Unmilled Powder Research Articles

ZnO varistors with reduced amount of additives prepared by direct mixing of constituent phases

The aim of this work was to reduce number of additives using new method of preparation, refered as “direct mixing of the constituent phases”. The method is based on sintering of a mixture of constituent phases with the following compositions: ZnO phase uniformly doped with 0.1 mol% Mn 2+ and 0.1 mol% Co 2+, γ-Bi 2O 3 stabilized with Mn 2+ and spinel phase of composition Zn 1.86Co 0.46Sb 0.67O 4 or Zn 1.971Ni 0.090Co 0.030Cr 0.247Mn 0.090Sb 0.545O 4. The varistors sintered at 1030 °C showed good electrical properties, with nonlinearity coefficient reaching 50 and low values of leakage current. To improve microstructure some powder mixtures were intensively milled and further processed on the same way as unmilled powder mixtures. Electrical and microstructural properties of milled and unmilled samples were compared.

Microstructure characterization of ball-mill-prepared nanocrystalline CaCu 3Ti 4O 12 by Rietveld method

Nanocrystalline powders of CaCu 3Ti 4O 12 (CCTO) have been prepared by high-energy ball milling the powder mixture of CaO, CuO, TiO 2 with molar ratio 1:3:4. The ball milling of powder mixture was performed in air medium for 5 min to 14 h. Microstructure characterization of all the ball-milled and unmilled powder mixtures was done by analyzing the X-ray powder diffraction data. Formation of cubic CCTO is noticed in the 2 h ball-milled powder mixture. With the progress of milling time, the cubic CCTO phase increases gradually. Complete formation of nanocrystalline CCTO was noticed within 8 h of ball milling. Formation mechanism, phase transition kinetics, structural and microstructural changes in terms of lattice imperfections and relative phase abundances of individual phases have been estimated from the analysis of X-ray powder diffraction data of unmilled and ball-milled powder mixtures by Rietveld's powder structure refinement method.

Amorphization and thermal stability of mechanical alloyed W–Ni–Fe

The tungsten heavy alloys with the composition of 80.7W–13.2Ni–6.1Fe (at.%) from the elemental powders of W, Ni and Fe were mechanically alloyed (MA-ed). A W–Ni base amorphous phase imbedded remnant nano-crystalline W with the grain size of 10–20 nm was achieved by suitable MA technology. Phase transition during MA and thermal stability of mechanically alloyed 80.7W–13.2Ni–6.1Fe alloys were investigated by using a combination of XRD, TEM, DTA and DSC analysis. Ni (Fe) first dissolves in crystalline W during milling, and then amorphization of W (Ni, Fe) supersaturated solid solution occurs after milling for 20 h. The volume fraction of amorphous in the as-milled powders increases with extending milling time up to 60 h, and the average composition of the amorphous phase is proximately 63W–37Ni (Fe). Different phase transformations occur in the powders milled for 20 and 60 h during subsequent annealing. For the powders milled for 60 h, besides the relaxation of crystal defects and lattice distortion, the crystallization of amorphous phase and the precipitation of NiW compound are confirmed during subsequent annealing. When temperature increasing above 1100 °C, NiW partially decompose in Ni solid solution and crystalline W through a peritectoid reaction. The primary melting temperatures of the nano-crystalline powders obtained by mechanically milled for both 20 and 60 h decrease by nearly 200 °C compared with that of the un-milled powder mixture.

Carbothermic Reduction of Anatase and Rutile.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Strain-enhanced sintering of iron powders

Sintering of ball-milled and un-milled Fe powders has been investigated using dilatometry, X-ray, density, and positron annihilation techniques. A considerable sintering enhancement is found in milled powders showing apparent activation energies that range between 0.44 and 0.80 eV/at. The positron annihilation results, combined with the evolution of the shrinkage rate with sintering temperature, indicate generation of lattice defects during the sintering process of milled and un-milled powders. The sintering enhancement is attributed to pipe diffusion along the core of moving dislocations in the presence of the vacancy excess produced by plastic deformation. Positron annihilation results do not reveal the presence of sintering-induced defects in un-milled powders sintered above 1200 K, the apparent activation energy being in good agreement with that for grain-boundary diffusion in γ-Fe.

Carbothermic reduction of anatase and rutile

The carbothermic reduction of anatase and rutile to TiC was investigated using a combination of thermogravimetric analysis and X-ray diffraction. Samples premilled for 50 h prior to heat treatment showed a much greater extent of reduction compared with the unmilled powders. Heating the rutile up to 800 °C and anatase up to 870 °C showed no evidence for reduction with the observed mass loss attributed to desorption of gas. After heating to 800 °C, most of the anatase has transformed to rutile. The reduction of rutile started at ∼830 °C whilst anatase underwent reduction at 870 °C. The initial identified products were Ti 5O 9 and Ti 4O 7 with the latter being the last apparently stable phase prior to TiC formation. The TiC formed from anatase was apparently stoichiometric whilst that from rutile was clearly sub-stoichiometric. Reasons for the difference in reduction behaviour are discussed.

Formation of a quasicrystalline phase in mechanically alloyed Al65Cu25Fe15

An elemental powder mixture with a nominal composition of Al65Cu20Fe15 was mechanically milled for times of up to 8 h. Subsequent annealing of as-milled powders was performed at different temperatures between 600 and 800 °C for 4 h. A combination of X-ray diffractrometry, differential scanning calorimetry and transmission and scanning electron microscopy has been used to characterise the microstructure of as-milled powders before and after annealing. The icosahedral quasicrystalline (QC)-phase did not form directly after milling. QC-phase was obtained from the un-milled powder by annealing above 700 °C for 4 h. However, a combination of milling and subsequent annealing could enhance the formation of QC at temperatures above 600 °C.

Effect of mechanical activation on synthesis of ultrafine Si3N4–MoSi2 in situ composites

Abstract Si 3 N 4 –MoSi 2 in situ composite has been synthesized by reacting powders of molybdenum (Mo) and silicon nitride (Si 3 N 4 ). Mo and Si 3 N 4 powders mixture in a molar ratio of 1:3 were ball milled for 0–100 h. The milled and unmilled powder mixtures were reacted at different temperatures between 1000 and 1600 °C in an argon atmosphere. The effect of mechanical activation (MA) induced by milling has been studied through X-ray diffraction (XRD), differential thermal analysis (DTA), and thermo-gravimetric analysis (TGA). No peaks of Mo in the XRD pattern have been observed after 70 h of milling. The crystallite size of the Mo has been found to be the lowest (41 nm) after milling for 30 h. Similarly, a 100 nm lowest size of crystallite of Si 3 N 4 was observed after milling for 50 h. DTA and TGA results show that the reaction between Mo and Si 3 N 4 enhances with increase in milling time. Milling for 10 h lowers the pyrolysis temperature by 150 °C. Additional milling upto 100 h does not lead to further reduction in the pyrolysis temperature. The intensities of peaks of MoSi 2 in the pyrolysed samples increased with increase in milling time. MoSi 2 particles of size less than 1 μm were observed to be uniformly distributed through out the Si 3 N 4 matrix.

Comparative study on the corrosion behavior of milled and unmilled magnesium by electrochemical impedance spectroscopy

The corrosion behavior of milled Mg prepared by high-energy ball milling for 10 h has been investigated in alkaline solutions by electrochemical impedance spectroscopy and compared with that of unmilled Mg. X-ray powder diffraction indicates a crystallite size of 34 nm for the milled Mg compared to >100 nm for the unmilled powder. Chemical analyses show no significant iron contamination in milled Mg powder, indicating the absence of tools erosion during the milling procedure. In contrast, significant MgO enrichment in the milled powder is observed (6.5 wt.% after 10 h milling compared to 1.0 wt.% before milling). The oxygen contamination is mainly attributed to the powder oxidation occurring during milling. From XPS analyses, no MgO enrichment is detected on milled Mg electrode surface, confirming that MgO is dispersed homogeneously in the bulk of the material rather than to segregate on its surface. Electrochemical impedance spectroscopy demonstrates clearly the better corrosion resistance of milled Mg compared to unmilled Mg in passive conditions (KOH solution, pH=14) and in more active corrosion conditions (borate solution, pH=8.4). This is illustrated by a nobler corrosion potential and by a significant increase of the interfacial resistance related to the film and charge-transfer reaction. Moreover, the variation of the different electrochemical parameters (corrosion potential, interfacial resistance and capacitance) with immersion time is less accentuated and tends more rapidly to a steady state with milled Mg, suggesting an enhancement of the Mg(OH) 2 formation kinetic. The origin of the distinctive passivation behavior of ball-milled Mg is discussed.

Ignition mechanism of mechanically activated Me–Si(Me = Ti, Nb, Mo) mixtures

The influence of mechanical activation on the characteristics and mechanism of ignition of self-propagating high-temperature synthesis processes of different silicides in the systems Me–Si (Me =Ti, Nb, Mo) was investigated. The results show that mechanical activation does not alter the mechanism involved but influences significantly the ignition characteristics. The influence, however, strongly depends on the stoichiometry of the mixtures. The composition Ti:Si = 1:2 shows the largest influence, with the ignition temperatures decreasing from 1400 °C for unmilled powders to about 600 °C for powders milled for several hours. The compositionsTi:Si = 5:3, Nb:Si = 1:2 show less pronounced decreases, while the compositionMo:Si = 1:2 shows no decrease. These differences are discussed in terms of the role of microstructure in the reaction mechanism and the different response of the systems to contamination, particularly from oxygen. The results suggest that for these systems self-ignition processes during milling cannot be explained only on the basis of the decrease in the ignition temperature.

Experiment and FEM Analysis of Tensile Behavior of Bimodal Nanocrystalline Al-Mg Alloys

AbstractThe tensile behavior of bimodal nanocrystalline Al-7.5Mg alloys was investigated using experiments and two-dimensional axisymmetric elastic-plastic finite element method (FEM). Cryomilled nanocrystalline powders blended with 15% and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk bimodal nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions. The calculated stress-strain curves have acceptable agreement with experimental curves of the bimodal structures. The bimodal Al-7.5Mg alloys show reasonable ductility while retaining enhanced strength compared to conventional alloys and nanocrystalline metals.

Microstructure characterization and phase transformation kinetics of polymorphic transformed ball milled a-TiO 2–10 mol% m-ZrO 2 mixture by Rietveld method

Nanocrystalline polymorphic TiO 2 composites and ZrTiO 4 phase mixture have been prepared by high-energy ball milling the mixture of anatase TiO 2–10 mol% monoclinic ZrO 2 and post-annealing the ball milled powder at different elevated temperatures. For comparison the phase transformation kinetics at elevated temperatures, unmilled powder mixture and ball milled anatase TiO 2 powders have been post-annealed with the same heat treatment schedule. High temperature annealing up to 1273 K of the ball milled nanocrystalline composites reveals the instability and stability of the srilankite and rutile TiO 2 polymorphic phases, respectively and formation of ZrTiO 4 at 773 K. However, in unmilled mixture, only a partial anatase to rutile phase transformation is observed. The prepared composites may be utilized within a wide range of particle sizes. Phase transformation kinetics as well as microstructure characterization of ball milled materials for different durations and post-annealed materials of 12 h milled sample at different temperatures for 1 h duration, in terms of several microstructure parameters have been studied in detail employing Rietveld’s X-ray powder structure refinement methodology. Particle size, rms lattice strain, change in lattice parameter, mol fraction of individual phases, etc. have been estimated from the microstructure analysis and used to interpret the results. Particle size and lattice strain values of some of the phases are found to be anisotropic in nature. A very interesting phase transformation kinetics has been noticed with progress of milling and post-annealing. Both the 12 h ball milled samples of anatase TiO 2 and TiO 2+ZrO 2 mixture composed of srilankite and rutile phases. The monoclinic ZrO 2 is supposed to utilize as a solute to form the polymorphic TiO 2 solid solution. Post-annealing of 12 h ball-milled anatase TiO 2 powder at 973 K for 1 h duration results in complete formation of rutile phase but post-annealing of 12 h milled sample of TiO 2–ZrO 2 powder mixture at 773 K for 1 h duration results in formation of ZrTiO 4 phase and its content with rutile phase increases in expense of srilankite phase clearly indicates its formation from srilankite TiO 2–monoclinic ZrO 2 solid solution.

Mechanochemical Activation of Solid State Reaction between Mg and TiO2

The mechanochemical reaction of TiO 2 and Mg via mechanical alloying of Mg, TiO 2 and Al has been investigated. Reduction of TiO 2 is evident from substantial formation of MgO. Weakening of Mg and TiO 2 diffraction peaks along with broadening suggests refinement in grain size. Mechanically milled powder shows completion of reaction upon subsequent heat treatment while the reaction of the unmilled powder is still incomplete. Analysis suggests that the milled powder undergoes self-propagation while unmilled powder does not.

Reactive hot pressing of Al2O3-Ni composites

Reactive hot pressing has been used to form Al2O3-Ni composites from Al and NiO. The effect of attrition milling on the precursor powder and subsequent composite formation was examined. The surface area, phase assemblage, reaction temperature, and morphology of precursor powders were characterized as a function of milling time, which ranged from 0 (unmilled) to 480 min (8 hrs). During milling, particle surface area increased from less than 1 to more than 11 m2/g as the size of the Al and NiO particles decreased. At the same time, the temperature at which Al and NiO reacted to form Al2O3 and Ni decreased from more than 1000°C to around 600°C. Formation of Al2O3 or Ni during milling was not detected, regardless of time. Precursor milling time also affected the morphology and phase assemblage of composites produced by reactive hot pressing. Composites formed from unmilled powders contained a small amount of unreacted NiO and had a Ni ligament size greater than 10 μm. The composite forming reaction went to completion when powders milled for one hour or more were hot pressed. Based on microstructural evidence and analogy to similar reactions, it appears that the composite forming reaction proceeds by Al diffusing into and reacting with NiO.

Bimodal Structured Bulk Nanocrystalline Al-7.5Mg Alloy

ABSTRACTThe microstructure, mechanical properties and deformation of bimodal structured nanocrystalline Al-7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al-7.5Mg powders, and then cryomilled nanocrystalline powders blended with 15% and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk nanocrystalline alloys. Bimodal bulk nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions, show balanced mechanical properties of enhanced yield and ultimate strength and reasonable ductility and toughness compared to comparable conventional alloys and nanocrystalline metals.

Activation of the carbothermic reduction of manganese ore

The effect of extended milling on the carbothermic reduction of a manganese ore has been examined using a combination of thermal analysis and X-ray diffraction (XRD). Thermodynamic modelling indicated that reduction of MnO 2 to MnO was possible at 25 °C, although no reaction was found to occur during milling of the ore with graphite for up to 10 h. For a physical mixture, cryptomelane, KMn 8O 16, reduced at 500 °C and braunite, Mn 7SiO 12, at 700 °C after 10 h milling these temperatures were reduced by 200 °C. The initial product was Mn 3O 4, although in the 10-h-milled powder, the reduction of braunite may have been directly to MnO. Reduction at 600 °C only formed Mn 3O 4 in the unmilled powder but the major product in the 10-h-milled powder was MnO. The increased extent of reaction after premilling may allow current processing plants to expand their throughput without increasing the size of reduction kiln.

Formation of Mg–Al–Ti/MgO composite via reduction of TiO 2

This paper investigates the possibility of reduction of TiO 2 through the mechanical alloying of Mg and TiO 2 powders. Elemental Mg and Al powders were first mixed with TiO 2 according to the nominal composition of Mg5wt.%–Al17.15wt.%–TiO 2. The powder mixture was mechanically milled in a planetary ball mill for different durations. Two types of DSC curves have been observed. The unmilled powders showed no reaction when heated to about 500 °C, while an exothermic reaction in the mechanically milled powders was observed at about 390 °C. Further investigation of the heat treatment of the mechanically milled powders showed the complete disappearance of TiO 2 and the formation of MgAl 2O 4 after annealing at 400 °C, implying that reduction of TiO 2 has occurred via the mechanical activation and annealing processes.

Synthesis of dense nanometric MoSi2 through mechanical and field activation

The effect of mechanical and field activation on the synthesis of dense nanometric MoSi2 was investigated. Powders of Mo and Si, milled separately or comilled in a planetary ball mill, were reacted in a spark plasma synthesis (SPS) apparatus under different electric current conditions. Milled powders reacted faster and required less current than unmilled powders. Mixtures of powders which were milled separately (to nanometric size) reacted in the SPS to produce micrometric α–MoSi2. Similar results were obtained for samples comilled to produce nanometric reactants which did not contain detectable amounts of the product phase. When products form during milling, they contain both the α and β modifications of MoSi2. The product after the SPS reaction was nanometric MoSi2 with a crystallite size of 140 nm.

Highly enhanced sinterability of commercial PZT powders by high-energy ball milling

High-energy ball milling technique was successfully applied to a commercial lead zirconate titanate (PZT) powder to decrease its sintering temperature by 100°C. The sinterability enhancement of commercial PZT powder through this technique was evidenced by experimental results. The commercial PZT powder is made up of ball-shaped particles of tens of micrometers in size, consisting of <1-μm grains. After ball milling for 10 h, the large particles were totally broken down and the grain size was reduced by two orders of magnitude. The milled PZT powder achieves its maximum sintering rate at about 800°C, while the sintering rate peak for unmilled powder is about 1150°C. Almost fully dense PZT ceramics, which show good dielectric and ferroelectric properties, can be achieved by sintering the milled powder at temperature as low as 950°C. On the other hand, the unmilled powder still exhibits porous structure after being sintered at 1000°C and shows very high dielectric loss and loose P–E hysteresis loop. It can be concluded that high-energy ball milling is an effective technique to improve the sinterability of commercially available PZT powders and may be also applied to other materials.

The Reaction‐Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid‐State Oxidation of Aluminum Powder

The effect of attrition milling on the solid‐state oxidation of aluminum powder is important for the reaction‐bonded aluminum oxide process. Attrition milling increased the surface area to 14.4 and 20.2 m2/g versus 1.2 m2/g for unmilled powder and smeared the Al particles, and the surface was hydrolyzed to form bayerite and boehmite. Upon heating the hydroxides decompose to form an 11–13 nm thick amorphous plus γ‐Al2O3 layer which subsequently retards oxidation kinetics. The oxidation per unit area decreases for the higher surface area powders at temperatures below the critical temperature but the total oxidation of the milled powder is ∼70% versus ∼9% for the as‐received powder because of the higher surface area. The critical temperature depends on Al particle surface characteristics and is defined as the transition temperature above which the initial rate of oxidation is linear, not parabolic. Above the critical temperature the oxidation per unit area decreases significantly. In addition, linear oxidation occurs faster than parabolic oxidation and thus the initial fast oxidation kinetics (i.e., linear) can cause thermal runaway during oxidation. Therefore, oxidation below the critical temperature is essential to maximize solid‐state oxidation and to prevent thermal runaway. The critical temperatures for the as‐received (1.24 m2/g), the 6 h (14.4 m2/g), and 8 h (20.2 m2/g) attrition‐milled Al powders were 500°, 475°, and 500°C, respectively. A model for oxidation during the parabolic and linear oxidation stages is presented.