Alon Research Articles

Formation mechanism of MgAlON phase in aluminum–magnesia composite materials

A magnesia–alumina spinel phase (MgAlSP) and a magnesium–aluminum oxynitride phase (MgAlON) were formed in the closed pores of Al–MgO composite materials after the composites were annealed at high temperature in an oxidizing atmosphere. The MgAlSP and MgAlON phases were detected and investigated by X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry. The results indicated that the MgAlON phase first formed nanoscale particles on the surface of the spinel phase. These particles then interconnected with each other to form particle chain structures. With increasing temperature, the sizes of the MgAlON nanoparticles gradually increased. However, when the temperature was increased to 1550 °C, the chain structure of the particles decomposed and formed a granular MgAlON phase.

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 3h in 200MPa 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 600nm and from 75.4% to 86.1% at 2000nm, respectively. The average grain size of the HIPed sample was about 47.9μm.

Aluminum oxynitride by SHS under high pressure of nitrogen gas

Aluminum oxynitride (γ-AlON) powders were prepared from low-caloric Al–Al2O3 mixtures by SHS under high pressure of nitrogen gas in the presence of heat-generating agent, Mg(ClO4)2, as a booster. The preparation procedure was characterized and optimized with respect to Al/Al2O3 ratio, amount of heatgenerating additive, and nitrogen gas pressure. The morphology of synthesized γ-AlON powders was found to depend on Al powder brand. Compared to γ-AlON powders obtained in chemical furnace, we managed to prepare less sintered arenaceous cakes that could be easily disintegrated into target γ-AlON powder. SHSproduced powders may find their application in production of optical ceramics.

Powders for fabricating polycrystalline optical ceramics: Synthetic procedures (an overview)

Overviewed are existing synthetic procedures for preparation of powders used as raw materials in fabricating optical polycrystalline ceramics such as aluminum oxynitride 9Al2O3 · 5AlN (AlON), yttrium aluminum garnet 5Al2O3 · 3Y2O3 (YAG), and magnesium aluminate spinel Al2O3 · MgO (MAS). Most promising for practical implementation seems to be solution-combustion synthesis from dehydrated gels containing appropriate inorganic salts and organic fuels.

Transparent polycrystalline cubic silicon nitride

Glasses and single crystals have traditionally been used as optical windows. Recently, there has been a high demand for harder and tougher optical windows that are able to endure severe conditions. Transparent polycrystalline ceramics can fulfill this demand because of their superior mechanical properties. It is known that polycrystalline ceramics with a spinel structure in compositions of MgAl2O4 and aluminum oxynitride (γ-AlON) show high optical transparency. Here we report the synthesis of the hardest transparent spinel ceramic, i.e. polycrystalline cubic silicon nitride (c-Si3N4). This material shows an intrinsic optical transparency over a wide range of wavelengths below its band-gap energy (258 nm) and is categorized as one of the third hardest materials next to diamond and cubic boron nitride (cBN). Since the high temperature metastability of c-Si3N4 in air is superior to those of diamond and cBN, the transparent c-Si3N4 ceramic can potentially be used as a window under extremely severe conditions.

Effect of Post-Annealing Treatment on the Structure and Luminescence Properties of AIN:Tb<sup>3+</sup> Thin Films Prepared by Radio Frequency Magnetron Sputtering

Terbium-doped aluminum nitride thin films have been deposited by radio frequency magnetron sputtering. The influence of annealing treatments on structural, morphological and luminescence properties of the films is examined with the aim to optimize post-deposition annealing conditions. Temperatures starting from 500 up to 1000°C using two annealing techniques were investigated: rapid thermal processing and quartz tube furnace. X-ray diffraction analysis revealed the formation of aluminum oxide and aluminum oxynitride phases at temperatures higher than 750°C. The oxygen content in the surface layer was measured with energy dispersive X-ray. The terbium emission was obtained after excitation either by photons or electrons. The films treated with rapid thermal processing at 750°C resulted in the highest emission.

Influence of substrate temperature and gas pressure on aluminum oxynitride coatings obtained by pulsed laser deposition

AbstractThe paper presents results of the investigation on the influence of deposition parameters, such as substrate temperature, total gas pressure and reactive gas composition on the structure, chemical composition and mechanical properties of aluminum oxynitride coatings obtained by pulsed laser deposition (PLD) method. Selection of process parameter ranges, which could be promising for aluminum oxynitride (ALON) coatings deposition, was the main objective of the work. Two series of experiments were carried out with varied pressure and temperature. It was found that from the chemical composition viewpoint, the most promising are atmospheres containing 20 % to 40 % oxygen. The nitrogen to oxygen ratios in the coatings can be controlled by increasing the total pressure or substrate temperature. However, increasing the pressure has a negative effect on the O + N:Al ratio, mechanical properties and quality of the coatings. The influence of temperature is much less drastic and more controllable. Increasing the deposition temperature is much more beneficial since it improves the mechanical properties and can compensate to some extent the negative effect of the total pressure. From the coating quality viewpoint, it is possible to establish an optimum temperature range for which the coatings are characterized by a compact structure and a limited number of droplets.

Structure and photoluminescence of polymer-derived Pr-doped SiAlON red phosphor

The praseodymium (Pr)-doped silicon based aluminum oxynitride (SiAlON) red phosphors were prepared by a novel polymer-derived method using polycarbosilane and acetylacetonates as the starting materials. The organic precursors containing Pr and Al were formed via polymer reactions at 320°C, followed by nitridizing with NH3 at 800°C for the organic-inorganic transformation, and finally by sintering under N2 at 1650°C for the crystallization. Accordingly, the chemical composition, surface morphology, crystal structure and photoluminescence property of the polymer-derived Pr-doped SiAlON phosphors were studied. The strong red emission was observed at 623nm under the excitation of 290nm with the average decay time of 30μs, corresponding to 1D2→3H4 transition. The empirical formula was determined to be Si3.0Al0.2O0.3N3.0C0.03Pr0.02, which is Si rich with trace amount of C. The main crystal phase was identified as hexagonal β-(Si,Al)3(O,N)4:Pr with a space group P63. Each Pr ion was located in the lattice along c-axis by coordinating with three nearest N/O atoms in the neighboring plane. In addition, the amorphous Si rich components were also presented. Possible impure phases, accounted for less than 5%, might include SiC, (Pr2(Al0.5Si2.5O3.5N3.5) and PrSi3N5.

СВС-металлургия оксинитрида алюминия и последующая его очистка от примесей

A promising direction in design of new materials is the development of high-strength transparent ceramics. Examples of fabricating transparent ceramics out of alumomagnesium spinel or leucosapphire are known. However, alumomagnesium ceramics have significantly lower strength than leucosapphire, while the latter is too expensive due to a complexity and low productivity of its technology. One of leading materials for manufacturing high-strength polycrystalline transparent ceramics is aluminum oxynitride composition Al23O27N5 (or Al5O6N). Introduction of nitrogen into aluminum oxide results in a formation of an isotropic cubic crystal structure of oxynitride, stabilizes its matrix and gives it a high strength and crack toughness. The present study aims to establish the scientific bases for a new high-performance low power high-purity aluminum oxynitride powder technology Al23O27N5 (Alon), which includes synthesis of ingots by SHS-metallurgy, subsequent disintegration of ingots to obtain powders as well as magnetic and chemical removal of impurities. In experiments in the process reactor SHS-30, it was found that an increase of the weight of a starting mixture from 100 g to 3 kg leads to a noticeable change in the desired product composition: nitrogen content increases from 7.7 to 10.8 % N, and Al and O content correspondingly decreases. According to X-ray phase analysis, the main phase in cast oxynitrides is Al5O6N. An appreciable content of Al7O3N5 was also detected. A method for disintegration of cast aluminum oxynitride has been developed, and optimal grinding parameters have been determined that make it possible to obtain powders with a particle size of less than 100 μm. After grinding, the particles are fragmented. In the integral composition of the particles there are traces of milled steel balls, the material of which is spread over the surface of Alon particles. The magnetic and chemical cleaning practically completely removes impurities (Mo, Al, Fe, AlN) arising due to the incompleteness of chemical transformation, product contamination and milling of steel when grinding.

Oxynitrides by aluminothermic SHS in nitrogen gas: Influence of nitrogen pressure

Cast aluminum oxynitrides were prepared by aluminothermic SHS under pressure of nitrogen gas and characterized by time-resolved XRD and chemical analysis. Burning velocity U, material loss caused by sputtering η1, and yield of target material into ingot η2 were measured as a function of nitrogen pressure P(N2). The nitrogen content of synthesized oxynitrides was made as high as 7 wt %. The results may turn helpful in designing high-strength transparent ceramics.

Wear resistance performance of boron nitride coatings on end milling cutters

For defence industry applications, the machining of ultra-high strength steels or aluminium with ballistic temper grades is a widely-used operation. Therefore, it is a great challenge to find an optimum solution for tool wear. In that sense, tool coatings provide various solutions for machining of hard materials or improving wear resistance. A cost-effective tool coating provides longer tool life, hence decreases the tool cost contribution in production. Boron nitride (BN) coating could be thought as a new generation coating method compared to titanium nitride (TiN), aluminium titanium nitride (AlTiN) or silicon aluminium oxy-nitride (SiAlON). BN coating material has a great strength, toughness and chemical stability whereas it has an important disadvantage of high intrinsic stress. In this industrial study, standard end mill cutters are coated with BN material by physical vapour deposition (PVD) method. The coated and uncoated milling tools are compared according to their wear resistance and the obtained workpiece surface roughness. First results of the study are presented in this paper.

Wear resistance performance of boron nitride coatings on end milling cutters

For defence industry applications, the machining of ultra-high strength steels or aluminium with ballistic temper grades is a widely-used operation. Therefore, it is a great challenge to find an optimum solution for tool wear. In that sense, tool coatings provide various solutions for machining of hard materials or improving wear resistance. A cost-effective tool coating provides longer tool life, hence decreases the tool cost contribution in production. Boron nitride (BN) coating could be thought as a new generation coating method compared to titanium nitride (TiN), aluminium titanium nitride (AlTiN) or silicon aluminium oxy-nitride (SiAlON). BN coating material has a great strength, toughness and chemical stability whereas it has an important disadvantage of high intrinsic stress. In this industrial study, standard end mill cutters are coated with BN material by physical vapour deposition (PVD) method. The coated and uncoated milling tools are compared according to their wear resistance and the obtained workpiece surface roughness. First results of the study are presented in this paper.

Tris(dimethylamido)aluminum(III): An overlooked atomic layer deposition precursor

Aluminum oxide and aluminum nitride-containing films were grown by atomic layer deposition (ALD) and plasma-enhanced atomic layer deposition (PE-ALD) by employing an under-utilized tris(dimethylamido)aluminum(III) precursor. This compound has not been reported as a precursor for ALD of alumina previously, and has only been reported as an AlN precursor for a thermal process using ammonia as a coreagent. Thermogravimetric analysis demonstrates its excellent volatility and thermal stability, both of which are ideal characteristics for an ALD precursor. Aluminum oxide films were deposited thermally using water as a coreagent. By x-ray photoelectron spectroscopy, the films appeared nearly pristine with only adventitious carbon on the surface accumulated postdeposition that was easily removed with 2 min of Ar+ sputtering. The rest of the films contained a very low 1.4% impurity of carbon. Aluminum nitride films were attempted using the same aluminum precursor with nitrogen plasma as a coreagent; they contained large amounts of oxygen due to ambient exposure, possible oxidation during characterization, or the presence of incidental oxygen during the deposition of AlN, which allowed the formation of an aluminum oxynitride. Though the composition was not stoichiometrically AlN, the films also contained ∼1% carbon impurities, which is an improvement over many other AlN films reported, particularly those using TMA as a precursor. This precursor shows great promise for the deposition of low-impurity or impurity-free aluminum nitride by PE-ALD.

Combining 27Al Solid-State NMR and First-Principles Simulations To Explore Crystal Structure in Disordered Aluminum Oxynitride.

The nuclear magnetic resonance (NMR) technique gives insight into the local information in a crystal structure, while Rietveld refinement of powder X-ray diffraction (PXRD) sketches out the framework of a crystal lattice. In this work, first-principles calculations were combined with the solid-state NMR technique and Rietveld refinement to explore the crystal structure of a disordered aluminum oxynitride (γ-alon). The theoretical NMR parameters (chemical shift, δiso, quadrupolar coupling constants, CQ, and asymmetry parameter, η) of Al22.5O28.5N3.5, predicted by the gauge-including projector augmented wave (GIPAW) algorithm, were used to facilitate the analytical investigation of the 27Al magic-angle spinning (MAS) NMR spectra of the as-prepared sample, whose formula was confirmed to be Al2.811O3.565N0.435 by quantitative analysis. The experimental δiso, CQ, and η of 27Al showed a small discrepancy compared with theoretical models. The ratio of aluminum located at the 8a to 16d sites was calculated to be 0.531 from the relative integration of peaks in the 27Al NMR spectra. The occupancies of aluminum at the 8a and 16d positions were determined through NMR investigations to be 0.9755 and 0.9178, respectively, and were used in the Rietveld refinement to obtain the lattice parameter and anion parameter of Al2.811O3.565N0.435. The results from 27Al NMR investigations and PXRD structural refinement complemented each other. This work provides a powerful and accessible strategy to precisely understand the crystal structure of novel oxynitride materials with multiple disorder.

Effect of nitrogen incorporation into Al-based gate insulators in AlON/AlGaN/GaN metal–oxide–semiconductor structures

The superior physical and electrical properties of aluminum oxynitride (AlON) gate dielectrics on AlGaN/GaN substrates in terms of thermal stability, reliability, and interface quality were demonstrated by direct AlON deposition and subsequent annealing. Nitrogen incorporation into alumina was proven to be beneficial both for suppressing intermixing at the insulator/AlGaN interface and reducing the number of electrical defects in Al2O3 films. Consequently, we achieved high-quality AlON/AlGaN/GaN metal–oxide–semiconductor capacitors with improved stability against charge injection and a reduced interface state density as low as 1.2 × 1011 cm−2 eV−1. The impact of nitrogen incorporation into the insulator will be discussed on the basis of experimental findings.

(Invited) Gate Stack Technology for Advanced AlGaN/GaN Mos-Hemt Power Devices

AlGaN/GaN high electron mobility transistor (HEMT) has gained much attention as next-generation high frequency and high power devices. Since AlGaN/GaN HEMT with Schottky gate is restricted in device application due to large gate leakage current and normally-on operation, metal-oxide-semiconductor (MOS) gate stack with deposited gate insulator has been widely investigated to overcome these limitations. Among various insulating materials, aluminum based oxide such as Al2O3 is one of the potential candidate because of its wide bandgap and high thermal stability. However, large amount of interface traps at MOS interfaces severely degrade both carrier mobility and threshold voltage (Vth) stability. Furthermore, significant electron injection into Al2O3 layer causing positive Vth shift was often observed for Al2O3/AlGaN/GaN MOS-HEMT. We have recently reported that N incorporation into Al2O3 (aluminum oxynitride) significantly reduces not only the negative fixed charges but also the electron traps in Si and SiC MOS devices. In this study, superior physical and electrical properties of AlON gate dielectrics on AlGaN/GaN substrates were demonstrated. We fabricated high-quality AlON/AlGaN/GaN MOS capacitors with improved stability against charge injection and a reduced interface state density. Moreover, the impact of nitrogen incorporation into the gate insulator will be discussed on the basis of results from physical characterizations.

(Cr,Al)N/(Cr,Al)ON Oxy-nitride Coatings deposited by Hybrid dcMS/HPPMS for Plastics Processing Applications

In plastics industry injection molding and extrusion tools are subjected to adhesive and abrasive wear by the flowing hot melt during extrusion process and adhering solidified melt. Due to their beneficial properties, Cr-based nitride hard coatings deposited by physical vapor deposition (PVD) are applied as protective coatings. In this regard, especially Cr-based oxy-nitride coatings with increased oxygen contents have a high potential to be used on plastic processing tools. In the presented work, five different oxy-nitride monolayer and bilayer coatings (Cr,Al)N/(Cr,Al)ON were synthesized on tool steel substrate AISI 420 (X42Cr13, 1.2083) by means of a hybrid direct current and high power pulsed magnetron sputtering (dcMS/HPPMS) process. Further, the coating architecture was varied in order to compare oxy-nitride monolayers with nitride coatings comprised of thin oxy-nitride top layers. All coatings were investigated with regard to their coating properties and, as defined by the corresponding coating/substrate interactions, with regard to their compound properties. The interactions between the coatings and Makrolon® 2408 polycarbonate (PC2408) were also characterized by means of high temperature contact angle and adhesive tensile strength measurements. Complementary, in-depth chemical analysis of the surface oxide regions of the coatings by means of X-ray photoelectron spectroscopy (XPS) was performed and revealed that the surface oxide composition is primarily determined by the oxygen concentration in the coating bulk. Further, this resulted into high contact angles of the polycarbonate melt, which is desirable for injection molding. All coated samples exhibited a better demolding behavior of the solidified plastic melt as compared to uncoated AISI 420.

Effect of carbon black content and particle size on the phase evolution of aluminum oxy nitride (AlON)

In this work aluminum Oxy-nitride (γ-AlON) crystals were synthesized by Co-precipitation processing from Al(NO3)3 solution, magnesium acetate and nanosized carbon particles as the source of Al3+, Mg2+ and reducing agent, respectively. The effects of the nanosized carbon and Mg2+ content and particle size on the formation of AlON phase were investigated by means of XRD, SEM and EDS analysis. Our results indicated that AlON powder could be synthesized when the resultant precursors were calcined at 1700°C for 2h under flowing N2. Under optimal additional content of C (7.1wt%), the resultant AlON powders exhibited the primary particle size of about 2–4µm with a specific surface area of 3m2/g.

In-situ hot pressing sintering behaviors of Y2O3-La2O3 co-doped AlON ceramic

Aluminum oxynitride (AlON) ceramic was fabricated by in-situ hot pressing sintering (HPS) from Al2O3and AlN doped with Y2O3 and La2O3 as well as without dopant for comparison. Dense AlON ceramic with a relative density of 99.7% was obtained by HPS doped with 0.4wt% Y2O3 and 0.1wt% La2O3 at 1700°C for 4h. The maximum fracture toughness of AlON ceramic with and without dopant was 3.74MPam1/2 and 2.27MPam1/2, respectively. The maximum Vickers hardness of AlON ceramic with and without dopant was 18.3GPa and 16.5GPa, respectively. Compared with those of undoped AlON ceramic, the relative density and the fracture toughness of Y2O3-La2O3 co-doped AlON ceramic were obviously improved. Overlarge grain size would reduce the Vickers hardness of AlON. The sintering activation energy of Y2O3-La2O3 co-doped AlON was about 57% of that of undoped AlON. The sintering activity of AlON was highly improved by co-doping with Y2O3 and La2O3.

Synthesis and characterization of aluminum Oxy nitride (AlON) from the nanosized gel precursor

The purpose of this research was the synthesis of aluminum Oxy-nitride (ALON) nanocrystals with through the alkoxide sol-gel processing method. For the synthesis of the nanosized precursor, sucrose (SU, as a source of reducing and gel agent), carbamide (CR, to adjust the pH and source of atomic nitrogen), aluminum buthylate (as the source of Al3+), and magnesium acetate (as the source of Mg2+ and the doping agent) were used. Various characterizations including X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), diffusion reflectance spectroscopy (DRS spectrum), energy dispersive X-ray spectroscopy (EDX) and field emission scanning electron microscopy (FESEM) were used to characterize the as-synthesized samples. Our results indicated that the precursor solution containing SU:CR with the molar ratio of 1:3M and 5% magnesium acetate led to AlON phase. This phase could be obtained after a special three-step calculation schedule of nanosized precursor consisting of 1h at 1000°C, 1h at 1300°C and 3h at 1700°C.