Effect of organic liquid process control agents on properties of ball-milled powders

Despite widespread use of liquid process control agents (PCA) during material processing via high-energy ball milling, their effect on the properties of the milled powders has not been quantified or understood. It is generally accepted that PCA affect the energy transfer between milling tools and powders being milled; however, which PCA properties are particularly important is not known. It is further unclear how to select a proper PCA depending on the specific objectives set for preparation of the milled materials. Here, an approach is offered to begin developing respective selection criteria and determine the correlations between different PCA properties and milling outcomes. Two powders, Bi₂O₃ and CuO, are milled using the same conditions and only varying liquid PCA. A set of thirteen organic liquids served as variable PCAs. Correlations between particle and crystallite sizes of the milled powders and multiple PCA properties were identified. PCA density affected both particle and crystallite sizes for both oxides. However, the effect of density was less significant when compared to those of other parameters uniquely important for specific milling outcomes for specific oxides. The significant correlations of particle sizes for Bi₂O₃ and CuO were with PCA proton affinity and surface tension, respectively. Crystallite sizes of Bi₂O₃ and CuO correlated respectively most strongly with the PCA dynamic viscosity and density. It is proposed that both physical and chemical interactions between PCA and powder affect the milling outcomes. Chemical interactions are specific for each powder/PCA pair and contribute most directly to changing particle and crystallite sizes of the powder being milled. Physical interactions are more generic. The approach developed here is found to be useful and, with a significantly expanded database, is expected to help developing a practical guide for selecting the PCA depending on the powder characteristics and the desired outcome of the material processing by milling.     

乙酸异戊酯共沸干燥法制备低团聚掺锑氧化锡纳米微粉

以SnCl4·5H2O和SbCl3乙醇溶液为原料,用阴离子树脂交换除氯水解法制备得到无氯离子的前驱体掺锑氢氧化锡胶体沉淀。首次对以含氧官能团为主的系列憎水有机溶剂进行了共沸干燥脱水研究,并对所得粉体团聚程度进行比较。实验发现掺锑氧化锡共沸脱水干燥效果与有机溶剂分子结构之间有密切关系,提出了选择共沸有机溶剂的3个原则,在一系列有机溶剂中选择了最符合的乙酸异戊酯进行干燥实验,与常用的正丁醇共沸溶剂进行了消除粉体团聚效果的比较。运用IR、BET、TEM、XRD等方法对掺锑氢氧化锡粉体的结构、比表面积、形貌、物相进行表征。结果表明,乙酸异戊酯溶剂是理想的共沸干燥有机溶剂,其干燥所得掺锑氢氧化锡蓬松粉体的比表面积为284.44m2/g,比用正丁醇处理的增大了22%。将乙酸异戊酯干燥所得的掺锑氢氧化锡微粉经热处理后得到了低团聚的掺锑氧化锡纳米微粉。     

Parametric optimization of NiFe2O4 nanoparticles synthesized by mechanical alloying

In this study, the Taguchi robust design method is used for optimizing ball milling parameters including milling time, rotation speed and ball to powder weight ratio in the planetary ball milling of nanostructured nickel ferrite powder. In fact, the current work deals with NiFe₂O₄ nanoparticles mechanochemically synthesized from NiO and Fe₂O₃ powders. The Taguchi robust design technique of system optimization with the L9 orthogonal array is performed to verify the best experimental levels and contribution percentages (% ρ) of each parameter. Particle size measurement using SEM gives the average particle size value in the range of 59–67 nm. X-ray diffraction using Cu Kα radiation is also carried out to identify the formation of NiFe₂O₄ single phase. The XRD results suggest that NiFe₂O₄ with a crystallite size of about 12 nm is present in 30 h activated specimens. Furthermore, based on the results of the Taguchi approach the greatest effect on particle size (42.10 %) is found to be due to rotation speed followed by milling time (37.08 %) while ball to powder weight ratio exhibits the least influence.     

A study on the synthesis of nanostructured WC–10 wt% Co particles from WO<Subscript>3</Subscript>, Co<Subscript>3</Subscript>O<Subscript>4</Subscript>, and graphite

The formation of the nanostructured WC–10 wt% Co powder from WO₃, Co₃O₄, and graphite is studied. The effects of the processing parameters of high-energy ball milling, reduction in H₂ atmosphere, and carburization in Ar/CO atmosphere are investigated. The crystallite size of the as-synthesized WC is 30–40 and 40–50 nm for 900 and 1000 °C carburized powders, respectively. The powder is agglomerated with the size of the primary particles ranging from 50 to 700 nm. High-energy ball milling of WO₃–Co₃O₄–C powder mixtures leads to finer particle and crystallite sizes with larger surface area. Such milled powders can be reduced to nanostructured W at 570 °C and carburized to form WC at temperatures as low as 900 °C. Crystal growth has taken place during carburization, particularly at 1000 °C, which results in the formation of truncated triangular prisms and nanoplates of WC at 1000 °C.     

Effect of ball milling on the physical and mechanical properties of the nanostructured Co–Cr–Mo powders

In the present study, Co-based machining chips (P1) and Co-based atomized alloy (P2) has been processed through planetary ball mill in order to obtain nanostructured materials and also to comprise some their physical and mechanical properties. The processed powders were investigated by X-ray diffraction technique in order to determine several microstructure parameters including phase fractions, the crystallite size and dislocation density. In addition, hardness and morphological changes of the powders were investigated by scanning electron microscopy and microhardness measurements. The results revealed that with increasing milling time, the FCC phase peaks gradually disappeared indicating the FCC to HCP phase transformation. The P1 powder has a lower value of the crystallite size and higher degree of dislocation density and microhardness than that of the P2 powder. The morphological and particle size investigation showed the role of initial HCP phase and chemical composition on the final processed powders. In addition results showed that in the first step of milling the crystallite size for two powders reach to a nanometer size and after 12 h of milling the crystallite size decreases to approximately 27 and 33 nm for P1 and P2 powders, respectively.     

Preparation and characterisation of ultrafine lead titanate (PbTiO3) powders

Ultrafine lead titanate (PbTiO₃) powders in tetragonal form have been successfully prepared via three processing routes, namely, conventional co-precipitation, microemulsion-refined freeze drying, and microemulsion-refined co-precipitation. The formation process of lead titanate from the resulting precursors was monitored using techniques such as thermal analyses and X-ray diffraction for phase identification. It was found that the two microemulsion-refined processing routes led to a lower formation temperature for lead titanate than that observed in the conventional co-precipitation route. The three lead titanate powders have also been compared for particle and agglomerate size distributions and specific surface area. It appears that the microemulsion-refined co-precipitation is the technique which results in the formation of the finest lead titanate powder amongst the three processing routes investigated in the present work. This revised version was published online in September 2006 with corrections to the Cover Date.     

Synthesis, characterization and annealing of mechanically alloyed nanostructured FeAl powder

Elemental powders of Fe and Al were mechanically alloyed using a high energy rate ball mill. A nanostructure disordered Fe(Al) solid solution was formed at an early stage. After 28 h of milling, it was found that the Fe(Al) solid solution was transformed into an ordered FeAl phase. During the entire ball milling process, the elemental phase co-existed with the alloyed phase. Ball milling was performed under toluene to minimise atmospheric contamination. Ball milled powders were subsequently annealed to induce more ordering. Phase transformation and structural changes during mechanical alloying (MEA) and subsequent annealing were investigated by X-ray diffraction (XRD). Scanning electron microscope (SEM) was employed to examine the morphology of the powders and to measure the powder particle size. Energy dispersive spectroscopy (EDS) was utilised to examine the composition of mechanically alloyed powder particles. XRD and EDS were also employed to examine the atmospheric and milling media contamination. Phase transformation at elevated temperatures was examined by differential scanning calorimeter (DSC). The crystallite size obtained after 28 h of milling time was around 18 nm. Ordering was characterised by small reduction in crystallite size while large reduction was observed during disordering. Micro hardness was influenced by Crystallite size and structural transformation.     

Effects of stearic acid on synthesis of nanocomposite WC–MgO powders by mechanical alloying

The effects of stearic acid as a process control agent (PCA) on the synthesis of WC–MgO by mechanical alloying have been investigated. 0–2.0 wt% of stearic acid is added into the mixture of WO₃, Mg, and graphite powders in high-energy planetary ball milling experiments, and the as-milled powders are characterized by XRD and TEM. Results show that the mechanochemical reaction among WO₃, Mg, and graphite to form WC–MgO can be changed from a mechanically induced self-propagating reaction (MSR) to a gradual reaction by the addition of stearic acid in the range from 1.2 to 1.8 wt%, when other milling parameters are maintained at the same level. It has also been found that with the addition of stearic acid, the crystallite and particle size of WC–MgO powders can be refined, the homogeneity of particle size can be improved and the powder yield can be increased.     

纳米四方多晶氧化锆粉体的低温制备及反应机理研究

采用引入有机添加剂的低温超强碱法制备出纳米四方多晶氧化锆粉体.采用X射线荧光光谱、X射线衍射、透射电镜对粉体物相组成和微观结构进行了分析.结果表明,有机添加剂-低温超强碱法在反应初期氧化锆晶核就已经形成,这与普通的湿化学法显著不同;同时有机添加剂的引入,在反应开始就减少了粉体的团聚,利于获得分散性好的高质量粉体;所制备的粉体具有粒度小、粒度均匀和分散性好等特点;该方法具有低温高效节能的特点.     

Processing of Cu-Fe and Cu-Fe-SiC nanocomposites by mechanical alloying

The Cu-Fe and Cu-Fe-SiC nanocomposite powders were synthesized by a two step mechanical alloying process. A supersaturated solid-solution of Cu-20 wt% Fe was prepared by ball milling of elemental powders up to 5 and 20 h and subsequently the SiC powder was added during additional 5 h milling. The dissolution of Fe into Cu matrix and the morphology of powder particles were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. It was found that the iron peaks in the XRD patterns vanish at the early stages of mechanical alloying process but the dissolution of Fe needs more milling time. Moreover, the crystallite size of the matrix decreases with increasing milling time and the crystallite size reaches a plateau with continued milling. In this regard, the addition of SiC was found to be beneficial in postponing the saturation in crystallite size refinement. Moreover, the effect of SiC on the particle size was found to be significant only if it is added at the right time. It was also found that the silicon carbide and iron particles are present after consolidation and are on the order of nanometer sizes.     

Aluminium nitride ceramics prepared by a pyrolytic route

Metallic aluminium was dissolved in a fluid organic electrolyte. Drying of the viscous solution obtained, by simply evaporating the organic compounds, resulted in a relatively coarse powder with a broad particle size distribution. By contrast, a very fine-grained powder with a mean particle size in the submicrometre range, was obtained by hot-paraffin drying of a precursor solution of controlled viscosity. Sintering experiments with hot-paraffin dried powders calcined in ammonia, showed that these powders can be densified at 1820 °C without sintering additives, to densities of >99% theoretical density.     

Metal oxide powder synthesized with amorphous metal chelates

Chelate powder consisting of amorphous particles was synthesized through the process in which the droplet of the chelate solution is dried in the gas phase and solidified in a moment using a splay-dry technique. To investigate the advantage of the use of amorphous chelate powder in the processing of metal oxide powder, this study provides following two routes: a conventional route of mechanically mixing of crystalline metal-ethylenediaminetetraacetic acid (EDTA) powders and spray-dry mixing of metal-EDTA. These routes were followed by calcinations of metal-EDTA powder to form metal oxide powder. In this study, morphology, crystallization and metal composition of resulting (Ba,Sr)TiO₃ and YBa₂Cu₃O₇ powders were investigated. The amorphous metal-EDTA powder involving several metal elements is appropriate for successful calcination at lower temperatures rather than the mixture of crystalline powders prepared by mechanically mixing.     

Thermally activated crystalline phase restoration in disordered barium ferrite powder prepared by ball milling

Depending on milling conditions (air or vacuum) for the same milling time, a different level of decomposition and structure disorder in BaFe₁₂O₁₉ ferrite powders can be obtained by ball milling. Air-milled material has a tendency to form a gas-saturated disordered structure (superoxide) and to transform into simple oxides (reduction process through oxygen-saturated disordered phase) as opposed to the vacuum-milled powder where a highly disordered phase occurs. Both powders have a different morphology (particle size and distribution). Annealing processes produce crystalline barium ferrite phase with interesting magnetic properties, but in the present paper only structural transformations are analysed. Scanning electron microscopy, X-ray diffraction and thermal analysis techniques were applied to study the effects of heat treatment (10 h at 773 or 1273 K) in disordered BaFe₁₂O₁₉ ferrite powders prepared by 1000 h ball milling in air and vacuum.     

Mechanical alloying and sintering of nanostructured tungsten carbide-reinforced copper composite and its characterization

Elemental powders of copper (Cu), tungsten (W) and graphite (C) were mechanically alloyed in a planetary ball mill with different milling durations (0–60 h), compacted and sintered in order to precipitate hard tungsten carbide particles into a copper matrix. Both powder and sintered composite were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and assessed for hardness and electrical conductivity to investigate the effects of milling time on formation of nanostructured Cu–WC composite and its properties. No carbide peak was detected in the powder mixtures after milling. Carbide WC and W₂C phases were precipitated only in the sintered composite. The formation of WC began with longer milling times, after W₂C formation. Prolonged milling time decreased the crystallite size as well as the internal strain of Cu. Hardness of the composite was enhanced but electrical conductivity reduced with increasing milling time.     

Synthesis of Cu–Zr–Al/Al2O3 amorphous nanocomposite by mechanical alloying

Two routes were used to produce Cu–Zr–Al/Al₂O₃ amorphous nanocomposite. First route included mechanical alloying of elemental powders mixture. In second route Cu₆₀Zr₄₀ alloy was synthesized by melting of Cu and Zr. Cu₆₀Zr₄₀ alloy was then ball milled with Al and CuO powder. It was not possible to obtain a fully amorphous structure via first route. The mechanical alloying of Cu₆₀Zr₄₀, Al and CuO powder mixture for 10 h led to the reaction of CuO with Al, forming Al₂O₃ particulate, and concurrent formation of Cu₆₂Zr₃₂Al₄ amorphous matrix. The thermodynamical investigations on the basis of extended Miedema’s model illustrated that there is a strong thermodynamic driving force for formation of amorphous phase in this system. Lack of amorphization in first route appeared to be related to the oxidation of free Zr during ball milling.     

Synthesis and characterization of V8C7 nanocrystalline powder by heating milled mixture of V2O5, C and Ca via mechanochemical activation

In this study, nano-crystalline vanadium carbide was synthesized through reduction of V₂O₅ by carbon and Ca using high energy ball milling and subsequent heat treatment. Vanadium pentoxide, calcium and carbon black were placed in a planetary ball mill and sampled after different milling times. The activated powders were synthesized by microwave heating at temperatures 800 °C. XRD and FESEM were used for characterization of synthesized powder. On the basis of obtained results, the synthesized V₈C₇ crystallites were in the scale of nanometers and the lattice parameter had some deviation from the standard value. Furthermore, investigations showed that at higher milling time, the amorphization degree of V₄C₃ phase increased, while the degree of crystallite decreased.     

Preparation of Al2O3–TiB2 nanocomposite powder by mechanochemical reaction between Al,B2O3 and Ti

Mechanochemical processing is a novel technique for the synthesis of nano-sized materials. This research is based on the production of Al₂O₃–TiB₂ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of aluminum, titanium and boron oxide powders was subjected to high energy ball milling. The structural evaluation of powder particles after different milling times was conducted by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that during ball milling the Al/B₂O₃/Ti reacted with a combustion mode producing Al₂O₃–TiB₂ nanocomposite. In the final stage of milling, the crystallite sizes of Al₂O₃ and TiB₂ were estimated to be less than 50 nm.     

Synthesis of nanocrystalline magnesium aluminate (MgAl2O4) spinel powder by the urea-formaldehyde polymer gel combustion route

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by the urea-formaldehyde (UF) polymer gel combustion route. A transparent gel formed from magnesium nitrate, aluminium nitrate and UF after drying underwet self-sustained combustion when initiated with a burning splinter. The combustion product on calcinations at 850 °C formed MgAl₂O₄ spinel. Calcination of the combustion product resulted in particle coarsening. The powder obtained by planetary ball milling of the spinel had a median particle size of 1.58 μm. The spinel particles are agglomerates of nanocrystallites of size in the range 10-30 nm. The compacts prepared by uni-axial pressing of the spinel powder sintered to >99% TD at 1600 °C.     

Synthesis of BF–PT perovskite powders by high-energy ball milling

0.7BiFeO₃–0.3PbTiO₃ (BF–PT) powders were synthesized from a mixture of the oxides Bi₂O₃, Fe₂O₃, PbO and TiO₂ using a Fritsch P4™ vario-planetary ball milling system. The perovskite structure of the BF–PT powder can be obtained well and the crystallite size of the powders was greatly reduced to 20–35 nm after milling for 8 h. The pre-calcined course shows a rhombohedral–tetragonal phase transition with the increasing temperature and shows the structure transition near the Curie temperature T_c.     

The influence of different drying conditions on powder properties and processing characteristics

A water-based suspension of submicron titania particles was dried using a variety of techniques. The resulting powders were fully characterized in order to observe the effect of drying conditions on particle agglomeration. Direct evaporation methods led to quite severe agglomeration, whilst removal of the water by a freeze-drying technique produced powders containing only weak-secondary clusters.

The consequences of the state of aggregation after drying on powder compaction, sintering rates and microstructural development were determined. Although all powders originated from a common starting suspension, samples isolated by freeze drying sintered most rapidly, reaching about 98% of the theoretical density after firing at 1150 °C for 2 h. Agglomerated powders obtained after drying by evaporation, using either a heat lamp or microwave oven to drive off the water, required twice as long to sinter to comparable density. Moreover there was evidence of a much finer-grained microstructure in ceramics fabricated from freeze-dried products.