Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Mechanochemical reduction of MoO3/SiO2 powder mixtures by Al and carbon for the synthesis of nanocrystalline MoSi2

In this investigation, MoSi₂ intermetallic compound has been synthesized by reducing of MoO₃/SiO₂ powder mixtures by Al and carbon via mechanical alloying (MA). Powder mixtures were ball milled for 0–100 h and structural evolutions have been monitored by X-ray diffraction. In the Al system, both β-MoSi₂ (high temperature phase) and -MoSi₂ (low temperature phase) were obtained after 3 h of milling and after 70 h of milling the β-phase transformed to -phase. The crystallite size of -MoSi₂ and Al₂O₃ after milling for 100 h was 12 and 17 nm, respectively. In reducing with carbon, two different compositions with nominal carbon content of 13.7 and 24 wt.% were used that in both compositions, -MoSi₂ forms during 10 h of milling. Higher carbon content increases the amount of MoSi₂.     

Properties of ZrO2–Al2O3 composite as a function of isothermal holding time

Zirconia and alumina based ceramics present interesting properties for their application as implants, such as biocompatibility, good fracture resistance, as well as high fracture toughness and hardness. In this work the influence of sintering time on the properties of a ZrO₂–Al₂O₃ composite material, containing 20 wt% of Al₂O₃, has been investigated. The ceramic composites were obtained by sintering, in air, at 1600 °C for sintering times between 0 and 1440 min. Sintered samples were characterized by microstructure and crystalline phases, as well as by mechanical properties. The grain growth exponents, n, for the ZrO₂ and Al₂O₃ were 2.8 and 4.1, respectively, indicating that different mechanisms are responsible for grain growth of each phase. After sintering at 1600 °C, the material exhibited a dependency of hardness as function of sintering time, with hardness values between 1500 HV (120 min) and 1310 HV (1440 min) and a fracture toughness of 8 MPa m^1/2, which makes it suitable for bioapplications, such as dental implants.     

Microstructural evolution of some MgO–TiO2 and MgO–Al2O3 powder mixtures during high-energy ball milling and post-annealing studied by X-ray diffraction

High-energy dry ball-mill and post-anneal processing were applied to synthesize MgTiO₃ and Mg₂TiO₄ single crystalline phases from the predetermined compositions of MgO–TiO₂ powder mixtures. Also, the experiments were performed to show that it is possible to prepare MgAl₂O₄ single crystalline phase from the predetermined composition of MgO–Al₂O₃ powder mixture only by employing high-energy dry ball milling, i.e. without post-annealing the milled samples. In contrast, fully developed single crystalline powders of MgTiO₃ and Mg₂TiO₄ were obtained after post-annealing the milled samples for 1 h at 900 and 1200 °C, respectively.     

Microstructure and magnetic properties of Co/Al2O3 nanocomposite powders

Nanocomposite powders of magnetic cobalt nanoparticles dispersed by nonmagnetic Al₂O₃ particles have been prepared by planetary ball milling. Ball milling of the CoO and Al mixture powder after a certain milling duration reduces CoO to (fcc and hcp) Co completely and oxidizes Al to -Al₂O₃ simultaneously. The average grain sizes of the nanocomposite powders are 19 nm for Co and 28 nm for -Al₂O₃ after the completion of the reduction reaction. By direct ball milling of the mixture of Co and Al₂O₃, the allotropic phase transformation of Co was observed and the average grain size of Co is reduced to 5 nm. For both the samples of the mechanochemical series and the direct milling series, the saturation magnetizations of the nanocomposite powders decrease with decreasing average grain size of Co. This may be due to the enhancement of the interface effects and the increase of the superparamagnetic particles with decreasing Co grain size. The coercivities of the Co/Al₂O₃ nanocomposite powders increase up to 380 Oe. The increasing grain boundaries with decreasing Co grain size result in the domain wall pinning which predicts the coercivity enhancement. In addition to the grain size effects, the reduction of the particle size toward the size region of single domain also contributes to the increase of coercivity.     

Development of moderate temperature CVD Al2O3 coatings

Chemically vapor deposited Al₂O₃ coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al₂O₃ coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al₂O₃. To overcome this limitation, alternate systems to deposit Al₂O₃ at moderate temperatures have been investigated. Systems using NO–H₂, H₂O₂, NO₂–H₂ and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al₂O₃ in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates.     

Formation and chemical leaching effects of nonequilibrium Al0.6(Fe25Cu75) alloy powder produced by rod milling

The formation and chemical leaching effects of a nonequilibrium Al_0.6(Fe₂₅Cu₇₅)_0.4 powder produced by rod milling is described. X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and vibrating sample magnetometry were used to characterize both the as-milled and leached specimens. After 400 h of milling, only the bcc AlFe phase with an amorphous phase was detected in the XRD patterns. The crystallite size for the bcc AlFe phase (110) after 400 h of milling was about 5.3 nm. The peak temperature and the crystallization temperature of the as-milled powders were 448.7 and 428.0 °C, respectively. Al atoms leaching from the as-milled bcc AlFe powders in the L1 condition did not alter the diffraction pattern significantly, even though Al atoms had been removed. After the L1 specimen was annealed at 500 °C for 1 h, the bcc AlFe phase transformed to the fcc Cu, Fe, and CuFe₂O₄ phases. The peak widths of L1 and L2 specimens were similar, but became broader than that of the as-milled powder. The saturation magnetization decreased with increasing milling time, and a value of 10.4 emu/g was reached after 400 h of milling. After cooling the specimen from 750 °C, the magnetization slowly increased at approximately 491.4 °C, indicating that the bcc AlFe phase had transformed to the fcc Cu and Fe phases.     

Synthesis of TiC–Al2O3 nanocomposite powder from impure Ti chips, Al and carbon black by mechanical alloying

The possibility of providing TiC–Al₂O₃ nanocomposite as a useful composite from low-cost raw materials has been investigated. Impure Ti chips were placed in a high energy ball mill with carbon black and aluminum powder and sampled after different times. XRD analysis showed that TiC has been synthesized after 10 h of milling. It could be observed from the width of XRD patterns’ peaks that the size of produced TiC crystallites is in the order of nanometer. In order to forming of TiC–Al₂O₃ composite, heat treatment was performed in different temperatures. Investigations have revealed that formation temperature of TiC as the dominant phase decreased for the milled specimens during heat treatment, also nanocrystalline TiC–Al₂O₃ composite was formed in this situation. Furthermore milling led to increase of strain and decrease of TiC lattice parameter while during heat treatment nanocrystalline grains grow up and strain decreases.     

Solid state reactions in highly dispersed praseodymium oxide–SiO2 system

This paper reports results of studies on the interaction of praseodymium oxide nanocrystals with an amorphous silica. Nano-sized (3–4 nm) amorphous precursor of praseodymium oxide synthesized using a microemulsion technique were supported onto a high surface SiO₂ or occluded into SiO₂ matrix. Solid state reactions occurring in these binary systems upon heat treatment in air, argon or hydrogen at 800–1100 °C were studied by TEM, XRD, FT-IR and UV–vis spectroscopy. It has been found that morphology of the sample as well as annealing atmosphere influence greatly the phase evolution. At temperatures above 900 °C, nanocrystalline praseodymium silicates of various morphology and crystal structure were obtained. In particular, a new polymorph of Pr₂Si₂O₇, isostructural with I-type Ln₂Si₂O₇ (Ln₆[Si₄O₁₃][SiO₄]₂) Ln = Ce, La, has been identified.     

Crystallization of magnesium niobate from mechanochemically derived amorphous phase

The effect of high-energy ball milling and subsequent annealing on the mixture of MgO and Nb₂O₅ has been investigated. X-ray diffraction (XRD) measurement indicates that an amorphous phase is produced after milling for 5 h, while traces of MgNb₂O₆ crystallized from the amorphous phase during prolonged milling. Significant crystallization of MgNb₂O₆ from the amorphous state is observed after annealing at 500 °C, while the reaction of the remaining MgO and Nb₂O₅ does not take place at this temperature. Single phase MgNb₂O₆ can be achieved for all the milled samples at 700 °C. No significant grain growth is observed when the milled powders were annealed at temperature below 900 °C. Almost fully dense MgNb₂O₆ ceramics are obtained after annealing at 1100 °C from the as-milled powders.     

Synthesis and characterization of molybdenum aluminide nanoparticles reinforced aluminium matrix composites

Aluminium matrix composites reinforced with molybdenum aluminide nanoparticles were synthesized by ball milling and reactive sintering of the mixture of aluminium and 10 wt% hydrated molybdenum oxide powders. Sintering the as milled powder in air below 750 °C produced MoAl₁₂ intermetallic compound nanoparticles, at 750 °C produced a mixture of MoAl₅ and MoAl₄ nanoparticles and at 800 °C under Argon atmosphere produced predominantly MoAl₄ intermetallic nano-particles in the Al matrix. The powder compacts sintered in air below 750 °C produced MoAl₁₂ whereas at 750 °C or above formed the Al matrix composite reinforced with the MoAl₅ nanoparticles. These nanoparticles become agglomerated to take up some irregular shaped flakes in the metal matrix. The reaction between Al and hydrated Mo oxide powders was found to be a favorable way to produce predominantly a particular Mo–Al intermetallic compound at a particular temperature. The Al₂O₃ particles formed as another reaction product, in all the above reactions, remain distributed in these composites. The composites thus formed were characterized by SEM-EDS, DTA, XRD and TEM analysis.     

Direct synthesis of A2(Ti(1 − y)Zry)2O7 (A = Gd, Y) solid solutions by ball milling constituent oxides

Different compositions in two solid solutions, A₂(Ti_(1 − y)Zr_y)₂O₇ (A = Gd³⁺, Y³⁺), with high oxygen ion conductivity, have been successfully prepared at room temperature via mechano-chemical synthesis. Stoichiometric mixtures of the constituent oxides were milled in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by using X-ray diffraction showing that in all cases and after milling for 19 h, the powders consisted of a single phase. Powders were also examined by scanning electron microscopy (SEM) finding out that they basically consist of sub-micron size agglomerates and aggregates of nanoparticles.     

Production of niobium carbide ceramic composites derived from polymer/filler mixtures: preliminary results

Studies have shown that Al₂O₃–NbC composites present a good potential to be used for metalworking. Manufacturing of composite ceramic material derived from polymer reactive filler mixtures were investigated. The present study reports the preliminary results of reaction bonded niobium carbide derived from polymer (polysiloxane), inert filler (Al₂O₃) and reactive filler (Nb). Niobium powder, alumina and polysiloxane mixtures were homogenized in a planetary ball milling and pressureless sintered in inert atmosphere at temperatures up to 1600 °C. Depending on the niobium content and pyrolysis conditions, ceramic materials with a porosity of 20–40%, a weight loss of 5–15%, a linear shrinkage of 2–4% and a flexural strength of maximum 80 MPa were obtained. X-ray diffraction (XRD) of a sample containing 60 wt% polymer + 40 wt% Nb showed the presence of new crystalline phases such as NbC, Nb₃Si and Nb₅Si₃.     

Fabrication of Bulk (Fe,Cr)_3Al/Al_2O_3 Intermetallic Matrix Nanocomposite Through Mechanical Alloying and Sintering

In the present study,(Fe,Cr)_3Al/20 vol% Al_2O_3 nanocomposite was prepared through mechanochemical reactions during ball milling and successfully bulked using a combination of cold isostatic press and sintering at 1400 ℃ for 1 h. Two processing approaches were utilized to produce(Fe,Cr)_3Al/Al_2O_3 nanocomposite: The first was milling of Fe, Cr,Al and Fe_2O_3, while the second one was milling of Fe, Cr, Al and Cr_2O_3, both in stoichiometric condition, to synthesize(Fe,Cr)_3Al/20 vol% Al_2O_3. Structural changes of powder particles during mechanical alloying were studied by X-ray diffraction. The microstructure and the morphology of powder particles and bulk samples were also studied by scanning electron microscopy and transmission electron microscopy. Microstructural analysis showed that mechanochemical reactions took place during milling, and nanometric Al_2O_3 was uniformly distributed in the matrix. The results also showed that the second approach required a considerably higher milling time to produce(Fe,Cr)_3Al/Al_2O_3 nanocomposite, as compared to the first one. For this reason, bulk samples were produced from the synthesized nanocomposite in the first approach. The microstructure of the sintered samples consisted of a network structure of(Fe,Cr)_3Al and Al_2O_3 phases with superior mechanical properties.     

Structural and electrical properties of SrBi2(Ta0.5Nb0.5)2O9 thin films

SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBTN) thin films were obtained by polymeric precursor method on Pt/Ti/SiO₂/Si(1 0 0) substrates. The film is dense and crack-free after annealing at 700 °C for 2 h in static air. Crystallinity and morphological characteristic were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The films displayed rounded grains with a superficial roughness of 3.5 nm. The dielectric permittivity was 122 with loss tangent of 0.040. The remanent polarization (P_r) and coercive field (E_c) were 5.1 μC/cm² and 96 kV/cm, respectively.     

Effect of annealing on the morphology evolution and densification of LiMn2O4 film from chitosan-containing solution

Dense LiMn₂O₄ films deposited on a Pt-coated silicon substrate were obtained by annealing the deposited Li–Mn–O-chitosan films under a two-stage heat-treatment procedure. It was demonstrated that the heat-treatment at 300 °C plays an important role in the subsequent densification of LiMn₂O₄ films. This is attributed to the formation and rearrangement of the nano-sized LiMn₂O₄ crystallites. The surface morphology of the calcined Li–Mn–O-chitosan films was highly related to the annealing temperature. Ridge-like bumps formed on the surface of the films after being heated at 200 °C for 1 h. With calcination at 400 °C or higher, the surface morphology turned into a wrinkle-like microstructure. This morphology transformation is ascribed to the flowing characteristics of the Li–Mn–O-chitosan films during heat-treatment and subsequent thermal decomposition of the precursor at higher temperatures. Moreover, the electrochemical tests showed that the 700 °C-annealed LiMn₂O₄ film possesses the highest discharge capacity of 56.3 μA h/(cm² μm) and best capacity retention of 90.7% after 50 charge/discharge cycles of all annealed films.     

Sintering of alumina-niobium carbide composite

Several studies have been focused on particulate-dispersed Al₂O₃ composites in order to improve both room and high temperature mechanical properties and wear resistance. In the present work Al₂O₃-NbC composites have been pressureless sintered and their microstructures analysed as a function of NbC and Y₂O₃ concentration, the latter added as sintering aid. The compositions used in this study were Al₂O₃-xNbC and (Al₂O₃ 3%Y₂O₃)-xNbC, (x = 10, 20 and 40 wt%) and the sintering was performed at 1650 °C/30 min and 1750 °C/15 min. A density greater than 96% of the theoretical density was reached even for those materials sintered at 1650 °C. The observed microstructure was more homogeneous for the samples with Y₂O₃ addition and the Y₃Al₅O₁₂ phase was detected. The Al₂O₃ grain growth restraining due to the NbC concentration was more pronouncedly in samples sintered at 1750 °C.     

Fabrication of carbon-coated cobalt nanoparticles by the catalytic method

A mass of carbon-coated cobalt nanoparticles were successfully synthesized by chemical vapor deposition of methane over Co/Al catalyst at 650 °C. The phases found in the as-prepared carbon-coated nanoparticles were fcc-Co. The diameter of these nanoparticles with 2–10 nm carbon coating shell was in the range of 5–80 nm. However, it is well-known that the Co nanoparticles are suitable for carbon nanotube synthesis, such as Co/Al₂O₃ and Co/SiO₂ catalysts. Here, we speculated that the formation mechanism of carbon-coated cobalt nanoparticles might due to that the formation of carbon nanotubes was inhibited by the catalyst supporter of Al. Compared with Al₂O₃ and SiO₂, Al possesses very low melting point (660 °C), very high diffusion coefficient and thermal conductivity, thus the isotropic precipitation of carbon species on the surface of cobalt particles took place, favoring the formation of carbon-coated Co nanoparticles.     

Study on phase formation and sintering kinetics of BaTi0.6Zr0.4O3 powder synthesized through modified chemical route

BaTi_0.6Zr_0.4O₃ powder was prepared from barium oxalate hydrate, zirconium oxy-hydroxide and titanium dioxide precursors. Barium oxalate hydrate and zirconium oxy-hydroxide were precipitated from nitrate solution onto the surface of suspended TiO₂. Phase formation behaviour of the materials was extensively studied using XRD. BaTiO₃ (BT) and BaZrO₃ (BZ) start forming separately in the system upon calcinations in the temperature range 600–700 °C. BT–BZ solid solution then forms by diffusion of BT into BZ from 1050 °C onwards. The precursor completely transforms into BaTi_0.6Zr_0.4O₃ (BTZ) at 1200 °C for 2 h calcination. The activation energy (AE) of BT (134 kJ mol⁻¹) formation was found to be less than that of BZ (167.5 kJ mol⁻¹) formation. BTZ formation requires 503.6 kJ mol⁻¹ of energy. The sintering kinetics of the powder was studied using thermal analyzer. The mean activation energy for sintering was found to be 550 kJ mol⁻¹.