Papers by Keyword: Cryomilling

Abstract: Rapid preparation of nanocrystalline γ-Fe₂O₃ powder with superparamagnetism was realized by cryomilling commercial Fe₂O₃ powder using liquid nitrogen. The effects of milling temperature and duration on the grain size, phase and microstructure of the nanocrystalline Fe₂O₃ powder were analyzed. Magnetic property of the nanocrystalline γ-Fe₂O₃ powder was also tested by magnetometer at room temperature. The results demonstrate that nanocrystalline γ-Fe₂O₃ powder with single phase can be prepared rapidly by cryomilling with liquid nitrogen. The mean particle size of γ-Fe₂O₃ powder can be reduced from 300 nm to 13 nm by cryomilling at −130 °C within 3 hours. The nanocrystalline γ-Fe₂O₃ powder shows superparamagnetism at room temperature.

Abstract: In order to prevent the oxidation of Ti, which ultimately leads to the generation of intermetallic compound Ti₃Al, a new method of cryomill in liquid nitrogen was used to deal with the Ti/Al₂O₃ powders. The size distribution, phase composite and microstructure of the powders were analyzed using laser particle size analyzer, XRD, and TEM, respectively. Then, the performances of Ti/Al₂O₃ cermet sintered using cryomilled powders and room temperature milled powders were compared. The results show that, with the increase of cryomilling time, the grain size decreases shapely and high reactivitive nanoscale powders are finally obtained. With the cryomilling in liquid nitrogen, the Ti-N bonds are formed, which successfully prevent the oxidation of Ti. Ti/Al₂O₃ cermet sintered using cryomilled powders shows higher density, better mechanical properties than that using RT milled powders.

Abstract: In this study, microstructural and mechanical properties of a Al-5 wt.% Si/2 wt% (NbB2, NbC) composite synthesized by sequentially milling (mechanical alloying (MA) and/or cryogenic milling (CM)) were investigated. Nb2O5, B2O3 and C powder blends were milled using high energy milling for 5 h and annealed at 1400 oC for 12 h to produce NbB2-NbC hybrid powders. The NbB2-NbC hybrid powders were mixed with the matrix Al-5 wt.% Si powders to constitute the Al-5 wt.% Si/2 wt.% (NbB2-NbC) powders blends which were mechanically alloyed (MA'd) for 4 h using SpexTM Mixer/Mill, cryo-milled for 10 min in a SpexTM 6870 Freezer/Mill and finally MA’d for 1h in SpexTM Mixer/Mill again. As-blended, MA’d and cryomilled powders were compacted in a hydraulic press with a uniaxial pressure of 450 MPa. Compacted samples were sintered at 570°C for 2 h under Ar gas atmosphere. Microstructural characterizations of the as-blended/MA'd powders and the sintered composites were performed using X-ray diffractometry (XRD) and scanning electron microscopy (SEM) techniques. Density and microhardness measurements and sliding wear tests were performed on the sintered composite samples. Sequentially milled and sintered Al-5 wt.% Si-2 wt.% (NbB2-NbC) samples had the highest mean microhardness value (2.29 ± 24.98 GPa) and the lowest wear volume loss (0.038 mm³).

Abstract: FeSiCr-epoxy composites consisting of different FeSiCr contents were prepared through the pressure molding after mixing by cryomilling at liquid nitrogen temperature. The electromagnetic properties including ac conductivity, permittivity, and permeability were investigated in detail. The results show that the frequency dispersions of ac conductivity followed the power law, indicating the hoping conduction behavior. The real permittivity of composites increased with the increase of FeSiCr contents, and the enhancement of permittivity may be attributed to the increase of the interface area between FeSiCr and epoxy, while Debye-like dielectric relaxations were observed, and the fitting analysis of Cole-Cole curve indicates a nonideal Debye relaxation. The frequency dispersions of permeability were also relaxation linetype, which is attributed to the domain wall displacement.

Abstract: The microstructure and dielectric properties of the graphene–epoxy composites prepared by cryomilling at liquid nitrogen temperature were studied by SEM and RF impedance material analyzer. The result indicated that both the dielectric constant and conductivity of the composites increased with the increase of the graphene content. The value of the dielectric constant of the composite with about 8 wt % of graphene was as high as 200. Moreover, the frequency dispersion behaviors of the conductivity within a certain frequency range accorded with the Jonscher's power law demonstrating that the conductive mechanism is hopping conduction. The negative reactance decreased with the increase of the testing frequency which indicated a capacitive character

Abstract: In this paper, epoxy resin was used as matrix to prepare composite cryomilling at liquid nitrogen temperature in which the content of CNTs powder vary from 25 wt% to 40 wt%. The microstructure and dielectric properties of the composite were analyzed by SEM and RF impedance material analyzer. The result showed that permittivity of the composites turned negative after the CNTs reached certain content. The plasma oscillation of delocalized electrons in CNTs conductive networks causes negative permittivity phenomenon.

Abstract: High performance nanostructured light metals and alloys are very interesting for replacing conventional heavier materials in many industrial components. High Energy Ball Milling and Cryomilling are useful techniques to obtain nanocrystalline powders. In this work the effect of several milling conditions such as rotation speed, time, ball to powder ratio and temperature on the crystallite and particle size and morphology in pure aluminum are presented. X-Ray Diffraction, Laser Diffraction and Scanning Electron Microscopy are used. High energy ball milling at ambient and cryogenic temperature of Al powders rapidly leads to a nanometer size down to about 35 nm. High ball to powder ratio promotes both low crystallite and particle size. Small crystallite size like 18 nm and particle size as 4 μm were achieved in the most energetic conditions at ambient temperature. Isopropyl alcohol used as liquid media and protective atmosphere has a strong influence on the results depending on the milling temperature of Al.

Abstract: Boron (B) has great potential to be the primary fuel in energetic systems for its high heating values per unit volume and mass. The existence of B₂O₃ layer on its surface holding the combustion of B back has limited its extensive utilization. Adding magnesium (Mg) into B can improve its poor combustion performance according to the previous research. A new technique, cryomilling, was employed to prepare Mg and B (Mg/B) composite powders. The powders were cryomilled with a ball-to-powder ratio (BPR) of 80: 1(w/w) and an impeller rotation speed of 400 rpm, 500 rpm and 600 rpm. The cryomilling time is 5 h, 6 h and 7 h. A small amount of ferrum (Fe) is introduced into the powders in spite that the main phases are Mg and B. The effects of cryomilling parameters, such as cryomilling time and rotation speed on Mg/B composite powders were investigated. The results show the amount of active Mg and B is over 80%. The bonding mechanism during the process is analogous to mechanical alloy.

Abstract: The operation with a combination of three processing routes: cryomilling, hot isostatic pressing (HIPping) and hot extrusion was adopted in the present study for preparation of the bulk nanocrystalline Al 7050. The microstructure and fractography of the bulk material were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Furthermore, the chemical composition, density and tensile properties of the material were also measured. Microstructural investigation showed that the grain size of the bulk nanocrystalline Al 7050 ranged from 100nm to 200nm. Numerous dispersoids with a diameter/length of ~50nm were observed on grain boundaries and inside the grains. Besides, one phase of these dispersoids existed in the bulk nanocrystalline Al 7050 was identified as Al₆(FeMn). These dispersoids dispersed within the bulk nanocrystalline Al 7050, to some extent, increased the mechanical properties and thermal stability of the material. The resulted sample exhibited ultimate strength of 412MPa with an elongation of 5.2% when tested under tensile load, which was a bit lower than that of the traditionally wrought Al 7050-T6. The present results suggested that improper selected starting powder and milling parameters resulted in the flake-like morphology of the cryomilled powder. The flake-like morphology made it difficult for the cryomilled powder to fill the can entirely and achieve a high density material, which led to the weak interface within the bulk material and in turn degraded the mechanical properties of the bulk nanocrystalline Al 7050 prepared in the present work.

Abstract: Two kinds of novel materials, Mg-1.6mol%Ni-0.4mol%NiO-2mol%MF (MF=NbF5, CrF3), along with Mg-1.6mol%Ni-0.4mol%NiO for comparison, were examined for their potential use in hydrogen storage applications, having been fabricated via cryomilling. The effects of NbF5 and CrF3 on hydrogen storage performance were investigated. A microstructure analysis showed that, aside from the main phase Mg, Ni and NiO phases, NbO, MgF2 and Mg2Ni were present in all samples after ball milling, MgH2 and NbH2 were observed in all samples after absorption. The CrF3-containing composite exhibited a good PCT results and a low onset desorption temperature under 0.1 MPa. The NbF5-containing composite exhibited a low absorption temperature of 323 K, a high hydrogen storage capacity of 4.03wt% at 373 K under the hydrogen pressure of 4.0 MPa, and it absorbed 90% of its full hydrogen capacity in 2700 sec and 100% in 5100 sec, it desorbed more than 1.8wt% in 3600 sec under vacuum environment. The CrF3-doped sample exhibited a low onset desorption temperature of 543 K under 0.1 MPa, and a low hysteresis coefficient of 0.25 at 573 K, and lower than 0.2 when temperature was 623 K. NbO and NbH2 played an important role in improving the absorption and desorption performance.