Comparison of deoxidation capability on the specific surface area of irregular titanium powder using calcium reductant

Irregular titanium powders of various particle size in the range 60–250?μm were deoxidized using calcium as the reductant, and the effect of specific surface area on the process was investigated. At 1273?K, the oxygen concentration was reduced from 0.2840?wt% to 0.0950?wt%, from 0.2050?wt% to 0.0825?wt%, and from 0.1700?wt% to 0.0825?wt% in titanium powders with average particle sizes of 60?μm, 125?μm, and 250?μm, respectively, and the corresponding removal degree of oxygen (RDO) for these specimens was determined to be 66.5%, 59.7%, and 51.4%; that is, the RDO was inversely proportional to the average particle size of the irregular titanium powder. It was confirmed that the specific surface area of the titanium powder available for reaction with calcium, was the critical factor in the deoxidation of titanium powder using calcium as reductant.     

The use of yttrium (III) isopropoxide to improve thermal conductivity of polycrystalline aluminum nitride (AlN) ceramics

Instead of Y₂O₃ powders, yittrium isopropoxide (YIP) was used as a sintering additive to sinter high thermal conductivity polycrystalline aluminum nitride (AlN). The reasons for using sintering additive in sol-gel form are due to the fact that the particle sizes are uniform in the nano scale and also they promote a better coating of AlN grains, being more effective during sintering process. The binder burn out was carried in two different atmospheres, N₂ (N₂ BBO) and air (air BBO). The thermal conductivity of dense polycrystalline aluminum nitride samples with the addition of Y₂O₃ (YIP formulation) ranging from 1.0 to 10.0 wt% with N₂ BBO and air BBO was measured by the laser-flash technique. The results of measured thermal conductivity exhibited higher values than those reported for samples of same yttria formulation (Y₂O₃ powder) and sintered conditions.     

Titanium diboride particle-reinforced aluminium with high wear resistance

A TiB₂ particle (61 vol%, 4 m mean size) reinforced aluminium fabricated by liquid-aluminium infiltration was subjected to unlubricated rolling wear and was found from the weight loss to be 1.5 times more wear resistant than 17-4 ph stainless steel, twice as wear resistant as 1020 steel, 7.5 times more wear resistant than 2024 aluminium, and 12.8 times more wear resistant than the aluminium matrix. This wear resistance is attributed to the lack of particle pull-out and the ability of the TiB₂ particles to protect the softer underlying matrix from abrasion. This composite was approximately three times more wear resistant than AlN particle (50 vol%)-reinforced aluminium. The greater wear resistance of Al/TiB₂ compared to Al/AlN is due to the slow wear of the TiB₂ particles and the AlN particle pull-out. A slight decline in tensile strength and no effect on the modulus was observed in Al/TiB₂ after heating at 300 or 600°C for 240 h. This high-temperature stability is attributed to the lack of reactivity between TiB₂ and the aluminium matrix.     

泡沫前驱体碳热还原氮化法合成氮化铝粉体

本研究采用碳热还原氮化法(CRN)合成AlN粉体。以γ-Al₂O₃和炭黑为原料, 采用直接发泡工艺与注凝成型相结合的方法制备出Al₂O₃/C泡沫, 作为合成AlN粉体的前驱体。泡沫孔隙尺寸从几十微米到几百微米, 总孔隙率56%~90%。具有通孔结构的泡沫前驱体实现了原料内部各处的均匀的固-气反应, 泡沫总孔隙率≥80%可显著提高CRN反应的速率。XRD分析结果显示: CRN过程中存在γ-Al₂O₃到α-Al₂O₃的相转变, 反应起始温度在1300℃以上, 并在1550℃反应完全。在1650℃合成得到的AlN颗粒平均粒径不超过1 µm, 氮含量为32.9wt%。     

Formation of AlN by nitrogen ion implantation

The phase composition of aluminium after bombardment with doses from 1 × 10¹⁶ to 1 × 10¹⁸ N⁺ ions cm^-2 is investigated by high voltage electron microscopy and selected area diffraction. This implantation always produced polycrystalline aluminium nitride (AlN). A thermal treatment to 600 °C did not yield new crystalline phases. At low temperatures the growth of AlN precipitations takes place mainly coherently as a result of a high vacancy density. Larger AlN precipitations grow similarly to the Ostwald ripening process. In this process incoherent precipitations are associated with a high dislocation density with an anisotropic distribution.     

Microstructural characteristics and evolution of Ti2AlN/TiAl composites with a network reinforcement architecture during reaction hot pressing process

The microstructural evolution of TiAl matrix composites with a novel network distribution of Ti₂AlN particle reinforcement was studied. The composites were synthesized by reaction hot pressing method using pure Al and nitrided Ti powders as initial materials. Pure Ti powders nitrided at 600 °C for a certain time in an atmosphere of flowing nitrogen turned into new compound Ti(N) powders, which have a shell of titanium nitrides (such as TiN, Ti₂N and TiN_0.3) and a core of Ti–N solid solution. Within the composites synthesized, Ti₂AlN particles, produced by in situ reaction, exhibit a network distribution. The special shell/core structure of the compound Ti(N) powders contributes to this architecture. Nitriding time of the Ti powders greatly affects the microstructure of the composites. Increasing the nitriding time is beneficial to the distribution of Ti₂AlN particles in a continuous network form. However, too long nitriding time can result in the aggregation of Ti₂AlN particles and thus destroy the uniformity of the network structure. The in-situ synthesized Ti₂AlN/TiAl composites with uniform network structure have a superior mechanical property, and their compressive strengths at 800 °C and 1000 °C are 1112 MPa and 687 MPa, respectively.     

纳米AlN/BN复相陶瓷的显微结构与性能

采用尿素与硼酸的反应, 在850℃氮气氛下, 合成了纳米BN?包裹的AlN粉料, 通过TEM观察, BN粒径为20～30nm. 通过热压烧结制备出纳米h-BN包裹的AlN/BN 复相陶瓷, SEM/TEM观察, h-BN呈细针状, 针长100～400nm, 针宽为30～50nm. 当h-BN含量≥20wt%时, 其硬度显著降低, 提高了材料的可加工性能.     

The adhesion of copper films deposited onto aluminum nitride

Good adhesion between copper film and AlN substrate is obtained when the surface of AlN is laser-irradiated prior to copper film deposition and post deposition annealing is conducted. Surface chemistry of AlN substrates before and after laser irradiation and the interfacial reactions of copper film/AlN couples were studied with Auger Electron Spectroscopy (AES) to understand the adhesion mechanisms. The surface of as-received AlN substrates was covered with a thin sheath of Al₂O₃. Laser irradiation removed the surface Al₂O₃ layers, smoothened the surface, and decomposed AlN leaving metallic aluminum on the surface. The interfacial reactions in the copper film/AlN couple are affected by the amounts of oxygen and metallic aluminum available at the interface. The adhesion mechanism is the formation of a Cu-O-Al compound at the interface of copper film/AlN couple. Since copper does not react with AlN, laser induced decomposition of AlN seems to be the driving force for the formation of the compound. This revised version was published online in September 2006 with corrections to the Cover Date.     

Dense AlN/FeSiAl composite ceramics with high thermal conductivity and strong microwave absorption

Journal of Materials Science: Materials in Electronics - A series of AlN/x wt% FeSiAl composite ceramics with 4 wt% Y2O3 sintering aid was fabricated by pressureless sintering in a nitrogen...     

Mechanism of in situ formation of AlN in Al melt using nitrogen gas

In situ processing of AlN particle reinforced aluminum composites was investigated using a gas bubbling method with nitrogen gas as the gaseous precursor and pure aluminum as the starting matrix in the temperature range of 1173–1573 K. The products were characterized using XRD, SEM, and EDS techniques. Experimental results showed that it is feasible to synthesize AlN particle reinforced Al composites in situ using purified nitrogen gas. Significant AlN was synthesized by bubbling deoxidized N₂ through Al melt. The AlN particles synthesized in situ were small in size (<10 m) and were enriched in the top part of the product formed in the crucible. Directly bubbling commercial purity nitrogen gas did not lead to formation of significant AlN due to the deleterious effect of the trace oxygen impurities in the bubbling gas. The deleterious effect of trace oxygen impurities on the mechanism of formation of AlN in the Al-N system was critically analyzed from both thermodynamic and kinetic points of view. Chemisorption of O₂ molecules at the gas bubble-Al melt interface is more favorable and much faster than that of N₂, thereby inhibiting chemisorption of N₂ molecules. Significant AlN can be formed only at the content of oxygen below a critical value in the N₂ bubbling gas.     

Ti/Al_2O_3界面的光电子能谱研究

用X射线光电子谱（XPS）和俄歇电子能谱（AES）研究了Ti／Al_2O_3界面形成的过程。研究表明，活性金属Ti在室温下能与衬底Al2O3（1102）形成约20nm强结合的界面区。从Al，O，Ti的光电子谱形状变化以及它们随着Ti覆盖度的增加而出现结合能位移表明，在界面处形成的反应层中，最初几个单层的Ti很容易被Al2O3表面的活性氧所氧化，从而使Ti／Al2O3界面逐步由具有更强相互作用的TiOx／Al2O3界面所代替，并形成由多相混合体［Ti-O相，（Ti，Al）2O3相以及金属Al相］所组成的界面区。就是说，Ti通过Al—O键的O2-离子转移其电子给Al3 并使它还原成金属Al，从而形成Ti－O键所致。本文用AES强度剖面分析观察到了这种被还原的Al。     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Study of interface phenomena between bone and titanium and alumina surfaces in the case of monolithic and composite dental implants

The interface between mandibular bone and dental implants was examined with the in vivo dog model. Implant/bone interfaces were investigated for three types of materials: Ti–30 wt% Ta/Al2O3, titanium and Al2O3 using microscopy techniques covering a large magnification range: scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis and Auger spectroscopy. During the interaction of the Al2O3 ceramic with bone, an interfacial layer about 15 m thick is formed. The same phenomenon was observed at the titanium bone interface, where the thickness of the layer was about 10 m. In all cases, interface layers were sharp with well-defined borders between bone tissue and implant materials. No calcification took place inside the interface layer. A chemical analysis performed on this layer shows the presence of titanium, calcium and phosphorus in the case of titanium implants, and aluminium, calcium and phosphorus in the case of alumina implants. A rapid decrease in metal composition with increasing distances from the implant surface is correlated to a slow increase in calcium and phosphorus in the direction of the bone. Direct contact between implant and bone was observed. No biocorrosive effects were detected at the Ti–30 wt% Ta/Al2O3 metal–ceramic interface.     

Some observations on the wetting and bonding of nitride ceramics

Several series of experiments have been conducted to gain information about the wettability of AlN, BN, Si₃N₄ and two sialon ceramics by potential braze materials. It was possible to achieve wetting of all five ceramics using aluminium, copper-titanium alloys, and a Ag-28Cu-2Ti alloy. Wetting by aluminium and the Ag-28Cu-2Ti alloy was usually good. Both wetting and non-wetting alloys containing titanium reacted to form TiN and it is argued that the achievement of wettability is associated with a certain degree of hypostoichiometry. While aluminium should also have reacted, no clear evidence was obtained. In supplementary experiments it was found that bonds formed by brazing with aluminium at 1000 °C could have shear strengths as great as 60MPa. Although the experimental work was preliminary in nature, it suggested that good brazing systems could be developed.     

Study of the Ti/Al2O3 interface

The Ti/Al₂O₃ (1 ¯1 0 2) interface formation has been investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy (AES). The results showed that when an active metal titanium was evaporated on to a room-temperature Al₂O₃ (1 ¯ 1 0 2) surface in ultrahigh vaccum, a Ti/Al₂O₃ interface region of about 200 nm was formed, and in the first several monolayers of titanium, the titanium was oxidized due to the active oxygen anions on the surface. Therefore, the pure Ti/Al₂O₃ interface was replaced gradually by a titanium oxides/Al₂O₃ interface, which has a stronger interaction than the former. The change of shape of the photoemission lines and the shift of binding energy of aluminium, oxygen and titanium with increasing coverage of titanium showed that the formation of the Ti-O bond at the interface is due to titanium transferring its electrons to Al³⁺ via O^2– anions in the Al-O bond, whereby the Al³⁺ was reduced to metallic aluminium, Al⁰. The AES intensity profile also proved the existence of the reduced species Al⁰. This suggests that the reaction layer consists of a multiphasic mixture: the Ti-O type phase, the (Ti, Al)₂O₃ phase and metallic aluminium phase.     

Synthesis of boride and nitride ceramics in molten aluminium by reactive infiltration

The infiltration of solid powder mixtures with molten aluminium has been investigated as a potential route for the synthesis of ceramic/metal composites. Either titanium or tantalum powder was mixed with boron nitride flakes for the reaction powder mixture. The infiltration occurred spontaneously at 1473K for both [Ti+BN] and [Ta+BN] powder mixtures. Owing to reactions between the starting materials, both boride and nitride ceramics were produced in molten aluminium. TiB2 and AlN were produced from the [Ti+BN] powder mixture, and TaB2 and AlN were produced from the [Ta+BN] powder mixture. When the [Ti+BN] powder mixture was used, a reaction producing Al3Ti took place immediately after the infiltration of the molten aluminium, and a subsequent reaction producing TiB2 and AlN proceeded gradually. The time required to convert BN flakes to TiB2 and AlN particles at 1473K was in the range of 1800–3600 s. On the other hand, when the [Ta+BN] powder mixture was used, there was an initial incubation period to allow the tantalum and molten aluminium to react with each other. The reaction between tantalum, BN and aluminium took place after this incubation period. This revised version was published online in November 2006 with corrections to the Cover Date.     

A study on the microstructure and property development of d.c. magnetron co-sputtered ternary titanium aluminium nitride coatings Part II Effect of magnetron discharge power

Advanced ternary (Ti,Al)N coatings were produced by reactive magnetron co-sputtering technique with separate titanium and aluminium targets at a 30° magnetron configuration. The aluminium magnetron discharge power was adjusted from 0 to 6.0 W/cm² to investigate the effect of magnetron discharge power on the microstructure and property development of the coatings. It was found that increasing the aluminium magnetron discharge power caused the deposition rate and the aluminium content to increase, and the grain size and surface roughness of the coatings to decrease substantially. Tighter packing of the grain columns occurred and the microstructure changed from a porous zone 1 to a densified zone T structure, resulting in a continuous increase of the coating hardness. The major texture component of the coatings changed from the (111) to (200) orientations. The (101) orientations of the AlN structure also developed. It was found that the microstructure and hardness enhancement of the coatings was associated with an increased formation of the TiAlN and AlN phases and a densified, fine grain structure at higher magnetron discharge powers.     

Growth of GaN and AlN thin films by laser induced molecular beam epitaxy

Hexagonal GaN and AlN thin films were grown by laser induced molecular beam epitaxy using Al or Ga metal as target material and N₂as nitrogen source. The films were deposited on sapphire (0001) and SiC (0001) substrates. Epitaxial growth of GaN has been achieved at 730°C and 10⁻³ mbar N₂ pressure. The AlN films were polycrystalline with predominant (0001) orientation.     

Effect of particle bombardment on the orientation and the residual stress of sputtered AlN films for SAW devices

We present a study of the effect of particle bombardment on the preferred orientation and the residual stress of polycrystalline aluminum nitride (AlN) thin films for surface acoustic wave (SAW) applications. Films were deposited on silicon (100) substrates by radio frequency (RF) sputtering of an aluminum target in an argon and nitrogen gas mixture. The main deposition parameters were changed as follows: the total pressure from 4 mTorr to 11 mTorr, the N2 content in the gas mixture from 20% to 80%, and the substrate self-bias voltage from -10 V to -30 V. If a sufficiently high negative substrate self-bias voltage is induced, (00.2)-oriented films are obtained over the full ranges of pressure and N2 content. Such films have values of residual stress ranging from -3 GPa to +1 GPa, depending on the deposition conditions. Our results suggest that the energy of the Ar ions colliding with the substrate controls the preferred orientation of the films, whereas the directionality of the ions (for the same energy) is the main factor determining the residual stress. To demonstrate the suitability of our material for the intended application, SAW filters with good electroacoustic response have been fabricated using AlN thin films with optimized (00.2) orientation and controlled residual stress.     

反应烧结法制备(AlN,TiN)-Al2O3复合材料的研究

以Ti,Al,Al₂O₃为初始粉料,通过750~800℃氮气保护下的中温焙烧,然后在1420~1550℃在氮气氛下反应烧结,制备了不同配比的(AlN,TiN)-Al₂O₃复合材料。研究了组成及烧结工艺对复合材料力学性能、显微结构等的影响。用XRD,SEM等方法分析粉体及烧结体的相组成及微观结构。分析结果表明:AlN,TiN的形成,有助于材料的致密化并使其力学性能提高。组成为20wt%(Al,Ti)-Al₂O₃的粉体在1520℃、30MPa、保温、保压30min热压烧结条件下,与N₂气反应可得到硬度(HRA)为 94.1的高硬度的(AlN,TiN)-Al₂O₃复合材料,该材料的抗弯强度为687 MPa,断裂韧性(K_IC)为6.5MPa·m1/2。