Ab initio molecular dynamics simulations of nano-crystallization of Fe-based amorphous alloys with early transition metals

The addition of early transition metals（ETMs） into Fe-based amorphous alloys is practically found to be effective in reducing the α-Fe grain size in crystallization process. In this paper, by using ab initio molecular dynamics simulations, the mechanism of the effect of two typical ETMs（Nb and W） on nano-crystallization is studied. It is found that the diffusion ability in amorphous alloy is mainly determined by the bonding energy of the atom rather than the size or weight of the atom. The alloying of B dramatically reduces the diffusion ability of the ETM atoms, which prevents the supply of Fe near the grain surface and consequently suppresses the growth of α-Fe grains. Moreover, the difference in grain refining effectiveness between Nb and W could be attributed to the larger bonding energy between Nb and B than that between W and B.

Microstructure Characteristic and Electrochemical Corrosion Behavior of Surface Nano-crystallization Modified Carbon Steel

The surface nano-crystallization （SNC） of carbon steel was achieved via a high-speed rotating wire-brush- ing process. Microstructure characteristics of SNC steel were systematically studied. The SNC steel surface exhibited marked deformed plastic flows and high surface roughness. Due to the accumulated strains, a deformed gradient lay er with thickness of 40-50 μm was produced, and the grain size of the topmost zone was about 50-100 nm. X-ray photoelectron spectroscopy （XPS） analysis indicated that enhanced Fe oxides and Cr oxides were generated. Electrochemical corrosion tests, including open circuit potential （OCP）, electrochemical impendence spectroscopy （EIS）, potentiodynamic polarization （PDP） and potentiostatic polarization （PP） were conducted to study the corrosion be- havior of SNC steel in 3.5 mass% NaC1 solution, where an improved corrosion resistance was observed. The resulted improvement resulted from the dominated positive effects （the attached Cr alloying element and enhanced oxide film） against the negative effects （the higher roughness and the improved corrosion activity of surface microstructure）.

A METHOD OF PREPARING SPHERICAL NANO-CRYSTAL CELLULOSE WITH MIXED CRYSTALLINE FORMS OF CELLULOSE Ⅰ AND Ⅱ

A new kind of nano-crysta cellulose (NCC) prepared from natural cotton fiber has been obtained by the method ofacid hydrolysis. Compared to most other nanophase materials that derive from inorganic materials, our products are preparedfrom natural cotton fibers. The products are of spherical shape with mixed crystal forms of cellulose I and II. The preparationconditions determine the properties of the products. Prior treatment is a critical procedure. The properties of the products arealso strongly affected by such conditions as the kinds of acids used, the ratio of the acid mixture, the acid concentration, theultrasonic agitation time and hydrolysis temperature. The number average molecular weight of NCC is determined by gelpermeation chromatography (GPC). The particle size and shape were determined by transmission electron microscopy(TEM). X-ray diffraction was used to detect the crystallinity and average crystallite size of the panicle.

Modified Scherrer Equation to Estimate More Accurately Nano-Crystallite Size Using XRD

Scherrer Equation, L=Kλ/β.cosθ, was developed in 1918, to calculate the nano crystallite size (L) by XRD radiation of wavelength λ (nm) from measuring full width at half maximum of peaks (β) in radian located at any 2θ in the pattern. Shape factor of K can be 0.62 - 2.08 and is usually taken as about 0.89. But, if all of the peaks of a pattern are going to give a similar value of L, then β.cosθ must be identical. This means that for a typical 5nm crystallite size and λ Cukα1 = 0.15405 nm the peak at 2θ = 170° must be more than ten times wide with respect to the peak at 2θ = 10°, which is never observed. The purpose of modified Scherrer equation given in this paper is to provide a new approach to the kind of using Scherrer equation, so that a least squares technique can be applied to minimize the sources of errors. Modified Scherrer equation plots lnβ against ln(1/cosθ) and obtains the intercept of a least squares line regression, ln=Kλ/L, from which a single value of L is obtained through all of the available peaks. This novel technique is used for a natural Hydroxyapatite (HA) of bovine bone fired at 600°C, 700°C, 900°C and 1100°C from which nano crystallite sizes of 22.8, 35.5, 37.3 and 38.1 nm were respectively obtained and 900°C was selected for biomaterials purposes. These results show that modified Scherrer equation method is promising in nano materials applications and can distinguish between 37.3 and 38.1 nm by using the data from all of the available peaks.

Characteristic Investigation of Nano-Crystal Hydrogen Storage Alloys

A Ml (NiCoMnA1)5 hydrogen storage alloys was prepared by double-roller rapid quenching.Its microstructure, electrochemically and kinetic characteristic were studied.A uniform crystal phase with CaCu5 structure could be detected by XRD analyses, whose average grain size is 30 ～ 50 nm and the ratio of c/a of nano-crystal hydrogen storage alloy is larger.The hydrogen absorption/desertion p - C isotherms of alloy show that its fiat-performance is perfect and the magnetic stagnant effect is very little.An simulate cell is used for electrochemical measurement.Electrode is 10C, the capacity decreasing rate via the 450 cycles at 7C is less than 20%.

Determination of the Nucleation Region Location of Si Nano-Crystal Grains Prepared by Pulsed Laser Ablation through Changing Position of Substrates

To determine the nucleation region location of Si nano-crystal grains, pulsed laser ablation of Si target is performed in Ar gas of 10 Pa at room temperature with laser fluence of 4 J/cm2, the substrates are located horizontal under ablation spot with different vertical distance. Characteristics of deposited grains are described by scanning electron microscopy, Raman scattering and X-ray diffraction spectra, the results indicate that deposition position on substrates in a certain range is relative to target surface, which changes according to different vertical distance of substrates to ablation spot. Grain size increased?at first and then decreased with addition of lateral distances to target in the range, but the integral distribution rule was independent of position of substrates. Combining with hydrodynamics model, nucleation division model, thermokinetic equation and flat parabolic motion, spatial nucleation region location of grains is obtained through numerical calculations, which is 2.7 mm-43.2 mm to target surface along the plume axis.

STUDY ON THE SHOCK WAVE CRYSTALLIZATION OF AMORPHOUS ALLOYS BY DSC

Shock wave and annealing crystallization of amorphous alloys FeSiB, FeMoSiB and FeCuNbSiB were studied by isothermal and non-isothermal DSC technique. It was found that the shock wave crystallization is very perfect, the fraction crystallized is very close to 100%, though the period of crystallization is very short, only about 10-4-10-6s. Their produced phases differ from the parent phase in structure and composition. The high velocity of the transformation is very difficult to explain by the diffusion theory of solid state phase transition.

High performance nano-structured stainless steel sheet

High-strength steels have been attracting more and more attention of people,Unfortunately.deterioration of ductility limited their applications.To solve this problem,a nano-structured stainless steel sheet is developed to combine high strength and high ductility.Processing of the surface mechanical attrition treatment(SMAT) was introduced to obtain a nano-grain layer on the double surface of the stainless steel sheet.The microstructure of the nanostructured steel sheet is characterized by an alternate distribution of coarse grained layer and nanocrystalline layer.Then the dual surface nano-crystallized stainless steel sheets were co-warm rolled at 500℃.The experimental results reveal that the mechanical properties of the nanostructured steel exhibit high yield strength in the range of 700 -950 MPa and tensi le strength higher than 930 MPa.Moreover,elongation to fracture reaches to 15%-48%, together with a uniform elongation stabilized to 13%-45%.

SYNTHESIS OF Ni-Mo ALLOYS BY MECHANICAL ALLOYING

Ni_(100-x)Mo_x(x=20,30,38)alloys have been synthesized by mechanical alloying.The structure evolution of powders in mechanical alloying process has been monitored by X-ray diffraction,scanning electron microscopy and transmission electron microscopy analyses.The results show that the Ni_(62)Mo_(38)sample becomes partially amorphous after high energy ball milling,while the Ni_(80)Mo_(20)and Ni_(70)Mo_(30)become non-equilibrium nanocrystals.

A novel synthesis method and up-conversion properties of hexagonal-phase NaYF_4:Er nano-crystals

A novel synthesis method for hexagonal （]3）ophase NaYF4：Er nano-crystals （NCs） which showed up-conversion （UC） from infrared to visible spectral region was developed. The NaYF4：Er NCs were synthesized in oleie acid （OA） and 1-0ctadeeene （ODE） with Y2（COa）3＇xH20, Er2（COa）3xH20, Na2CO3 and NH4F as precursors. This proposed method was simple and less toxic compared with generally used method so far. The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control ofNaYF4：Er NCs. The UC emission spectra were obtained with the emission wavelength at about 980 nm （4111/2--4115/2）, 800 nm （41912--415/2）, 660 nm （4F912--4115/2） and 540 nm （453/2----4115/2） from Er3＋ ions, by excitation wavelength of 1550 nm. The slope values, n, in the pump-power dependence, showed that the emission at 980 and 800 um were generated by 2-step UC and at 660 nm and 540 um were 3-step UC. The optical process for the UC excitation was discussed.

Structural and mechanical properties of nano-crystal TiN coatings

The structural and mechanical properties of TiN coatings prepared by ion beam assisted deposition (IBAD) were studied. The coatings have a polycrystal structure with grain size of ～10 nm or less. The hardness of the coatings increases with increasing grain size of TiN crystallites. The coating with grain size of 10.3 nm even has a superhardness of 44.7GPa. The relationship between the hardness and the grain size in the nano-crystalline coatings was discussed on the basis of grain-boundary triple junctions.

A novel 2 T P-channel nano-crystal memory for low power/high speed embedded NVM applications

We introduce a novel 2 T P-channel nano-crystal memory structure for low power and high speed embedded non-volatile memory（NVM） applications.By using the band-to-band tunneling-induced hot-electron （BTBTIHE） injection scheme,both high-speed and low power programming can be achieved at the same time. Due to the use of a select transistor,the ＂erased states＂ can be set to below 0 V,so that the periphery HV circuit （high-voltage generating and management） and read-out circuit can be simplified.Good memory cell performance has also been achieved,including a fast program/erase（P/E） speed（a 1.15 V memory window under 10μs program pulse）,an excellent data retention（only 20%charge loss for 10 years）.The data shows that the device has strong potential for future embedded NVM applications.

Preparation and luminescence properties of BaWO_4:Yb^(3+)/Tm^(3+) nano-crystal

This research investigated the effect of different contents of Tm3＋ and different concentration ratios of Yb/Tm on the lumi- nescent properties of BaWO4：yb3＋/Tm3＋ nano-crystal synthesized by the hydrothermal method. The results indicated that lumines- cence intensity reached the strongest when CTm=1.0 mol.% and Cyb/CTm=2：1. X-ray diffraction revealed the tetragonal system of BaWO4：Yb3＋/Tm3＋ and the grain sizes were between 31 and 45 nm according to Scherrer equation. The results of X-ray diffraction and scanning electron microscopy were similar. Four emission peaks at 454, 475, 647 and 790 nm including two blue emission peaks and two red emission peaks, corresponding to ID2→3F4, 1G4→3H6, IG4→3F4 and 3H4→3H6 transitions of Tm3＋, were observed under excitation of 980 nm semiconductor laser. Blue emissions at 454 and 475 um were four-photon and three-photon absorption respec- tively in accordance with the relationship between luminescence intensity and pump power.

Improved Plasticity of Fe_(25)Co_(25)Ni_(25)(Si_(0.3)B_(0.7))_(25) High Entropy Bulk Metallic Glass through the Addition of Cu

This work was focused on enhancement in the plasticity of Fe_(25)Co_(25)Ni_(25)(Si_(0.3)B_(0.7))_(25) high entropy bulk metallic glass(HE-BMG)by adding minor Cu(0–1.2,in at.%).It is found that the appropriate addition of Cu can efficiently improve the plasticity of the present HE-BMG while the excessive addition of Cu will deteriorate again the plasticity,and especially,the plastic strain of the present HE-BMG is improved from 0.8%for the Cu-free alloy to 4.7%for the 0.3 at.%Cu-added alloy.The transmission electron microscopy observation shows that there are manyα-Fe(Co,Ni)clusters with sizes of less than 5 nm dispersed in the glassy matrix in the 0.3 and 0.6 at.%Cu-added alloys,which may account for the significant improvement in the plasticity of the two alloys,and further increase in Cu content more than 0.9 at.%leads to the precipitation ofα-Fe(Co,Ni)and Fe(Co,Ni)B compound grains with diameters of tens of nanometers,in which the plasticity of the present HE-BMGs has deteriorated rapidly.

Ion diffusion,and hysteresis of magnesium hydride conversion electrode materials

MgH_(2),owing to a high theoretical capacity of 2038 mAh g^(−1),is regarded as a promising anode material for lithium-ion batteries(LIBs).However,the application of MgH_(2) is still far from satisfactory due to its poor cycling stability.Herein,nano-crystallization of MgH_(2) as an anode is applied for all-solid-state lithium-ion batteries(ASSLIBs)using LiBH4 as a solid-state electrolyte.The self-assembly designed MgH_(2) electrode on graphene could effectively alleviate the volume expansion,prevent the agglomeration of active substances,improve the electron transfer,and enhance the electrochemical performance of the anode material.As a result,a reversible capacity of 1214 mAh g^(−1) after 50 cycles is obtained.Significantly enhanced cycle life with a notable capacity of 597 mAh g^(−1) at a current density of 400 mA g^(−1) is delivered after 200 cycles.Further investigation on full cells also exhibits great application potential on ASSLIBs.