脉冲激光诱导液-固界面反应合成金刚石纳米晶中的结构相变模型

探讨了脉冲激光诱导液固界面反应法（ＰＬＩＲ：ｐｕｌｓｅｄｌａｓｅｒｉｎｄｕｃｅｄｌｉｑｕｉｄｓｏｌｉｄｉｎｔｅｒｆａｃｅｒｅａｃｔｉｏｎ）制备金刚石纳米晶的物理化学机制，提出了金刚石纳米晶的成核机理，即由激光诱导石墨六方结构原子团过渡到石墨菱方结构、然后转变成立方金刚石晶核，以及由石墨六方结构直接转变成六方金刚石结构的相变模型，并讨论了基于液固界面反应的纳米晶生长动力学，较好地从动力学上解释了ＰＬＩＲ合成金刚石纳米晶的物理化学机制。

纳米晶Fe_（73．5）Cu_1Nb_3Si_（13．5）B_9合金中缺陷及内应力的分布研究

用正电子湮没技术和微观磁学方法研究了纳米晶Ｆｅ_（７３．５）Ｃｕ_（１）Ｎｂ_（３）Ｓｉ_（１３．３）Ｂ_９合金结构缺陷在退火过程中的变化规律，表明晶化初期形成的界面结构只引入少量的缺陷，晶化后期由于出现大量晶界，缺陷浓度急剧上升；非晶制备过程中以及晶化时引入的准位错偶极子造成的应力场对其软磁性能有很大的影响，并可以解释在最佳退火温度的磁性行为。

Microstructure refinement of a dual phase titanium alloy by severe room temperature compression

Microstructure refinement of a dual phase titanium alloy, Ti-3AI-4.5V-5Mo, by severe room temperature compression was investigated. Nanocrystalline grains were observed in the sample with 75% reduction, in which the grain sizes of a phase and β phase were approximately 50 and 100 nm. Conversely, the average thicknesses of a phase and β phase in as-received microstructure were measured to be 0.7 and 0.5 μm, respectively. TEM and XRD methods were used to analyze the microstructure and texture changes after severe deformation. Microstructure refinement was deduced to the complex interaction among slip dislocations in the a phase, the complex interaction among slip dislocations and martensites in the β phases. In addition, the interaction between the a phase and the β phase also contributed to the microstructure refinement.

Role of plastic deformation in tailoring ultrafine microstructure in nanotwinned diamond for enhanced hardness

Nanotwinned diamond（nt-diamond）,which demonstrates unprecedented hardness and stability,is synthesized through the martensitic transformation of onion carbons at high pressure and high temperature（HPHT）.Its hardness and stability increase with decreasing twin thickness at the nanoscale.However,the formation mechanism of nanotwinning substructures within diamond nanograins is not well established.Here,we characterize the nanotwins in nt-diamonds synthesized under different HPHT conditions.Our observation shows that the nanotwin thickness reaches a minimum at ~20 GPa,below which phase-transformation twins and deformation twins coexist.Then,we use the density-functional-based tight-binding method and kinetic dislocation theory to investigate the subsequent plastic deformation mechanism in these pre-existing phase-transformation diamond twins.Our results suggest that pressure-dependent conversion of the plastic deformation mechanism occurs at a critical synthetic pressure for nt-diamond,which explains the existence of the minimum twin thickness.Our findings provide guidance on optimizing the synthetic conditions for fabricating nt-diamond with higher hardness and stability.

Atomistic simulation study of tensile deformation in bulk nanocrystalline bcc iron

In the present work,the mechanical properties of bulk nanocrystalline(NC) bcc Fe under tensile deformation have been studied by molecular dynamics(MD) simulations.Average flow stress was found to decrease with grain refinement below 13.54 nm,indicating a breakdown in the Hall-Petch relation.A change from grain boundary(GB) mediated dislocation activities to GB activities may be a possible explanation of the breakdown in the Hall-Petch relation.The results also indicate that the average flow stress increases with increasing strain rates and decreasing temperatures.Stress induced phase transformations were observed during the tensile deformation of NC Fe,and such phase transformations were found to be reversible with respect to the applied stress.The maximum fraction of the cp atoms was also found to increase with increasing applied stress.Significant phase transformation occurred in the stacking fault zone due to dislocation activities for large grain size(13.54 nm),while significant phase transformation occurred in the GBs due to GB activities for small grain size(3.39 nm).At deformation temperature of 900 K and above,no apparent phase transformation occurred because all atoms at GBs and grain interior could easily rearrange their position by thermal activation to form local vacancies/disordered structures rather than ordered close packed(cp) structures.

Unexpected luminescence enhancement of upconverting nanocrystals by cation exchange with well retained small particle size

Highly luminescent upconversion nanoparticles （UCNPs） with small sizes are highly desirable for bioapplications. A facile in situ cation exchange strategy has been developed to greatly enhance the UC luminescence of hexagonal phase NaYF4 NPs while maintaining their small particle size and shape. Via a cation exchange treatment by hot-injecting Gd3＋ precursors into the as-prepared NPs solution without pre-separation, the naked-eye visible UC emission of the NPs was enhanced about 29 times under 980 nm near infrared （NIR） excitation with unchanged particle size. The cation exchange process was further demonstrated for the case of NaYF4 nanorods （NRs）. After the cation exchange, the nanorod was broken into two NPs with stronger emission. The cation exchanged hydrophobic UCNPs were further encapSulated with poly（amino acid） and successfully applied for targeted cancer cell UC luminescence imaging.

Effects of grain size on phase transition behavior of nanocrystalline shape memory alloys

We report recent advances in the experimental and theoretical study of grain size(GS)effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy(SMA).It is shown that when GS<60 nm,the superelastic stress-strain hysteresis loop area(H)of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers.At the same time,the temperature dependence of the transition stress also decreases with GS and eventually approaches zero,leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship.Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization.It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces(grain boundary and Austenite-Martensite(A-M)interface)in the polycrystal.In particular,with GS reduction,interfacial energy terms will gradually become dominant over the bulk energy of the crystallite,eventually bring fundamental changes in the phase transition responses of the material.Modelling strategy leading to the establishment of quantitative relationships among GS,grain boundary,A-M interfaces and the macroscopic responses of the material are outlined.

Deformation study of bicrystalline and nano-polycrystalline structures using phase field crystal method

Deformation behaviors of bicrystalline and nano-polycrystalline structures of various tilt angles and inclination angles in two dimensions are investigated in detail using a two-mode phase field crystal model.The interaction between grain boundary(GB)and dislocation is also examined in bicrystals and nano-polycrystals that both contain asymmetric and symmetric tilt GBs,with energy analysis being carried out to analyze these processes.During deformation simulations,we assume the volume of each simulation cell at every time step is coincident with that of the initial state just before deformation.Our simulation results show that the behaviors of symmetric and asymmetric GBs in bicrystals and nano-polycrystals differ from each other depending on tilt angle and inclination angle.A new dislocation emission mechanism of interest is observed in bicrystals which contain low angle symmetric tilt GBs.Low angle GB has a higher mobility relative to high angle GB in both bicrystalline and nano-polycrystalline structures,as does asymmetric GB to symmetric GB.The generation,motion,pileup and annihilation of dislocations,grain rotation and grain coalescence are observed,which is consistent with the simulation results obtained by molecular dynamics.These simulation results can provide strong guidelines for experimentation.

Magnetic and optical properties of NaGdF4：Nd^3＋, Yb^3＋, Tm^3＋ nanocrystals with upconversion/downconversion luminescence from visible to the near-infrared second window

We have designed and synthesized NaGdF4：Nd^3＋, Yb^3＋, Tm^3＋. magnetic nano- phosphors with combined dual-mode downconversion （DC） and upconversion （UC） photoluminescence upon 800 run excitation. Hexagonal-phase NaGdF4：Nd^3＋, Yb^3＋, Tm^3＋ nanocrystals （NCs） with an average size of 21 nm were synthesized using a solvothermal approach. Nd^3＋, Yb^3＋, Tm^3＋ triple-doped NaGdF4 NCs exhibit a broad range of photoluminescence peaks covering a near infrared first/second window （860-900, 1,000, and 1,060 nm）, and visible emission including blue （475 nm）, green （520 and 542 nm） and yellow （587 nm） after excitation at 800 nm. A mechanism involving circulation of energy over Gd^3＋ sublattices as bridge ions and final trapping by the initial activator ions （Nd^3＋） has been proposed. Penetration depth studies indicate that NIR emission is easily detected even at a large tissue thickness of 10 mm. These paramagnetic nanophosphors demonstrate a large magnetization value of 1.88 emu/g at 20 kOe and longitudinal relaxivity value of 1.2537 mM-1.S-1 as a Tl-weighted magnetic resonance imaging contrast agent. These NaGdF4：Nd^3＋, Yb^3＋ Tm^3＋ NCs are promising for applications in biological and magnetic resonance imaging.

Small onion-like BN leads to ultrafine-twinned cubic BN

Nanotwinned cubic boron nitride(nt-cBN) with remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN(oBN) nano-precursor and induce phase transition under high pressure. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from^320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafinetwin substructure in the synthetic products. This study reveals that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this important material.

Quantitative analysis of nanoscale deformation fields of a crack-tip in single-crystal silicon

A mode II crack in single-crystal silicon was investigated experimentally using high-resolution transmission electron microscopy.Geometric phase analysis and numerical moiré method were employed to map the deformation fields of the crack-tip area.The normal strain field maps of the crack-tip area indeed showed the deformation occurs primarily in the vicinity of the dislocations and the normal strains are near zero in the crack-tip area.The shear strain field map shows that the relatively large shear strain is in the crack-tip area.The experimental results were compared with the predictions of linear elastic fracture mechanics.The comparison shows that measured strain distribution ahead of the crack-tip agrees with the predictions of linear elastic fracture mechanics up to 1 nm from the crack-tip.

Structural and optical verification of residual strain effect in single crystalline CdTe nanowires

Single crystalline CdTe nanowires have been synthesized using Au-catalyzed chemical vapor deposition. X-ray diffraction reveals the existence of non- negligible inhomogeneous compressive strain in the nanowires along the 〈111〉 growth direction. The effect of the strain on the electronic structure is manifested by the blue-shifted and broadened photoluminescence spectra involving shallow donor/acceptor states. Such residual strain is of great importance for a better understanding of the optical and electrical behaviors of various semiconductor nanomaterials as well as for device design and applications.