Room-temperature dislocation plasticity in SrTiO3 tuned by defect chemistry

Dislocations have been identified to modify both the functional and mechanical properties of some ceramic materials. Succinct control of dislocation-based plasticity in ceramics will also demand knowledge about dislocation interaction with point defects. Here, we propose an experimental approach to modulate the dislocation-based plasticity in single-crystal SrTiO₃ based on the concept of defect chemistry engineering, for example, by increasing the oxygen vacancy concentration via reduction treatment. With nanoindentation and bulk compression tests, we find that the dislocation-governed plasticity is significantly modified at the nano-/microscale, compared to the bulk scale. The increase in oxygen vacancy concentration after reduction treatment was assessed by impedance spectroscopy and is found to favor dislocation nucleation but impede dislocation motion as rationalized by the nanoindentation pop-in and nanoindentation creep tests.     

A physical model of the pressure dependence and biaxial mechanical properties of solid polymers

We give here a model for the pressure dependent, biaxial mechanical behavior of glassy polymers based on the thermally activated growth of deformation zones (Somigliana dislocation loops). The Coulomb criterion of plasticity, σ_c = S ? mσ_n, is found as the critical threshold needed to propagate Somigliana loops, in the same way as yield in crystals is found as the stress to move Volterra dislocation loops. While S is the shear strength, it is proposed that m follows basically from chain spacing fluctuations in the polymer glass; the temperature dependences of both parameters are derived. Application to tensile and compressive tests under a confinement pressure P is developed, with the aim to derive the pressure dependent (biaxial) strain-rate law. In particular, the pressure effect on dislocation density, that is, on plasticity defect nucleation, is shown to have a definite role in the plasticity of these solids. It introduces in the strain-rate law a normal stress dependent term (exp Dσ_n), which may have a decisive importance in a number of situations like multiaxial solicitations, solid state polymer shaping, second phase effects in polymer blends, and so on. Finally, a set of constant strain rate experiments is presented on an unsaturated polyester resin crosslinked with styrene. Measurements fit reasonably well with the predictions of the above model up to ～50 K below the glass transition, at which collective molecular motions invalidate its basic assumptions. The fit includes: (i) the Coulomb Criterion and its temperature dependence; and (ii) the dilative and shear apparent activation volumes at yield at all pressures.     

Polymer spherulites: A critical review

Polymeric and non-polymeric materials often crystallize as spherulites when crystallized from viscous melts or solutions at large undercooling. The essential component of a spherulite is fibrillar crystals that grow in predominantly radial directions and branch irregularly. We review the growth, branching and twisting of crystals in the light of theoretical and experimental advances of the last decade, while maintaining an appreciation for historical context.The crucial role of self-generated fields ahead of the crystal–melt interface is developed. Pressure gradients from volume contraction have been treated, as well as impurity gradients ahead of a growing crystal; fibril width W is predicted and found to be proportional to δ^1/2, where the diffusion length δ = D/G, the quotient of diffusivity and growth rate, conveys the extent of the field gradient. Ribbon-like spherulite radii grow at a constant rate under diffusion-coupled interface control.Non-crystallographic branching is required to maintain the volume occupied by fibrillar crystals as the spherulite radius increases. Topological giant screw dislocations and induced nucleation at cilia tethered to crystals are observed mechanisms leading to branching normal to the wide dimension of lamellar crystals; but the relative importance of each of these is not yet established. Repetitive tip splitting by kinetic interface instability has been suggested as a branching mechanism in the wide dimension of lamellar crystals.Larger molecular mass reduces the spherulite growth rate, more so at low undercoolings, for reasons that remain unresolved. Miscible diluents often profoundly reduce G by lowering both thermodynamic driving force and local transport dynamics that govern the secondary nucleation rate. Spherulite blend morphology is linked to the competition between radial growth rate G and diffusivity D of the diluent, expressed as the diffusion length δ.Polymer crystals in which chain helices all have the same sense show banded spherulites, as do crystals in which the chain axes are not perpendicular to the basal surfaces. Recent analyses with optical birefringence and X-ray micro-diffraction support the presence of helicoidally twisted ribbons, although other structural arrangements have sometimes been revealed by microscopy. Assessments of twist directions in spherulites of chiral polymers point to unbalanced basal surface stress as the source of twisting, although a general mechanical analysis is lacking. Another twisting model employs regular arrays of isochiral giant screw dislocations; results are mixed for this model.     

Numerical analysis for the dynamics of the oxidation-induced stacking fault in czochralski-grown silicon crystals

The continuum model of point defect dynamics to predict the concentration of interstitial and vacancy is established by estimating expressions for the thermophysical properties of point defects and the point defect distribution in a silicon crystal and the position of oxidation-induced stacking fault ring (R-OiSF) created during the cooling of crystals in Czochralski silicon growth process are calculated by using the finite element analysis. Temperature distributions in the silicon crystal in an industrial Czochralski growth configuration are measured and compared with finite volume simulation results. These temperature fields obtained from finite volume analysis are used as input data for the calculation of point defect distribution and R-OiSF position. Calculations of continuum point defect distributions predict the transition between vacancy and interstitial dominated precipitations of microdefects as a function of crystal pull rate (V). The dependence of the radius of R-OiSF (R_OiSF) on the crystal length with fixed growth rate for a given hot zone configuration is examined. The R_OiSF is increased with the increase of crystal length. These predictions from point defect dynamics are well agreed with experiments and empirical V/G correlation qualitatively, where G. is the axial temperature gradient at the melt/crystal interface.     

Atomistic Simulations of Materials Fracture and the Link between Atomic and Continuum Length Scales

The macroscopic fracture response of real materials originates from the competition and interplay of several atomic-scale mechanisms of decohesion and shear, such as inter-planar cleavage and dislocation nucleation and motion. These phenomena involve processes over a wide range of length scales, from the atomic to the macroscopic. We briefly review the attempts to span these length scales in dislocation and fracture modeling by (1) fully atomistic large-scale simulations of millions of atoms or more, approaching the continuum limit from the "bottom-up"; (2) directly coupling atomic-scale simulations and continuum mechanics, in a "top-down" approach; and (3) by defining a set of variables common to atomistic simulations and continuum mechanics and feeding the results of atomistic simulations into continuum-mechanics models in the form of constitutive relations . For this latter approach we discuss in detail the issues crucial to ensuring the consistency of the atomistic results and continuum mechanics. A case study of the constitutive-relation approach is presented for the problem of dislocation nucleation from a crack tip in a crystal under stress; a comparison of the results of atom-istic simulations to the Peierls–Nabarro continuum model is made.     

Growth regimes and the time development of lateral habits in polyethylene crystals

On the assumption that in regime I the growth rate of the lateral face of a lamellar crystal is proportional to the length of the face, and that in regime II the growth rate is independent of the length of the face, the time development of the size and aspect ratio of a polyethylene lamellar crystal is calculated. The aspect ratio is defined as the ratio of the length of the crystal in the b crystallographic direction to that in the a direction. It is assumed that steady state growth obtains, i.e. the solution concentration is constant. When both {110} and (200) faces are in regime I, the dimensions of the crystal increase exponentially in time, and lozenge shape (i.e. a crystal bounded by {110} faces) cannot be obtained under experimentally realizable conditions. When one of the faces is in regime I and the other in regime II, novel time dependencies of the crystal size and shape are derived, none of which has heretofore been observed. Despite the fact that the experimental conditions have never been realized in growth from solution, the now well-known fact that in regime I the growth rate is not proportional to the length of the growing face is re-established. Some new ways by which this assumption might be removed are suggested, and qualitative values for the upper bound for the ‘substrate length’ over which growth takes place from a nucleation event are discussed.     

Prediction and measurement of crystal size distributions for size-dependent growth

Predictions of size distributions, mean sizes and coefficients of variation in mixed-suspension mixed-product removal crystallizers are presented for the case when crystal growth rate is given by the expression G = G⁰ (1 + γL)^b as suggested by Abegg, Stevens and Larson (ASL). Crystal size distributions were measured at three scales of operation (1, 5 and 30 1.) for a system which exhibits size-dependent growth (potash alum/water). Experimental median sizes were in good agreement with the predictions and coefficients of variation significantly greater than 50% were obtained. The observed variation of crystal growth rate with size however was only moderately well represented by the ASL equation.     

Nucleation and growth of single crystal graphene on hexagonal boron nitride

Direct graphene growth was demonstrated on exfoliated hexagonal boron nitride (h-BN) single crystal flakes by low pressure CVD. The size of the hexagonal single crystal graphene domain increases with deposition time, with maximum size of ～270 nm. Most domains were found to nucleate at screw dislocation sites, and a step-flow growth mechanism was observed at atomic steps on the h-BN surface. Understanding the nucleation and growth mechanisms is an important step towards the synthesis of large single crystal graphene on h-BN substrates.     

Influence of thermal effect on dislocation loop evolution in Fe+-irradiated CeO2 during in-situ annealing

Cerium dioxide (CeO₂) is widely used as a surrogate fuel for uranium dioxide (UO₂) because of their same cubic fluorite crystal structure, nearly identical lattice parameters and similar physical properties. The degree of irradiation damage is related not only to the irradiation dose but also to the thermal effect. The influence of the thermal effect on the microstructure of Fe⁺-irradiated CeO₂ was investigated using in-situ transmission electron microscopy analysis during annealing. Shrinkage of the dislocation loops was observed for the first time in the irradiated CeO₂ foil. The threshold size determining whether the dislocation loop shrank was found to be approximately 19 nm through experimental measurements. When the loop size was less than this value, the dislocation loop shrank or even disappeared as the annealing time increased. Moreover, the smaller the loop size, the larger the shrinkage. The shrinkage rate measured in the in-situ annealing experiment was approximately 0.08 nm/s, which was almost consistent with the theoretical calculation (about 0.11 nm/s). However, when the loop size was larger than 19 nm, the loop grew rapidly at the beginning of annealing and then remained stable.     

Genomic Architecture of Phenotypic Plasticity in Response to Water Stress in Tetraploid Wheat

Phenotypic plasticity is one of the main mechanisms of adaptation to abiotic stresses via changes in critical developmental stages. Altering flowering phenology is a key evolutionary strategy of plant adaptation to abiotic stresses, to achieve the maximum possible reproduction. The current study is the first to apply the linear regression residuals as drought plasticity scores while considering the variation in flowering phenology and traits under non-stress conditions. We characterized the genomic architecture of 17 complex traits and their drought plasticity scores for quantitative trait loci (QTL) mapping, using a mapping population derived from a cross between durum wheat (Triticum turgidum ssp. durum) and wild emmer wheat (T. turgidum ssp. dicoccoides). We identified 79 QTLs affected observed traits and their plasticity scores, of which 33 reflected plasticity in response to water stress and exhibited epistatic interactions and/or pleiotropy between the observed and plasticity traits. Vrn-B3 (TaTF1) residing within an interval of a major drought-escape QTL was proposed as a candidate gene. The favorable alleles for most of the plasticity QTLs were contributed by wild emmer wheat, demonstrating its high potential for wheat improvement. Our study presents a new approach for the quantification of plant adaptation to various stresses and provides new insights into the genetic basis of wheat complex traits under water-deficit stress.     

Plastic deformation of polyethylene crystals as a function of crystal thickness and compression rate

Plastic deformation of polyethylene (PE) samples with crystals of various thickness was studied during uniaxial compression with initial compressive strain rates of 5.5×10⁻⁵, 1.1×10⁻³ and 5.5×10⁻³ s⁻¹. Samples with a broad range of crystals thickness, from usual 20 up to 170 nm, were obtained by crystallization under high pressure. The samples underwent recoverable compression below the compression ratio of 1.05-1.07. Following yield, plastic flow sets in above a compression ratio of 1.12. At a compression rate of 5.5×10⁻⁵ s⁻¹ the yield stress increases with the increase of crystal thickness up to 40 nm. For crystals thicker than 40 nm the yield stress levels off and remains constant. This experimental dependence was compared with the model developed on the basis of classical crystal plasticity and the monolithic nucleation of screw dislocations from polymer crystals. In that model contrary to the experimental evidence, the yield stress does not saturate with increase of crystal thickness. The activation volumes determined from strain rate jump experiments and from stress relaxation for crystals thicker than 40 nm are nearly constant at a level of 8.1 nm³. This activation length agrees very well with 40 nm for crystal thickness above which the yield stress levels off. It is proposed, as shown in a companion communication, that for PE crystals thicker than 40 nm two other modes of dislocation emission in the form of half loops of edge and screw dislocations begin to govern the strain rate, which no longer depend on lamella thickness.     

Fabrication of nanopatterned metal layers on silicon by nanoindentation/nanoscratching and electrodeposition

The present work illustrates a novel approach for the maskless and resistless fabrication of nanopatterned metal layers on Si substrates, based on the combination of nanomechanical surface modification techniques (such as nanoindentation and nanoscratching) and electrodeposition. Single crystal (1 0 0) n-doped Si substrates were first cleaned from native oxide. Nanoindentation and nanoscratching were then used to locally change the substrate microstructure and create regions with reduced electrical conductivity. The substrates were finally mounted as cathode electrodes in a three-electrode electrochemical cell to potentiostatically deposit a Ni layer. Electrodeposition was prevented in regions with modified microstructure, enabling the formation of a patterned Ni layer. The fabrication of several patterns including continuous Ni lines of 200 nm width and several microns length was obtained.     

Insights into High‐Temperature Uniaxial Compression Deformation Behavior of Ti3AlC2

We report on strain‐rate‐dependent compression deformation behavior of Ti₃AlC₂ at 1000°C–1200°C. At 1000°C and high strain rate (10^?2 or 10^?3 s^?1), Ti₃AlC₂ deforms in a nonplastic manner. Upon increasing temperature and reducing strain rate, Ti₃AlC₂ exhibits a limited plasticity. For instance, the true plastic strain at 1200°C and 10^?4 s^?1 is only 3%, beyond which strain softening following a short hardening regime occurs. The softening results from the formation of localized microvoids and microcracks. Decreasing the strain rate further to 10^?5 s^?1 at 1200°C, strain hardening instead of softening is identified. Under such conditions, the plastic strain remarkably increases, reaching a value as high as 27%. Postdeformation microstructural analyses of the dislocation configurations explicitly evidence the dislocation reactions, formation of hexagonal dislocation networks and dislocation entanglements. These account for the strain hardening. The extraordinary plasticity at 1200°C and 10^?5 s^?1 benefits from the initiation of nonbasal slip systems. Finally, a complete high‐temperature deformation scenario for nanolaminated Ti₃AlC₂ is elaborated.     

Osteogenic Potential of Mesenchymal Stem Cells from Adipose Tissue,Bone Marrow and Hair Follicle Outer Root Sheath in a 3D Crosslinked Gelatin-Based Hydrogel

Bone transplantation is regarded as the preferred therapy to treat a variety of bone defects. Autologous bone tissue is often lacking at the source, and the mesenchymal stem cells (MSCs) responsible for bone repair mechanisms are extracted by invasive procedures. This study explores the potential of autologous mesenchymal stem cells derived from the hair follicle outer root sheath (MSCORS). We demonstrated that MSCORS have a remarkable capacity to differentiate in vitro towards the osteogenic lineage. Indeed, when combined with a novel gelatin-based hydrogel called Osteogel, they provided additional osteoinductive cues in vitro that may pave the way for future application in bone regeneration. MSCORS were also compared to MSCs from adipose tissue (ADMSC) and bone marrow (BMMSC) in a 3D Osteogel model. We analyzed gel plasticity, cell phenotype, cell viability, and differentiation capacity towards the osteogenic lineage by measuring alkaline phosphatase (ALP) activity, calcium deposition, and specific gene expression. The novel injectable hydrogel filled an irregularly shaped lesion in a porcine wound model displaying high plasticity. MSCORS in Osteogel showed a higher osteo-commitment in terms of calcium deposition and expression dynamics of OCN, BMP2, and PPARG when compared to ADMSC and BMMSC, whilst displaying comparable cell viability and ALP activity. In conclusion, autologous MSCORS combined with our novel gelatin-based hydrogel displayed a high capacity for differentiation towards the osteogenic lineage and are acquired by non-invasive procedures, therefore qualifying as a suitable and expandable novel approach in the field of bone regeneration therapy.     

The Crystal and Molecular Structures of the Centrosymmetric Hexa-o-Phenylene

The crystal structure of the centrosymmetric isomer of hexa-o-phenylene has been solved by direct methods from counter data collected with MoKα radiation on a computer-controlled diffractometer by the balanced-filter technique and has been refined to R = 0.046. The deviation of the observed structure from an idealized regular geometry is assigned to intra- and intermolecular repulsions. The length of the bond linking the benzene rings is 1.498 Å, the angle of twist between the planes of the benzene rings is 77.4°. The packing arrangement shows a pseudo-trigonal pattern.     

Studies on nonisothermal crystallization of HDPE/POSS nanocomposites

The nonisothermal crystallization of HDPE/POSS nanocomposites (POSS content varying from 1 to 10 wt%) was studied using differential scanning calorimetry (DSC) technique. The Ozawa approach failed to describe the crystallization behaviour of nanocomposites, whereas the modified Avrami analysis could explain the behaviour of HDPE/POSS (90:10) nanocomposite only. The value of Avrami exponent n for HDPE/POSS (90:10) nanocomposite ranged from 2.5 to 2.9 and decreased with increasing cooling rate. It is postulated that the values of n close to 3 are caused by spherulitic crystal growth with heterogeneous nucleation while simultaneous occurrence of spherulitic and lamellar crystal growth with heterogeneous nucleation account for lower values of n at higher cooling rates. A novel kinetic model by Liu et al. was able to satisfactorily describe the crystallization behaviour of HDPE/POSS nanocomposites. Presence of POSS did not cause significant change in the activation energy for the transport of polymer segments to the growing crystal surface. POSS molecules exhibit nucleation activity only at 10 wt% loading in HDPE and are not effective nuclei at lower loadings.     

3D Dislocation structure evolution in strontium titanate: Spherical indentation experiments and MD simulations

In the present work, the dislocation structure evolution around and underneath the spherical indentations in (001) oriented single crystalline strontium titanate (STO) was revealed by using an etch‐pit technique and molecular dynamics (MD) simulations. The 3D defect structure at various length scales and subsurface depths was resolved with the help of a sequential polishing, etching, and imaging technique. This analysis, combined with load‐displacement data, shows that the incipient plasticity (manifested as sudden indenter displacement bursts) is strongly influenced by preexisting dislocations. In the early stage of plastic deformation, the dislocation pile‐ups are all aligned in 〈100〉 directions, lying on {110}₄₅ planes, inclined at 45° to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile‐ups along 〈110〉 directions appear, lying on {110}₉₀ planes, perpendicular to the (100) surface. MD simulations confirm the glide plane nature and provide further insights into the dislocation formation mechanisms by tracing the evolution of the complete dislocation line network as function of indentation depth.     

双扭旋叶片组合形式对管内湍流换热性能的影响

在圆管内一个截面上安装两个扭旋叶片的组合形式会产生多个纵向涡,纵向涡的形式与强弱受相邻两组叶片旋向和错位角的综合影响。为探究各种组合形式对湍流换热性能的影响,研究了6种不同组合形式某一截面上速度与温度梯度和压力梯度的协同程度,得出结论：错位角对Nu和压力降的影响明显大于叶片旋向的改变。在近壁区,增加错位角可以提高速度与温度梯度的协同程度,而且相邻两组叶片旋向相反形式优于旋向相同的形式;但旋向相同形式的速度与压力梯度的协同程度优于旋向相反形式,然而这种差距会随着错位角的增加而减小,当错位角为90°时,在y/R<0.75时协同程度非常接近。     

The Emerging Roles of JNK Signaling in Drosophila Stem Cell Homeostasis

The Jun N-terminal kinase (JNK) pathway is an evolutionary conserved kinase cascade best known for its roles during stress-induced apoptosis and tumor progression. Recent findings, however, have identified new roles for this pleiotropic pathway in stem cells during regenerative responses and in cellular plasticity. Here, we provide an overview of recent findings about the new roles of JNK signaling in stem cell biology using two well-established Drosophila models: the testis and the intestine. We highlight the pathway’s roles in processes such as proliferation, death, self-renewal and reprogramming, and discuss the known parallels between flies and mammals.     

Rate mechanisms of plasticity in semi-crystalline polyethylene

Based on our experiments on polyethylene where we have observed a constant level of plastic resistance, independent of lamella thickness exceeding 40 nm, we have fundamentally re-considered the rate controlling mechanisms of crystal plasticity in semi-crystalline polymers. In this we have not only re-examined and made modifications to the widely accepted mechanism of Young (Young RJ. Mater Forum 1988;11:210.) of monolithic nucleation of screw dislocations from edges of crystalline lamellae predicting an increase in plastic resistance with increasing lamella thickness, but we are proposing here two new modes of nucleation of both edge and screw dislocation half loops from lamella faces that are independent of lamella thickness. These two new modes of dislocation nucleation explain well the observed transition from a plastic resistance increasing with lamella thickness to one of constant resistance above a lamella thickness of ca. 35 nm in polyethylene. They also provide a more satisfactory framework to explain the temperature and strain rate dependence of the plastic resistance of polyethylene and predict the observed levels of activation volumes determined by us.