Milling Time Dependence Research Articles

Quick single-step mechanosynthesis of ZnO nanorods and their optical characterization: milling time dependence

We report on the growth of ZnO nanorods (NRs) by a quick single-step mechanochemical process and investigated the milling time dependence on the structural and optical properties of the ZnO NRs. Mechanochemical reactions are carried out in a planetary ball mill for the time durations ranging from 30 min to 5 h. XRD and TEM studies revealed wurtzite structure of the as-grown ZnO NRs with length of several hundreds of nanometers to few micrometers after 30 min of reaction. Average diameter of the as-grown ZnO NRs decreases from 40 to 15 nm with increasing reaction time. Micro-Raman spectra show red- shift in the characteristic Raman modes, indicating pres- ence of milling induced strain. As-grown NRs show blueshift in the excitonic absorption peak with increasing milling time due to decrease in size and induced strain. Room temperature photoluminescence (PL) spectra show strong band-edge related UV emission and other three major emission peaks, two in the UV-blue region and one at the visible region. Post-growth annealing of the as- grown ZnO NRs completely eliminates the defect related visible PL band. Low-temperature PL studies show an additional sharp peak related to donor-bound excitonic transition, revealing the n-type nature of the as-grown NRs.

Dependence of Milling Time on Electrochemical Properties of Nano Si Electrodes Prepared by Ball-Milling

Morphology, crystallinity and electrochemical properties of Si powders fabricated by a mechanical milling process with milling time of 6-40 h were investigated by means of FESEM, XRD, Raman, TEM and cell test. Average size of Si powders was reduced to sub-micrometer scale after 10 h-milling and 40 nm-Si powders could be obtained at 48 h-milling. With increasing milling time, Si powders mostly changes from crystalline Si to amorphous one and the content of amorphous Si was increased. TEM result showed that a negligiable amount of crystalline Si remained even after 48 h-milling. Si electrode with 48 h-milled Si powders exhibited the best electrochemical properties in terms of capacity, efficiency and cycle performance: initial capacity of 3025 mAh g(-1), coulrombic efficiency of 79.4% and the capacity retention of 1000 mAh g(-1) (at 130 cycles). The good electrochemical properties of nano- and amorphous-Si are due to the high resistance against volume change and good reversibility of Li ion.

XRD and TEM characterizations of the mechanically alloyed CuIn0.5Ga0.5Se2 powders

The CuIn0.5Ga0.5Se2 (CIGS) nanocrystalline powders were prepared by mechanical alloying method. Effect of various milling times and higher milling speed on the structure of CIGS nanoparticles was investigated by X-ray diffraction measurements. The Rietveld method was used to refine the XRD data using the MAUD program. Refinement process reveals that the main phase of the CIGS powders milled for different milling times is of chalcopyrite structure. Milling time dependence of the unit-cell parameters and crystallite size has also been reported. The TEM observations demonstrated that the size of agglomerated CIGS powder is about 140 nm. The EDAX analysis of various grains of the milled powder shows that the compositions vary from one grain to another. However, the global composition was found slightly copper rich.

The kinetics of conductivity type conversion in HgCdTe by ion beam milling

It is well established that dry etching of HgCdTe by Ar ion-beam milling (IBM) converts vacancy-doped p-type HgCdTe to n-type. Less well established is the understanding of the observed rate of conversion. This paper shows that the observed variations of the depth of conversion with milling time can be accounted for in terms of two rate processes operating in tandem. The first process is the diffusion of Hg interstitials from the surface to the conversion boundary followed by the second, or conversion, process that is the capture of the Hg interstitials by cation vacancies. Assuming steady-state conditions exist a quadratic equation is derived for the conversion depth and its milling-time dependence for two cases: when the initial p-type layer is electrically intrinsic at the milling temperature and when it is extrinsic. Good agreement is found with the available experimental Ar-ion-milling data. Good agreement is also found for type conversion due to etching with a radio-frequency Ar plasma. The effect of capture of Hg interstitials by traps, in addition to capture by cation vacancies, is also considered.

Enhancement of low-field magnetoresistance in Fe 3O 4 particles induced by ball milling

A series of Fe 3O 4 particles with different size have been obtained by mechanical ball milling from t=0–450 h. Crystal structure and microstructure of the samples are analyzed by XRD and SEM. An emphasis has been placed on magnetic and transport properties. The experimental results indicate that the sample t=350 exhibits an enhancement of magnetoresistance (MR) comparing with initial powder compress sample ( t=0). The low-field magnetoresistance reaches MR=−6.04% at Verwey temperature 120 K and MR=−2.54% at 290 K. Thermal behavior TGA analysis and investigation of magnetic properties have revealed that there is an oxide layer on surface of Fe 3O 4 particles. It is considered that the enhanced magnetoresistance can be taken into account in terms of spin-dependent tunneling effect between Fe 3O 4 particles. Temperature dependence of magnetization and resistivity are measured in order to study electrical and magnetic behavior near Verwey transition. In addition, we also discuss ball milling time dependence of coercivity H c and specific magnetization M s of these samples.

Melting behavior and origin of strain in ball-milled nanocrystalline Al powders

Ball-milled aluminum powders have been investigated by differential scanning calorimetry and high resolution X-ray line profile analysis. Ball-milling was carried out for different milling times ranging from 45 min to 32 days. The milling time dependence of the average grain-size and of the density of lattice defects, mainly dislocations, were determined by the modified Williamson-Hall and Warren-Averbach procedures based on the dislocation model of mean square strain. Characteristic grain sizes of ball-milled Al-powders decreases with increasing milling time and simultaneously, the grain-size distribution becomes sharper.

Magnetic properties of ball milled Cu70Fe15Mn15

We report on the structural and magnetic properties of mechanically alloyed samples having (Fe50Mn50)30Cu70 nominal composition. Our study included the evaluation of the milling time dependence of the average grain size and lattice parameter and that of the temperature dependences of the Mössbauer spectra, AC susceptibility, low field magnetization and coercivity. From the corresponding data we concluded about the presence of a canonical spin glass (CuMn), ferromagnetic (FeCuMn or FeCu) clusters and an antiferromagnetic FCC phase (γ-Fe) in our inhomogeneous samples.

Dependence of hydrogen storage characteristics of mechanically milled carbon materials on their host structures

We investigated whether the hydrogen storage characteristics of carbon materials prepared by mechanical milling in an H2 atmosphere were dependent on their host structures. We used natural graphite (NG) and activated carbon fibers (ACF) and compared them with activated carbon (AC) powders. The XRD patterns of NG and ACF milled for over 20 h and SEM images of these samples milled for 80 h were almost the same as those of AC. The hydrogen storage capacities of NG and ACF estimated by the inert gas fusion-thermal conductivity method increased with the mechanical milling time up to 10 h and showed little milling time dependence thereafter. The capacities of NG and ACF reached about 3.0 wt.% and were similar to that of AC. However, it should be noted that the hydrogen storage mechanism of NG and ACF mechanically milled in an H2 atmosphere might be different because the changes in their specific surface areas with milling time were opposite. Thermal desorption mass spectroscopy (TDS) revealed that the desorption spectra of the hydrogen molecules (mass number=2) of NG and ACF milled for 10 h in the same way as AC contained two peaks at about 500 and 800 °C. The desorption activation energies of hydrogenated NG and ACF at these peaks calculated from a Kissinger plot were almost with the same as those of hydrogenated AC. This suggests that the state of the hydrogen trapped in the structural defects in NG introduced by the mechanical milling may be almost the same as that of AC. In addition, we assumed the possibility that the state of the hydrogen in ACF hydrogenated by mechanical milling could be almost the same as that in hydrogenated AC. We considered that the nanocarbon materials hydrogenated under our milling conditions had very similar physical shapes and hydrogen storage capacities, independent of their host structures.

Dependence of hydrogen storage characteristics of mechanically milled carbon materials on their host structures

We investigated whether the hydrogen storage characteristics of carbon materials prepared by mechanical milling in an H 2 atmosphere were dependent on their host structures. We used natural graphite (NG) and activated carbon fibers (ACF) and compared them with activated carbon (AC) powders. The XRD patterns of NG and ACF milled for over 20 h and SEM images of these samples milled for 80 h were almost the same as those of AC. The hydrogen storage capacities of NG and ACF estimated by the inert gas fusion-thermal conductivity method increased with the mechanical milling time up to 10 h and showed little milling time dependence thereafter. The capacities of NG and ACF reached about 3.0 wt.% and were similar to that of AC. However, it should be noted that the hydrogen storage mechanism of NG and ACF mechanically milled in an H 2 atmosphere might be different because the changes in their specific surface areas with milling time were opposite. Thermal desorption mass spectroscopy (TDS) revealed that the desorption spectra of the hydrogen molecules (mass number=2) of NG and ACF milled for 10 h in the same way as AC contained two peaks at about 500 and 800 °C. The desorption activation energies of hydrogenated NG and ACF at these peaks calculated from a Kissinger plot were almost with the same as those of hydrogenated AC. This suggests that the state of the hydrogen trapped in the structural defects in NG introduced by the mechanical milling may be almost the same as that of AC. In addition, we assumed the possibility that the state of the hydrogen in ACF hydrogenated by mechanical milling could be almost the same as that in hydrogenated AC. We considered that the nanocarbon materials hydrogenated under our milling conditions had very similar physical shapes and hydrogen storage capacities, independent of their host structures.

The Temperature and Milling-Time Dependence of Coercivity in Ball-Milled Nd2Fe14B-Gd

Exchange coupled 20 vol% Nd 2 Fe 14 B-80 vol% Gd magnetic powders were synthesised by ball milling. Magnetometer measurements at 5 and 300 K showed that coercivity increased with milling time. The temperature dependence of the coercivity of a 30 h milled sample showed a gradual decrease from 5 K until approximately 270 K, after which the coercivity increased markedly to a peak before decreasing again.

Dependence of nanocrystalline phase formation and magnetic properties in Fe25Ni75 on ball milling time

Nanocrystalline Fe25Ni75 powders were prepared by the mechanical alloying process. They were investigated by X-ray diffraction and magnetic measurements. The ball milling time dependence of grain size, internal strain and magnetic properties has been discussed. The results show that the formation of Fe25Ni75 alloy phase is almost completed after milling for 30 hours. The reduction of grain size accompanied by the growth of internal strain was observed after extended milling. An average grain size about 10–20 nm of alloyed powders has been determined by Sherrer formula estimation. The saturation magnetisation Ms has a slight increase when the average grain size continues to decrease by intensive milling. This fine size of mechanically alloyed powders (10–20 nm) results in a single domain magnetic structure and the formation of superparamagnetic phase.

Mössbauer studies of the phase decomposition process of YFe 2 by mechanical grinding

Mechanically ground YFe 2 was studied by means of X-ray diffraction and Mössbauer effects. It is shown that YFe 2 decomposes into amorphous Y-Fe and bcc Fe on mechanical grinding. Milling time dependence of the fraction of Fe atoms in each state is determined by Mössbauer analyses.