Thermodynamic Properties of Nanograin Boundary and Thermal Stability of Nanograin Structure

The thermal features of the nanograin boundary were described by a developed thermodynamic model. Using the nanocrystalline Cu as an example, the pressure, the bulk modulus, and the volume thermal expansion coef- ficient were calculated to characterize the thermodynamic properties of the grain boundaries on the nanoscale. Based on the parabola-type relationship between the excess free energy and the excess volume of the nanograin boundary, the thermal stability, as well as its evolution characteristics, was analyzed. The experimental re- sults of the temperature-varying grain growth in the nanocrystalline Cu, which exhibited the discontinuous nanograin growth behavior, verified the thermodynamic predictions. In addition, the quantitative relationships correlating the excess volume and the lattice expansion with the nanograin size were discussed.

Coupling stiffening effect of carbon nanotubes on modulus of nanograined metals:Mechanism and multiscale modeling

A quantitative relationship between grain size and Young's modulus of metal composites has not been established by considering the coupling effect of the reinforcements on microstructural deformation.The objective of this study is to investigate the mechanism of deformation coordination between the carbon nanotube and the composite microstructure using molecular dynamics simulations.Through the analysis of stress contributions from grain boundaries and grains,a grain size-dependent continuum model for the pure metal modulus is established.On this basis,a predictive model for the modulus of metallic composites is developed by considering the coupling deformation between carbon nanotubes and the grains.The proposed model is capable of accurately capturing the relationship between grain size and the modulus of metal composites.This study provides a guideline for microstructure-dependent multiscale modeling.

High-dielectric-constant nanograin BaTiO_(3)-based ceramics for ultra-thin layer multilayer ceramic capacitors via grain grading engineering

Four BT-based ceramic samples were prepared using a grain grading approach.The bigger-grained(~100 nm)and smaller-grained(~70 nm)BaTiO_(3)(BT)powders were mixed.The smaller-grained BT powder controlled the average grain size and guaranteed the reliability,while the bigger-grained powder enhanced the dielectric constant.Various percentages of bigger-grained BT powder were introduced to balance the average grain size and the dielectric constant.As the proportion of bigger grains increased,the dielectric constant(εr)improved significantly.The room-temperatureεr of 25%bigger-grain mixed BT(2623)was~50%higher than that of the sample with a similar average grain size without grain grading.The ceramic mixed with 15%bigger-grained BT showed comprehensive dielectric performance,which met the EIA X5R standard and provided a considerableεr of 1841 along with a low dielectric loss of 0.78%.Notably,the average grain size was 90 nm,which favors the applications in ultra-thin multilayer ceramic capacitors.

Achieving a high energy storage density in Ag(Nb,Ta)O_(3) antiferroelectric films via nanograin engineering

Due to its lead-free composition and a unique double polarization hysteresis loop with a large maximum polarization(Pmax)and a small remnant polarization(Pr),AgNbO_(3)-based antiferroelectrics(AFEs)have attracted extensive research interest for electric energy storage applications.However,a low dielectric breakdown field(Eb)limits an energy density and its further development.In this work,a highly efficient method was proposed to fabricate high-energy-density Ag(Nb,Ta)O_(3) capacitor films on Si substrates,using a two-step process combining radio frequency(RF)-magnetron sputtering at 450℃and post-deposition rapid thermal annealing(RTA).The RTA process at 700℃led to sufficient crystallization of nanograins in the film,hindering their lateral growth by employing short annealing time of 5 min.The obtained Ag(Nb,Ta)O_(3) films showed an average grain size(D)of~14 nm(obtained by Debye-Scherrer formula)and a slender room temperature(RT)polarization-electric field(P-E)loop(Pr≈3.8 mC·cm^(−2) and P_(max)≈38 mC·cm^(−2) under an electric field of~3.3 MV·cm^(−1)),the P-E loop corresponding to a high recoverable energy density(W_(rec))of~46.4 J·cm^(−3) and an energy efficiency(η)of~80.3%.Additionally,by analyzing temperature-dependent dielectric property of the film,a significant downshift of the diffused phase transition temperature(T_(M2-M3))was revealed,which indicated the existence of a stable relaxor-like AFE phase near the RT.The downshift of the T_(M2-M3) could be attributed to a nanograin size and residual tensile strain of the film,and it led to excellent temperature stability(20-240℃)of the energy storage performance of the film.Our results indicate that the Ag(Nb,Ta)O_(3) film is a promising candidate for electrical energy storage applications.

Thermal stability of nanograins with grain boundary relaxation in microalloyed Cu-Sb and Cu-Fe

Grain refinement into nanoscale significantly enhances the strength and hardness of metallic materials but generally reduces the thermal stability by the introduced strong tendency for grain boundary(GB)migration.Stabilizing nanograins through GB relaxation had been proven as an effective way in several metals recently when the grain size is below a critical value.Here,we discovered that the abnormally enhanced thermal stability induced by GB relaxation can be realized in 0.5%Sb or Fe doped Cu,similar to that in pure Cu.The observed critical grain sizes for GB relaxation in the two Cu alloys are almost the same as pure Cu.However,the GB relaxation effect on thermal stability is kind of suppressed compared to pure Cu with similar grain sizes because of the segregation/precipitation of Sb/Fe during the annealing process,which accelerates the detwinning and the failure of relaxed GBs.

The frequency-dependent polarization switching in nanograined BaTiO_(3) films under high-strength electric field

The polarization reorientation in ferroelectric nanomaterials under high-strength AC electric fields is intrinsically a frequency-dependent process.However,the related study is not widely seen.We report a phase-field investigation regarding the dynamics of polarization switching and the electromechanical characteristics of a polycrystalline BaTiO_(3) nanofilm under applied frequency from 0.1 to 80 kHz.The grain boundaries and the in-plane strains are considered in the model.The obtained hysteresis and butterfly loops exhibit a remarkable variety of shapes with the changing frequency.The underlying mechanism for the observed frequency-dependent physical properties was discussed via domain structure-based analysis.In addition,we examined the influence of the kinetic coefficient in the Ginzburg-Landau equation as well as the influence of the electric-field amplitude to the frequency dependency.It was found that a higher value of kinetic coefficient or field amplitude tends to enhance the mobility of polarization switching and to transform high-frequency characteristics to low-frequency ones.

Thermally-triggered grain boundary relaxation in a nanograined Ni-Mo-W alloy

Conventionally,nanograined metals and alloys can be stabilized through segregating foreign elements at grain boundaries(GBs).Yet such an effect may be offset by formation of second phase at elevated temperatures.In this paper,by introducing minor W into a binary Ni-Mo alloy,we found precipitation of intermetallic phases was suppressed,enhancing thermal stability of the nanograined structure.Characterized faceted GBs and a high-fraction ofΣ3 coincidence site lattice(CSL)boundaries illustrate that GB structures are relaxed by formation of copious annealing twins.Adding W reduces stacking fault energy of the solid solution and facilitates the thermally-triggered GB relaxation.Suppressed precipitation of the intermetallic phases may be attributed to depletion of solutes at relaxed GBs.

碳纳米管-铜复合薄膜材料的抗辐照损伤性能研究

利用磁控溅射沉积技术获得的碳纳米管-金属复合材料是一种新型的纳米多孔材料,涂覆碳纳米管-金属复合材料的结构组件在新型核反应堆中具有广阔的应用前景。但是目前的报道中对碳纳米管-金属复合材料抗辐照损伤性能的研究还很少。因此,本工作通过磁控溅射沉积技术,制备了两种不同厚度的碳纳米管-铜复合薄膜,结合室温下的He离子辐照对石蜡基矿物油有机蒸汽的脱氢效应在复合薄膜表面包覆碳层,研究了碳层包覆前后碳纳米管-铜复合薄膜的抗辐照损伤性能。结果表明,随着辐照剂量和辐照温度升高,样品内逐渐产生孔洞等缺陷,且孔洞尺寸较小,这与碳纳米管-铜复合薄膜较高的晶界体积分数和比表面积以及降低的缺陷迁移速率有关。与平面基底上沉积的铜薄膜相比,复合薄膜在室温和高温辐照后,晶粒尺寸明显减小,这与一维碳纳米管基底可显著限制碳纳米管轴向垂直方向上的晶界迁移有关。碳层包覆后的碳纳米管-铜复合薄膜的高温结构稳定性和抗辐照损伤性能明显增强,这得益于碳包覆碳纳米管-铜复合薄膜较高的晶界体积分数和比表面积以及稳定的纳米晶粒。

定向凝固条件下Ti_(3)Al合金中梯度纳米结构的形成机制与力学行为——原子尺度研究

通过分子动力学(MD)模拟方法对Ti_(3)Al合金在定向凝固条件下的生长机理进行系统研究,并通过对比纳米晶(NG)、粗晶(CG)和梯度纳米晶(GNG)合金,研究Ti_(3)Al合金的结构力学性能。结果表明,在凝固过程中Ti_(3)Al相优先生长为等轴晶组织,随后逐渐演变为柱状粒,并最终形成梯度纳米晶结构。此时,晶粒在与凝固方向平行的方向优先生长。此外,研究还发现,相较于NG和CG结构,GNG结构定向凝固合金具有更高的抗拉强度和更好的延展性。GNG结构不仅有效抑制了小晶粒区域的应变局域化和晶粒生长,而且促进了较大晶粒区域的位错形成,从而获得更好的力学性能。

Growth of BeO Nanograins Synthesized by Polyacrylamide Gel Route

BeO nanoparticles were synthesized by polyacrylamide gel route.The effects of the processing parameters on the morphology and size of the synthesized BeO nanoparticles were investigated.The calcination temperature of the gel precursor containing beryllium sulfate was determined by thermogravimetry and differential scanning calorimetry（TG-DSC）,which is around 690 C and 160 C lower than the general temperature.Xray diffractometry（XRD）,transmission electron microscopy（TEM）,and specific surface area measurements（BET） showed that the synthesized nanoparticles under 700 C were pure,globular and about ～5-20 nm with narrow distribution.Interestingly,the nanograins coalesced and grew under higher calcination temperatures and longer calcination time.The influence of calcination temperature on the morphology and growth behavior is greater than that of its duration.The activation energy for grain growth was estimated to be 24.53 kJ/mol,and the dominant growth mechanism was most likely to be related to the vapor transport in pore control mode and grain-rotation-induced grain coalescence（GRIGC） mechanism.

Grain boundary stability governs hardening and softening in extremely-fine nanograined metals

Subject Code:E01 Conventional polycrystalline materials become harder with decreasing grain size,following the classical Hall—Petch relationship,i.e.,strength increases reversely proportional to the square root of the grain size.Strengthening occurs due to dislocation pileups at grain boundaries that prevent the dislocations

Nanograins on Ti-25Nb-3Mo-2Sn-3Zr alloy facilitate fabricating biological surface through dual-ion implantation to concurrently modulate the osteogenic functions of mesenchymal stem cells and kill bacteria

Surface mechanical attrition treatment(SMAT)method is an effective way to generate nanograined(NG)surface on Ti-25 Nb-3 Mo-2 Sn-3 Zr(wt.%)(named as TLM),a kind ofβ-type titanium alloy,and the achieved nanocrystalline surface was proved to promote positive functions of osteoblastic cells.In this work,to further endow the NG TLM alloy with both good osteogenic and antibacterial properties,magnesium(Mg),silver(Ag)ion or both were introduced onto the NG TLM surface by ion implantation process,as a comparison,the Mg and Ag ions were also co-implanted onto coarsegrained(CG)TLM surface.The obtained results show that subsequent ion implantation does not remarkably induce the surface roughness and topography alteration of the SMAT-treated layers,and it also has little impact on the microstructure of the SMAT-derivedβ-Ti nanograins.In addition,the implanted Mg and Ag ions are observed to exist as MgO and metallic Ag na noparticles(NPs)embedding tightly in theβ-Ti matrix with grain size of about 15 and 7 nm,respectively.Initial cell adhesion and functions(including proliferation,osteo-differentiation and extracellular matrix mineralization)of rabbit bone marrow mesenchymal stem cells(rBMMSCs)and the bacterial colonization of Staphylococcus aureus(S.aureus)on the different surfaces were investigated.The in-vitro experimental results reveal that the Mg and Ag single-ion implanted NG surface either significantly promotes the rBMMSCs response or inhibits the growth ofS.aureus,whereas the Mg/Ag coimplanted NG surface could concurrently enhance the rBMMSCs functions as well as inhibit the bacterial growth compared to the NG surface,and this efficacy is more pronounced as compared to the Mg/Ag co-implantation in the CG surface.The SMAT-achieved nanograins in the TLM surface layer are identified to not only play a leading role in determining the fate of rBMMSCs but also facilitate fabricating dualfunctio nal surface with both good osteogenic and antibacterial activities through co-implantation of Mg and Ag ions.Our investigation provides a new strategy to develop high-performance Ti-based implants for clinical application.

Suppression of grain boundary migration at cryogenic temperature in an extremely fine nanograined Ni-Mo alloy

Microindentation creep tests on an electrodeposited extremely fine(4.9 nm) nanograined(ng) Ni-14.2 at.% Mo(Ni-14.2 Mo) at both room temperature(RT) and liquid nitrogen temperature(LNT) demonstrated that lowering temperature retarded softening in the ng Ni-Mo alloy. The obtained strain rate sensitivity at LNT was one order of magnitude lower than that at RT. Microstructural characterization revealed that mechanically-driven grain boundary(GB) migration was greatly suppressed by lowering temperature,which might be ascribed to the presence of solute Mo atoms that significantly retarded coupled GB motion at LNT. Deformation was instead carried by shear bands.

Comparing the Fatigue and Corrosion Behavior of Nanograin and Coarse-Grain IF Steels

In the present work,a nanograin layer of about 150 μm thick was formed on the surface of an interstitial-free（IF） steel via friction stir processing.Then,the fatigue and corrosion behaviors of IF steel with nanograin layer were compared with that of coarse-structure counterpart.More than threefold increase in the hardness was observed due to the formation of nanograin layer.The size of nanograms in the stir zone was within 30-150 nm.This resulted in 50%increase in the fatigue strength of nanostructured specimen.Furthermore,the fracture surfaces were characterized using field emission scanning electron microscopy and scanning electron microscopy.As for the fatigue behavior of nanograin IF steel,the fracture surface was characterized by the formation of nanospacing striations and nanodimples.Besides,the nanograin structure pronounced the passivity and exhibited higher corrosion resistance.

Formaldehyde vapour sensing property of electrospun NiO nanograins

Beads free polyvinyl alcohol(PVA)/NiO nanofibers with an average diameter of 400 nm were successfully prepared through the electrospinning method.NiO nanograins were formed along the axis of the nanofiber due to the calcination of asspun fibers for 24 h at 450℃ and their presence was confirmed by FESEM.NiO nanograins were characterized by XRD,XPS and FTIR.The characterization results showed the presence of NiO in nanograins and its polycrystalline nature with ionic states.The sensing studies of NiO nanograins were performed towards the pulmonary disease breath markers and they showed better response towards formaldehyde vapour at 350℃.Calcined NiO grains showed a good response towards the 11-1145 ppm of formaldehyde vapour at the operating temperature of 350℃.NiO nanograins also showed quick response time(37 s)and recovery time(14 s)towards 46 ppm of formaldehyde.A sensing mechanism was proposed for the formaldehyde vapour interaction at 350℃ with NiO nanograins.

Transition of deformation mechanisms from twinning to dislocation slip in nanograined pure cobalt

Deformation mechanisms of nanograined and submicron-grained pure cobalt processed by means of high strain rate shear deformation at cryogenic temperatures were studied.Microstructural analysis revealed a transition of governing deformation mechanism from deformation twinning and dislocation slip in submicron-grains to and dislocations slip in nanograins.Microhardness tests illustrated that the Hall-Petch relation slope changes consequently with the transition of deformation mechanism.

Insight into the growth behaviors of MoS_(2)nanograins influenced by step edges and atomic structure of the substrate

The step edges and intrinsic atomic structure of single-crystal substrate play a critical role in determining the growth pathways of transition metal dichalcogenide(TMD)grains,particularly whether the TMDs will grow into wafer-scale single-crystal or anisotropic nanoribbons.Hereby,we investigate the growth behaviours of the MoS_(2)nanograins on(0001)and()sapphire substrates.On one hand,the step edges formed on the(0001)surface after thermal treatment are found to promote the macroscopic aggregation of MoS_(2)nanograins and to form unidirectional large triangular islands along with the<>steps in the annealing process,while on the pristine(0001)surface,the MoS_(2)nanograins grow into a random network-like pattern.Moreover,oxygen treatment on the substrate can further enhance the growth of MoS_(2)nanograins.Transmission electron microscopy and fast Fourier transform patterns reveal that the substrate could modulate the orientation of MoS_(2)nanograins during their growing process.On the other hand,the MoS_(2)nanograins on the surface could self-assemble into one-dimensional nanoribbons due to the strong structural anisotropy of the substrate.In addition,the ratio of Raman intensities for peaks that correspond to the and A1g phonon modes shows a linear relationship with the grain size due to the change of the“phonon confinement”.Moreover,new peaks located at 226 and 280 cm−1 can be observed in the off-resonant and resonant Raman spectra for the MoS_(2)nanograin samples,respectively,which can be attributed to the scatterings from the edges of as-fabricated MoS_(2)nanostructures.

基于热力学函数的纳米晶粒长大Cellular Automaton仿真研究

基于纳米晶热力学特性表征函数,将纳米晶热力学性质对晶界迁移的影响引入Cellular Automaton算法,对纳米晶粒长大行为进行了定量化和可视化的仿真研究.模拟结果表明,纳米晶粒长大的动力学与传统粗晶材料不同,在恒温条件下,纳米晶粒的长大指数n不是常数(传统粗晶材料的晶粒长大指数n=2为常数),随纳米晶粒长大过程的进行,n值从1.70至6.59发生变化.作为纳米晶粒长大的驱动力,纳米晶界的过剩自由能与纳米晶粒尺寸的变化直接相关.由于纳米晶材料强烈的小尺寸效应,纳米晶组织的热力学性质较大地影响纳米晶界的结构和能量状态,从而影响纳米晶粒长大的动力学特征.因此,只有结合纳米晶热力学特性的仿真研究才能获得对纳米晶粒长大行为本质性的认识.

纳米晶粒多晶Si薄膜的低压化学气相沉积

利用低压化学气相沉积(LPCVD)方法,以充Ar的S iH4作为反应气体源,在覆盖有热生长S iO2层的p-(100)S i衬底上制备了具有均匀分布的纳米晶粒多晶S i膜(nc-poly-S i)。采用扫描电子显微镜(SEM)、原子力显微镜(AFM)和拉曼谱等检测手段,测量和分析了沉积膜层的表面形貌、晶粒尺寸与密度分布等结构特征。结果表明,nc-poly-S i膜中S i晶粒的尺寸大小和密度分布强烈依赖于衬底温度、S iH4浓度与反应气压等工艺参数。典型实验条件下生长的S i纳米晶粒形状为半球状,晶粒尺寸约为40nm,密度分布约为4.0×1010cm-2和膜层厚度约为200nm。膜层的沉积机理分析指出,衬底表面上S i原子基团的吸附、迁移、成核与融合等热力学过程支配着nc-poly-S i膜的生长。

TiO_2纳米晶的制备

本文采用一种新的制备工艺制备了二氧化钛纳米晶 ,研究了二氧化钛的晶型和晶粒尺寸与工艺条件的关系 。