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.     

An investigation on the application of process control agents in the preparation and consolidation behavior of nanocrystalline silver by mechanochemical method

In this paper, the effect of various amounts and types of process control agent (PCA), i.e., stearic acid (SA) and ethylene bis-stearamide (EBS), in the production and consolidation behavior of nanocrystalline silver prepared by mechanochemical reduction of Ag₂O by graphite was studied. The structural evolution and morphology of powders were investigated using XRD, HRSEM and particle size analyzer techniques. The results showed the nanocrystalline Ag formed after 25 h of milling and the addition of PCA prolonged the synthesis process time. Also, the effect of EBS on prevention of the excessive cold welding of ultra-fine Ag particles in the final stages of milling was more serious than SA. In fact, the presence of PCA effectively inhibited the creation of coarse Ag particles and finally decreased the crystallite size to 14 nm. Moreover, with the addition of PCAs, the Brinell hardness of sintered Ag samples was considerably increased.     

Mechanical alloying of Cu–Mo powder mixtures and thermodynamic study of solubility

A set of Cu-based powder mixtures containing up to 8 at.% Mo has been processed through mechanical alloying for a range of milling times up to 75 h. The milling operation was carried out using two types of milling media, stainless steel and tungsten carbide balls, keeping a constant powder to balls weight ratio of 10:1. The alloyed powders obtained were characterised by X-ray diffraction and scanning electron microscopy in order to determine the mean crystallite size and particles morphology after milling, respectively. The crystallite size produced after milling was quantitatively determined based on both, the Scherrer and Williamson–Hall methods. An estimate of the increased solubility of Mo in Cu, produced by the microstructural refinement during milling, was carried out through a thermodynamical analysis based on the influence of crystallite size reduction on chemical potentials.     

Characteristics of W-20Cu nano-crystallite composites fabricated by mechanical alloying

In the present study, the microstructure and properties characteristics of W-20Cu nano-crystallite composites were investigated. Characterization techniques like XRD and SEM have been used to study the crystallite size of W-Cu powder obtained by mechanical alloying. As well as, the effect of milling time on the microstructure and properties of W-20Cu composites was discussed. The results show that with increasing milling time, the crystallite size of W-Cu composite powder decreased and kept steady at last, and the crystallite size of W(Cu) solid solution was 6.6 nm for milling 20 h. The microstructure of W-20Cu composites became homogeneous and tungsten crystallite size became fine. The relative density and bending strength of W-20Cu composites increased. The value of thermal conductivity peaked when milling time was 20 h.     

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.     

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.     

Elaboration and compressibility behavior of nanostructured SiC

Nanostructured SiC has been elaborated from coarse grained SiC by mechanical attrition. Milling time ranged from 1 h to 14 h. Crystallites sizes and microstrains have been determined from X-ray diffraction peak profile analysis using a “one peak method”. According to this method, the average crystallite size calculated from the (11.0) and (10.4) 6H-SiC reflections is less than 50 nm with a milling time of one hour. Up to 6 h milling time, crystallite sizes determined from these two peaks are very close, indicating that crystallite size evolution versus milling time is nearly identical for these two crystallographic directions. We observe a strong decrease of the crystallite size up to 3 h milling time. A saturation value is reached after 6 h milling time, associated with a strong increase of microstrains.From density-pressure measurements performed in uniaxial compression, the powder's compressibility behavior appears to be strongly related to changes in powder morphology and granularity during ball milling. The evolution of the powder morphology was followed by scanning electron microscopy. The iron contamination level by the steel vial and balls was controlled by X-ray microanalysis.     

Microstructural characterization of amorphous and nanocrystalline boron nitride prepared by high-energy ball milling

Microstructural parameters like crystallite size, lattice strain, stacking faults and dislocation density were evaluated from the X-ray diffraction data of boron nitride (BN) powder milled in a high-energy vibrational ball mill for different length of time (2-120 h), using different model based approaches like Scherrer analysis, integral breadth method, Williamson-Hall technique and modified Rietveld technique. From diffraction line-broadening analysis of the successive patterns of BN with varying milling time, it was observed that overall line broadening was an operative cause for crystallite size reduction at lower milling time (∼5 h), whereas lattice strains were the prominent cause of line broadening at higher milling times (>19 h). For intermediate milling time (7-19 h), both crystallite size and lattice strain influence the profile broadening although their relative contribution vary with milling time. Microstructural information showed that after long time milling (>19 h) BN becomes mixture of nanocrystalline and amorphous BN. The accumulations of defects cause this crystalline to amorphous transition. It has been found that twin fault (β′) and deformation fault (α) significantly contributed to BN powder as synthesized by a high-energy ball-milling technique. Present study consider only three ball-milled (0, 2 and 3 h) BN powder for faults calculation because fault effected reflections (1 0 1, 1 0 2, 1 0 3) disappear with milling time (>3 h). The morphology and particle size of the BN powders before and after ball milling were also observed in a field emission scanning electron microscope (FESEM).     

Nature and density of lattice defects in ball milled nanostructured copper

Copper powder of 99.9% purity with particle size in the micrometer range was subjected to high energy ball milling by milling times between 2 and 24 h applying stearic acid as surfactant. The nature and density of lattice defects were determined using differential scanning calorimetry (DSC) and X-ray line profile analysis (XPA). The DSC measurements exhibit a considerable drop of the total stored energy with increasing ball milling time indicating a surprising decrease of lattice defect concentrations by more than one order of magnitude. The results from XPA, however, show that neither the dislocation density, nor the crystallite size can account for this behavior. Rather it is to be attributed to a high concentration of deformation induced vacancy type defects, with their density gradually decreasing during ongoing milling.     

Particle fracture and plastic deformation in vanadium pentoxide powders induced by high energy vibrational ball-mill

An X-ray powder profile analysis in vanadium pentoxide powder milled in a high energy vibrational ball-mill for different lengths of time (0–250 h), is presented. The strain and size induced broadening of the Bragg reflection for two different crystallographic directions ([001] and [100]) was determined by Warren-Averbach analysis using a pattern-decomposition method assuming a Pseudo-Voigt function. The deformation process caused a decrease in the crystallite size and a saturation of crystallite size of ∼ 10 nm was reached after severe milling. The initial stages of milling indicated a propensity of size-broadening due to fracture of the powder particles caused by repeated ball-to-powder impact whereas with increasing milling time microstrain broadening was predominant. WA analysis indicated significant plastic strain along with spatial confinement of the internal strain fields in the crystallite interfaces. Significant strain anisotropy was noticed in the different crystallographic directions. A near-isotropy in the crystallite size value was noticed for materials milled for 200 h and beyond. The column-length distribution function obtained from the size Fourier coefficients progressively narrowed down with the milling time.     

Influence of process control agent type on the mechanosynthesis of Fe3O4 particles

The influence of the nature of the process control agent (PCA) used in the mechanosynthesis of the magnetite nanoparticles has been studied. The two-step route used here for obtaining nanocrystalline/nanoparticles Fe₃O₄ consists of a heat treatment, to prepare well-crystallised magnetite, followed by the mechanosynthesis process. Dry milled magnetite samples have been obtained as a reference, using the same conditions (duration and energy), to determine the influence of the process control agents (PCA). Three different PCAs have been used: benzene, ethanol and oleic acid. The characterisation of the magnetite particles has been performed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), magnetic measurements M(H), differential scanning calorimetry (DSC) and thermogravimetric (TG) measurements and Scanning Electron Microscopy (SEM). XRD and SEM analysis revealed a different processing mechanism for the two milling modes, wet and dry. In the case of dry milling, even for short milling times, iron contamination and formation of a wüstite – FeO phase is noticed. The use of the PCA during the milling process limits the above-mentioned contamination. Ethanol and benzene uses as PCA lead to synthesis of fine uniform sized particles. SEM images reveal the presence on nanoparticles. In the case of oleic acid, DSC, TG and magnetic measurements revealed the presence of a thin layer of oleic acid adsorbed on the particles. FTIR analysis highlighted the presence of both free and bonded oleic acid. The magnetisation of the samples was found to be linked to the powder contamination (FeO or oleic acid), structural defects or finite size effects.     

TiCN超细粉末的制备工艺研究

采用湿式高能球磨法制备超细TiCN粉体,考察不同球磨时间下粉体粒度的分布情况,分析球料质量比和球磨时间对TiCN粉体粒度的影响,并利用扫描电镜观察制备粉体的形貌。结果表明:随着球料质量比和球磨时间的增加,TiCN粉体平均粒径均出现先减小后增大的趋势,当球料质量比为8∶1时,球磨50h可获得平均粒径为0.84μm的类球形TiCN粉体。     

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.     

The use of X-ray diffraction peak-broadening analysis to characterize ground Al2O3 powders

X-ray peak broadening has been used to study the milling behaviour of a number of commercial alumina powders. It is shown that the milling behaviour is dependent upon the original particle size, internal defects in particles and the milling liquid used. Peak-broadening studies allow the effects of milling upon reduction of crystallite size and increase in stored energy to be separated. The effect of these two parameters was separated using the Cauchy correction method. Measurement of the particle size of the unmilled alumina powders in the transmission electron microscope was used to determine that the Cauchy method gave the most correct estimation of crystallite size. Both alumina crystallite size and stored energy are expected to enhance sintering of the powder to a high density. Attempts are made to predict the sintering ability of the materials studied in terms of the above parameters.     

Investigation of microstructural characteristics of nanocrystalline 12YWT steel during milling and subsequent annealing by X-ray diffraction line profile analysis

The variations of dislocation density, character of dislocations, and crystallite size as a function of milling time and post-heat-treating temperature were investigated for 12YWT nanocomposite ODS ferritic steel using X-ray diffraction line profile analysis. The modified Williamson–Hall and the modified Warren–Averbach methods, which are based on the dislocation model of the strain anisotropy, were utilized to characterize the microstructural parameters of the nanocomposite material and the matrix alloy. The presence of nano-oxide particles in the ODS steel caused an initially sharp decrease in the average crystallite size; however, with increasing milling time, the crystallite size of the unreinforced alloy reached the comparable value of that of the reinforced material. The subsequent heat treating on the powders milled for 80 h showed that the presence of Y₂O₃ dispersoids increased the recrystallization temperature and suppressed the grain growth up to 800 °C in the 12YWT alloy as compared to the matrix alloy which occurred about 700 °C. The results of X-ray diffraction line profile analysis also showed that the contribution of edge components of the dislocations increased at the initial milling stages, while the screw components tended to increase after 40-h milling time.     

Synthesis of sea urchin-like carbon nanotubes on nano-diamond powder

Carbon nanotubes (CNTs) have unique atomic structure and properties, such as a high aspect ratio and high mechanical, electrical and thermal properties. On the other hand, the agglomeration and entanglement of CNTs restrict their applications. Sea urchin-like multiwalled carbon nanotubes, which have a small aspect ratio, can minimize the problem of dispersion. The high hardness, thermal conductivity and chemical inertness of the nano-diamond powder make it suitable for a wide range of applications in the mechanical and electronic fields. CNTs were synthesized on nano-diamond powder by thermal CVD to fabricate a filler with suitable mechanical properties and chemical stability. This paper reports the growth of CNTs with a sea urchin-like structure on the surface of the nano-diamond powder. Nano-diamond powders were dispersed in an attritional milling system using zirconia beads in ethanol. After the milling process, 3-aminopropyltrimethoxysilane (APS) was added as a linker. Silanization was performed between the nano-diamond particles and the metal catalyst. Iron chloride was used as a catalyst for the fabrication of the CNTs. After drying, catalyst-attached nano-diamond powders could be achieved. The growth of the carbon nanotubes was carried out by CVD. The CNT morphology was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mean diameter and length of the CNTs were 201 nm and 3.25 microm, respectively.     

Pb1.5Nb2O6.5型微波介质陶瓷粉体的焙烧工艺研究

探讨了合成Pb1.5Nb2O6.5型微波介质陶瓷粉体的焙烧温度、焙烧时间、添加剂等不同工艺因素对Pb1.5-Nb2O6.5粉体物相变化、晶粒生长及形貌的影响,采用X射线衍射(XRD)和扫描电子显微镜(SEM)对制备的粉体进行物相与晶体形貌检测.实验结果表明在其他工艺因素控制得当时,焙烧温度对粉体粒度和物相变化的影响最大,而不同类型的添加剂不仅可以使晶粒生长更为完整,尺寸增大,同时也可以不同程度地改变粉体颗粒的生长趋势与形貌.实验制备出的Pb1.5Nb2O6.5粉体物相单一无杂质、晶粒尺寸较小,适于进一步制备微波介质陶瓷元件.     

X-ray studies of structure defects in nanostructured FeAl alloy

The evolution of the microstructure of the mixture iron and aluminium powders during the high energy mechanical alloying process was investigated by X-ray line profile analysis. Analysis of line breadths was carried out to get an insight into the interrelated effects of grain size, lattice strain and dislocations. The final product of the MA process was the nanocrystalline Fe(Al) solid solution with a mean crystallite of 10 nm. On the basic on the modified Williamson-Hall plots, the root-mean squared strains were explained by the presence of dislocations, with a dislocation density of about 6 × 10¹⁶ m^− 2. The identified steady-state saturation values of these parameters can be related to accumulate strain hardening of the powder material during longer milling times.     

X-ray line profile analysis of the ball-milled Fe–30Co alloy

This work deals with the microstructural properties of the Fe–30Co alloy prepared by ball milling of elemental iron and cobalt powders. The obtained mixed powder has been characterized by means of scanning electron microscope, X-ray microanalysis, laser diffraction, X-ray diffraction and microhardness measurements. X-ray line profile analysis based on the Rietveld method and adopting two different models has been used for the microstructural study. The refinement of the X-ray patterns shows that after 3 h of high energy milling the Fe(Co) is formed. The obtained Fe(Co) solid solution is characterized by body centered cubic structure with a lattice parameter a = 0.28564 ± 0.00004 nm and an ellipsoidal crystallite and microstrain field. The dissolution of cobalt in iron matrix is accompanied by the compression of the crystalline lattice by 0.37%. The progress of milling process produces an increase of the Debye–Waller factor and the dislocation density leading to the hardening of the powder. The variation of microhardness with milling time shows a change in hardening mechanisms.