. SEM results showed that the length and diameter of these Cr³⁺ doped alumina microfibers by hydrothermal route were about 2–5 μm and 100–300 nm, respectively; the obtained α-Al₂O₃ based powders via the microwave solvothermal method were microspheres with an average diameter about 1–2 μm. PL spectra showed that the Al₂O₃:Cr³⁺ samples presented a broad R band at 696 nm. It is shown that the 0.3 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ is optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 24 Å. It is found that the curve followed the single-exponential decay. Furthermore, the luminescence properties of the samples are also dependent on the morphology.     

Controllable preparation of alumina nanorods with improved solid electrolyte electrochemical performance

Alumina powders have been widely used in lithium-ion batteries such as separator coating, electrode surface modification and electrolyte fillers. Rod-like alumina with its special aspect ratio is expected to open up a new application direction. In this work, alumina nanorods were prepared by a facile hydrothermal method. The aspect ratio and morphology of alumina nanorods were optimized by adjusting the hydrothermal temperature, hydrothermal synthesis time, volume ratio, directing agent, and sintering temperature. γ-Al₂O₃ nanorods with a diameter of 200–300 nm and a mean length of 5 μm and α-Al₂O₃ with a diameter of 100–200 nm and mean length of 5 μm were obtained by calcining the alumina precursor (AACH) at 800 °C and 1200 °C, respectively. The prepared alumina nanorods were added into polymer solid electrolyte, which promoted the dissociation of the lithium salt and stabilized the propylene polycarbonate (PPC) polymer, resulting in an improved potential window (4.5 V) and ionic conductivity (3.7 × 10⁻⁴ S/cm) of the PPC-based polymer solid electrolyte (SE). An NCM622/SE/Li solid-state battery showed enhanced electrochemical performance at ambient temperature with an initial discharge capacity of 188.5 mAh/g and a retention capacity of 158.1 mAh/g after 200 cycles at a current density of 0.5 C. These alumina nanorods have potential to be widely used in high-performance solid electrolytes.     

High strength and high light transmittance sapphire/sapphire joints bonded using a La2O3-Al2O3-SiO2 glass filler

A novel La₂O₃-Al₂O₃-SiO₂ (LAS) glass was used as filler to join transparent sapphire for obtaining high strength and high light transmittance joints. The results show that the LAS glass filler had compatible coefficient of thermal expansion (CTE) with sapphire and excellent wetting ability on sapphire. During the joining process, no interfacial reaction occurred and the brazing seams were in a completely amorphous state under fast cooling conditions (~50 °C/min). With increased joining temperature, the mutual dissolution and diffusion between sapphire and the LAS filler were enhanced. The flexural strength of joints first increased and then decreased with an increase in the joining temperature from 1400 °C to 1550 °C. The optimal flexural strength of joint reached 325 MPa, which almost was the same as the strength of sapphire substrate. At 500 nm, the in-line transmittance of this joint was 80.5%, which was close to that of sapphire (84.2%).     

Narrowing size distribution widths of small α-Al2O3 nanoparticles by addition of large nanoparticles in coagulation separation

α-Al₂O₃ nanoparticles separated by fractionated coagulation still have broad size distributions which limit their wider applications. By adding 20-time mass of large α-Al₂O₃ (40.5 nm) into α-Al₂O₃ nanoparticles to be separated in coagulation separation, the average size of separated α-Al₂O₃ nanoparticles decrease from 6.6 nm without addition of large α-Al₂O₃ NPs to 4.4 nm, and the size distribution changes from 3–10 nm without addition of large α-Al₂O₃ NPs to 3–6 nm. With increasing amount of large α-Al₂O₃ NPs added, separated α-Al₂O₃ NPs exhibit smaller average sizes and narrower size distribution widths at the same separation concentrations. This approach may be applied to narrow size distribution widths in large-scale size-selective separations of other nanoparticles.     

Sintering of glass matrix composites with small rigid inclusions

We investigated the effect of dispersed crystalline particle volume content Φ on sintering of glass matrix composites (GMC) for low-temperature co-fired ceramics (LTCC) applications. Such composites typically consist of alumo-borosilicate glass and α-Al₂O₃ powders of similar average particle size (D₅₀ ≈ 3 μm). Sintering shrinkage was observed by dilatometry and heating microscopy and was backed up by glass viscosity measurements. Microstructure analysis revealed that α-Al₂O₃ particles do neither show significant dissolution into the liquid phase nor detectable crystallization throughout LTCC firing schedules. Therefore, in this study α-Al₂O₃ particles were treated as small rigid inclusions. It was found that Φ lowers the shrinkage rate of GMC. While the lowering is small for small Φ and at the early stage of densification it progressively increases during sintering, and final shrinkage shifts up to 170 K to higher temperatures for Φ = 0.45. The behaviour observed could be explained assuming that sintering is controlled by the effective viscosity, which progressively increases non-linearly during densification due to the gradually wetting of the surface area of corundum particles. We could demonstrate that Al₂O₃ cluster can cause residual pores and reduce the attainable shrinkage. The reduction of attainable shrinkage is found to depend on Φ³, reaching about 8% at Φ = 0.45.     

Novel synthesis and characterization of nanosized γ-Al2O3 from kaolin

This paper reports the synthesis of nanosized γ-Al₂O₃ from acid-leachates of calcined kaolin. Al (hydr)oxide was precipitated with ammonia from the leachate in the presence of polyethylene glycol. A white powder of nanosized γ-Al₂O₃ particles was obtained after calcination. X-ray diffraction (XRD), different scanning calorimetries and thermogravimetry (DSC-TG), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and magic angle spinning nuclear magnetic resonance (MAS NMR) were used to characterize the samples. Typical nanosized γ-Al₂O₃ particles showed rod-like morphology with width of about 7 nm and length of approximately 20 nm. A possible mechanism from kaolin to nanosized γ-Al₂O₃ is proposed.     

Influence of fluorides on phase transition of α-Al2O3 formation

Homogeneous α-Al₂O₃ particles have been synthesized by introducing fluorides in the alumina precursor. The effects of LiF, ZnF₂ and AlF₃ additives on the phase transformation and the micrograph of the prepared α-Al₂O₃ particles are investigated. It is shown that, at a heating rate of 5 °C min⁻¹, addition of 2% fluoride relative to the initial mass of the precursor decreases the transformation temperature by 300 °C for LiF and AlF₃ and well-dispersed α-Al₂O₃ powders with average particle size of ∼2 μm were obtained. The mechanism of the influence can be explained by the formation of intermediate compound (AlOF), which was considered to accelerate the mass transportation from θ- to α-Al₂O₃. The addition of ZnF₂ can slight reduce the θ- to α-Al₂O₃ phase transition temperature. However, the effect on the phase transition of α-Al₂O₃ formation is not obvious.     

Synthesis and characterization of nanocomposite Fe3O4/SiO2 core–shell abrasives for high-efficiency ultrasound-assisted magneto-rheological polishing of sapphire

A functional Fe₃O₄/SiO₂ core–shell abrasive was synthesized via hydrolysis of tetraethyl orthosilicate. A silica shell was successfully coated on a Fe₃O₄ core, resulting in a core-shell particle with an average diameter of 140 nm. The prepared core–shell abrasives was utilized for ultrasound-assisted magneto-rheological polishing (UAMP) of sapphire substrate. The experimental results showed that the Fe₃O₄/SiO₂ core–shell abrasives exhibited a remarkable polishing performance for the sapphire material, resulting in smooth and detect-free surfaces with a high material removal rate (MRR) compared to mixed abrasives (Fe₃O₄ and SiO₂) and pure Fe₃O₄ particles. The application of ultrasonic vibration to the sapphire wafer further improved the MRR, which was approximately 3.4 times higher than that of traditional magneto-rheological polishing. The largest MRR (1.974 μm/h) and comparatively low surface roughness (0.442 nm) of the polished sapphire wafer were achieved by UAMP with the Fe₃O₄/SiO₂ core–shell abrasives. The polishing mechanism of the sapphire wafer is discussed in terms of chemical reactions and mechanical polishing.     

CO2 laser peeling of Al2O3 ceramic and an application for the polishing of laser cut surfaces

A laser controlled fracture peeling technique is demonstrated to smooth the Al₂O₃ ceramic surface without thermal damages. It was found that a chip can be separated and curled from the ceramic surface during a focused CO₂ continuous wave (CW) laser dual-scanning. The thickness of the curled chip is ～50 μm and the formed subsurface roughness (R_a ≈ 2 μm) is close to the surface machined by mechanical breaking (R_a = 1.84 μm). The chip formation is attributed to the controlled fracture by the residual tensile stress in the recast layer, whereas the chip curling only occurs when the melting depth is shallower than the position of lateral cracks. The peeling technique can be applied to polish the cut surface of laser fusion cutting in ceramics. The polished cut surface (R_a = 2.18 μm) is free from recast, crack and heat effects. The microstructure is similar to the base material. The material removal rate during polishing is up to 0.125 mm³/s.     

Intense emission at 2.9 μm from Yb3+/Ho3+ co-doped TeO2–Ga2O3–ZnO tellurite glasses

Mid-infrared lasers have important applications in infrared countermeasures, sensing, environmental monitoring, biomedicine, and many military and civilian fields. In this work, an intense emission at 2.9 μm from Yb³⁺/Ho³⁺ co-doped TeO₂-Ga₂O₃-ZnO (TGZ) glass was reported. The 2 μm, 1.2 μm and visible emissions were also performed to understand the competitive luminescent mechanism. With the increase in Yb³⁺ concentration, all the emissions of Ho³⁺ increased, whereas the emission of Yb³⁺ decreased due to the phonon-assisted energy transfer from Yb³⁺ to Ho³⁺. The lifetimes of optimized 3 mol% Yb₂O₃ and 1 mol% Ho₂O₃ co-doped TGZ glass, which has the maximum emission intensity, are 548 μs and 1.7 ms at 2.9 and 2 μm, respectively. The Judd–Ofelt intensity parameters, absorption, and emission cross sections were calculated to evaluate the mid-infrared fluorescence properties of this new glass matrix material. The gain coefficients show that the 2 and 2.9 μm laser gain can be realized by small pump energy, indicating that this glass is a promising medium for the mid-infrared optical fiber laser.     

Structure characterisation and mechanical properties of crystalline alumina coatings on stainless steel fabricated via sol–gel technology and fibre laser processing

The surface modification of stainless steel by coating with alumina (Al₂O₃) was carried out using sol–gel coating technology in combination with laser processing. Alumina coatings have been synthesised via a sol–gel route and deposited on stainless steel substrates by dip coating. The coated substrates were then treated with pulsed ytterbium fibre laser radiation (λ = 1064 nm) in continuous wave mode with different specific energies. The composition and structure of the coated surfaces after laser processing were characterised by ATR-FTIR, XRD, SEM and contact angle measurements, whilst the mechanical properties of modified surfaces were determined using nano-indentation. The results showed that the alumina xerogel films coated on the substrates are successfully converted into crystalline alumina ceramic coatings by the laser irradiation, the structure of resulting coatings being dependent on the irradiation conditions, with increase of laser specific energy leading to the formation of initially γ-Al₂O₃ with increasing amounts of α-Al₂O₃ at higher energy. Nano-indentation results reveal that the laser processing results in significant improvement in hardness and Young's modulus of the alumina-coated surface and, at optimum, can achieve the mechanical properties at the same level as pure α-alumina ceramic, much higher than those of the as-dried xerogel coating.     

Preparation of Al2O3–ZrO2 nanocomposite films by laser chemical vapour deposition

Alumina (Al₂O₃)–zirconia (ZrO₂) nanocomposite films were prepared by laser chemical vapour deposition. α-Al₂O₃–ZrO₂ and γ-Al₂O₃–t-ZrO₂ nanocomposite films were prepared at 1207 and 1000 K, respectively. In the nanocomposite films, 10-nm-wide t-ZrO₂ nanodendrites grew inside the α- or γ-Al₂O₃ columnar grains. The γ-Al₂O₃–t-ZrO₂ nanocomposite films exhibited high nanoindentation hardness (28.0 GPa) and heat insulation efficiency (4788 J s^−1/2 m⁻² K⁻¹).     

Microstructure and thermoelectric properties of α-and γ-Al2O3 doped ZnO under high pressure

Using high pressure and high temperature (HPHT) technology to improve the thermoelectric properties of oxides is a feasible solution. To further understand the micro-physical mechanism improving the thermoelectric performance of ZnO in a higher pressure environment, the effects of α and γ lattice structure Al₂O₃ (α-Al₂O₃, γ-Al₂O₃) on doped ZnO were systematically studied. ZnO samples with different Al doping ratios (α-x, γ-x; x = 0.02, 0.04, 0.06, 0.08) were prepared by HPHT. Test results show that the high pressure and high temperature synthesis method effectively improves the solid solubility of α-Al₂O₃ and γ-Al₂O₃ in ZnO, among which γ-Al₂O₃ is more easily dissolved into the zinc oxide crystal lattice. Under the same doping ratio, the electronic conductivity of the sample synthesized with γ-Al₂O₃ is higher than that of the sample synthesized with α-Al₂O₃. In addition, the ZnAl₂O₄ phase precipitated out of the sample with increased Al doping. Although the ZnAl₂O₄ phase decreases the electrical properties of the sample, a small amount of ZnAl₂O₄ is uniformly dispersed in the ZnO sample, which can inhibit the growth of crystal grains, and assist grain refinement. The optimized high pressure sintering temperature was 1123 K, and the zT value of γ-0.04 was 0.17 at 973 K.