Mechanical properties,thermal conductivity and defect formation energies of samarium immobilization in Gd_(2)Zr_(2)O_(7):First-principles study and irradiation experiment

A density functional theory(DFT)study was employed to investigate the mechanical property,thermal conductivity,Debye temperature,electronic structure and defect chemistry of(Gd_(1-x)Sm_(x))_(2)Zr_(2)O_(7).All the(Gd_(1-x)Sm_(x))_(2)Zr_(2)O_(7) compounds exhibit an excellent structural and mechanical stability(Gd_(0.25)Sm_(0.75))_(2)Zr_(2)O_(7) has the lowest Young’s modulus of 213.7 GPa,the largest Possion’s ratio of 0.292,the lowest Debye temperature of 491.8 K and the lowest thermal conductivity.The calculated thermal conductivities of(Gd_(1-x)Sm_(x))_(2)Zr_(2)O_(7) are 1.17-1.21 W/(m·K)by the Clark’s model and 1.32-1.36 W/(m·K)by the Cahall’s model,respectively.The formation energies of O vacancies at 48f site are negative,which increase with the Sm content,however,the formation energies of O vacancies at 8b site are almost invariable.In addition,Sm partly occupying the Gd-site reduces distinctly the formation energies of defects such as A-site vacancies,cation antisite defects,anion Frenkel pairs of oxygen at 8b site and cation interstitials,which suggests that Sm-doped Gd_(2)Zr_(2)O_(7),especially equimolar GdSmZr_(2)O_(7),has a better irradiation tolerance.After the 16 MeVTa-ion irradiation at a fluence of 1×10^(14) or 2×10^(14) ions/cm^(2),the crystal structure of GdSmZr_(2)O_(7) transforms from pyrochlore to a defect fluorite without obvious amorphous phase.

Lattice strain suppresses point defect formation in halide perovskites

We computationally investigate the impact of crystal strain on the formation of native point defects likely to be formed in halide perovskites;A-site cation antisite(I_(A)),Pb antisite(I_(Pb)),A-site cation vacany(V_(A)),I vacancy(V_(I)),Pb vacancy(V_(Pb)),and I interstitial(I_(i)).We systematically identify compressive and tensile strain to CsPbI_(3),FAPbI_(3),and MAPbI_(3)perovskite structures.We observe that while each type of defect has a unique behaviour,overall,the defect formation in FAPbI3 is much more sensitive to the strain.The compressive strain can enhance the formation energy of neutral I_(Pb)and I_(i)up to 15%for FAPbI_(3),depending on the growth conditions.We show that the strain not only controls the formation of defects but also their transition levels in the band gap:A deep level can be transformed into a shallow level by the strain.We anticipate that tailoring the lattice strain can be used as a defect passivation mechanism for future studies.

A low-temperature synthesis-induced defect formation strategy for stable hierarchical porous metal–organic frameworks

A stable hierarchical porous metal-organic framework PCN-56 with abundant Lewis acid sites(denoted as Defective-PCN-56) was synthesized by the low-temperature synthesis-induced defect formation method.The existence of mesopore in structure was confirmed by N2 sorption isotherm and the successful encapsulation of large dye molecules.The Defective-PCN-56 has higher loading capacity toward anti-cancer drug Doxo compared with that of "nearly ideal-crystal"(denoted as Ideal-PCN-56)synthesized at high temperature,showing potential application as drug carrier.The low-temperature synthesis-induced defect formation strategy presented here provides a new and facile way to synthesize stable MOFs with the combination of intrinsic micropore and additional mesopore as well as abundant Lewis acid sites.

Mechanical force enhanced bony formation in defect implanted with calcium sulphate cement

To improve the osteogenic property of bone repairing materials and to accelerate bone healing are major tasks in bone biomaterials research. The objective of this study was to investigate if the mechanical force could be used to accelerate bone formation in a bony defect in vivo. The calcium sulfate cement was implanted into the left distal femoral epiphyses surgically in 16 rats. The half of rats were subjected to external mechanical force via treadmill exercise, the exercise started at day 7 postoperatively for 30 consecutive days and at a constant speed 8 m·min-1 for 45 min·day-1, while the rest served as a control. The rats were scanned four times longitudinally after surgery using microcomputed tomography and newly formed bone was evaluated. After sacrificing, the femurs had biomechanical test of three-point bending and histological analysis. The results showed that bone healing under mechanical force were better than the control with residual defect areas of 0.64±0.19 mm2 and 1.78±0.39 mm2（P〈0.001）, and the ultimate loads to failure under mechanical force were 69.56±4.74 N, stronger than the control with ultimate loads to failure of 59.17±7.48 N（P=0.039）. This suggests that the mechanical force might be used to improve new bone formation and potentially offer a clinical strategy to accelerate bone healing.

Evolution of Joint Formation in Resistance Microwelding of Crossed Pt-10%Ir and 316 LVM Stainless Steel Wires

The surface morphology, cross-sections, and joint break force（JBF） of joints welded under different electrode forces were studied. The defects, such as electrode sticking, notch, and excessive expulsions, were observed in the joints. No desirable joints were achieved with the consideration of weld geometries and joint performances. From the cross-sectional morphology, the joint evolution during the RMW of Pt alloy and 316 LVM SS wires was developed, which involved cold collapse and heat promoted set-down of Pt alloy wire, unbalanced heating at interface, molten phase squeezed out, and defect formation. Finally, the defect formation was also discussed.

First-principles Study of Divalent ⅡA and Transition ⅡB Metals Doping into Cu_2O

Divalent IIA metals such as Be, Mg, Ca, Sr, Ba and transition IIB metals such as Zn, Cd were investigated as possible n-type dopants into the Cu2 O theoretically by using the first-principles calculations based on density functional theory. By systematical analyses of the lattice parameters, the bond length, the electronic structure, the local density of states and the defect formation energy for various doping systems, it is revealed that Ca, Sr, Ba and Be are more suited for n-type doping into Cu2O as shallow donors, compared to Mg which introduces a relatively deep donor level in Cu2O. Meanwhile, Zn and Cd can hardly be doped into Cu2O due to the positive formation energy of relevant defects.

Study of High-Temperature Stability of Pyrochlore Zirconates

Based on defect chemistry theory and molecular dynamics,the defect formation energy and its relationship with the mechanism of pyrochlore-fluorite phase change were investigated,so as to reveal the underlying mechanism of high-temperature stability of pyrochlore zirconates.Results showed that with the rise of the atom mass of A,the defect formation energies decreased that meant the crystal structure tended to become more disordered.Noticeably,the first nearest cation antisite dominated the pyrochlore disorder transformation process.In addition,it was found that the diffusion of oxygen atoms was far higher than that of cations,and was increased with the temperature,thus also promoting the pyrochlore-fluorite transformation process.

Formation of defects in selective laser melted Inconel 718 and its correlation with mechanical properties through dimensionless numbers

This paper presents a profound study on the formation of three typical types of defects(i.e.,lack of fusion,keyholes,and gas pores)observed in selective laser melting(SLM)printed Inconel 718 samples,along with their correlations with mechanical properties of the samples.Computed tomography,scanning electron microscopy,and mechanical property tests revealed that the three types of defects fall into three stages of porosity evolution classified by recently-proposed dimensionless numbersηm(melting efficiency)andηv(vaporization efficiency).Meanwhile,experimental tests verified that the mechanical properties of products,such as strength and elongation,are remarkably sensitive to lack of fusion.However,these properties are slightly affected by the keyholes and gas pores.An optimal process window characterized by dimensionless numbers is realized by adjusting the processing parameters and employing different powders.This process window allows products to have relatively low defects and high mechanical performances.A quantitative relation between processing parameters,dimensionless numbers,defects,and mechanical properties is established based on these observations.This relation,along with the optimal process window,is believed to enhance the quality of SLM products of Inconel 718 alloy and can be further extended to SLM with other metal materials.

Curvature and Size Effects on Reactivities of Mono-to Octa-vacancies in a(5,5) Single-walled Carbon Nanotube

Defect curvature was developed based on our previously proposed direction curvature theory. Defect curvature, as a universal criterion, was used to identify vacancy formation energies E_f of mono-vacancies to octa-vacancies in a（5,5） tube. An ab initio calculation results showed that E_f decreased with increasing the defect curvature K_（V_s）（s = 1~8）. The structures with removed carbon atoms along zigzag chain or the tubular axis were the most stable in each kind of Vs, because their corresponding K_（V_s） was the largest. In addition, local product structures disturbed the variation rule of E_f as K_（V_s）. There was an odd-even oscillation rule in the smallest E_f among each kind of Vs as the s value and vacancies V2, V4 and V6 were more stable. The stabilities of the related vacancy structures were confirmed by two dissociation processes.

Manufacturing process of water-soluble salt-based ceramic cores based on vat photopolymerization

Water-soluble salt-based ceramic cores can be recycled and have excellent high-temperature chemical stability.In this work,vat photopolymerization was successfully applied to water-soluble salt-based ceramic cores for the first time.The powder raw materials of the printing suspension were sodium chloride and alumina.High-precision green bodies were manufactured by optimizing suspensions and parameters.In addition,the postprocessing method was optimized according to the microstructure and mechanical properties.The sintered part had a high bending strength and smooth surface.Finally,the dissolution rate and moisture resistance were compared under different dissolution and storage conditions.Compared to traditional manufacturing methods,vat photopolymerization enables the production of complex structures without molds and reduces production costs.This technology is suitable for the rapid iteration of complex structural parts and can be applied to precision parts in aerospace,military,and other technical fields with high cost-effectiveness and sustainability.

Effect of energy density on defect evolution in 3D printed Zr-based metallic glasses by selective laser melting

In this study, the defects in 3D printed Zr-based bulk metallic glasses(BMGs) fabricated by selective laser melting(SLM) under different energy densities have been investigated via both experimental and simulation approaches. Different defects, including balling, interlayer pores, open pores and metallurgical pores, are detected in the 3D-printed Zr-based MGs depending on the energy inputs. Balling mainly occurs at a relatively low energy density(E<8.33 J/mm^3) due to the incomplete melting of particles, while interlayer pores and open pores are formed at modest energy densities(E=13.89-16.67 J/mm^3) because of incomplete welding and insufficient filling of molten liquid between layers. Fine metallurgical pores appear on the upper surface at relatively high energy densities(E=20.83-27.78 J/mm^3), which originate from gas escaping from molten pools during rapid solidification of the melt. Computational fluid dynamics(CFD) simulations are carried out to verify the experimental observations. The CFD simulations reveal that the various defects formed in the 3D-printed Zr-based BMG are related to the melt flow behaviours in the molten pools under different energy densities. The present work provides in-depth understandings of defect formation in the SLM process and provides methods for eliminating these defects in order to enhance the mechanical performance of 3D printed BMGs.

First-principles Study of Point Defects in Stoichiometric and Non-stoichiometric Y_4Al_2O_9

By using the first-principles calculation, we studied the mechanisms of point defects in Y4AI209 （YAM）, a promising ternary oxide with excellent optical and thermal properties. It is found that the predominant native defect species is closely dependent on the chemical potentials of each constituent. In the case of O-rich condition, the oxygen interstitial has the very low defect formation energy, followed by the anti-site defects and AI vacancy; in the case of AI-rich condition, the oxygen vacancy yields the lowest defect formation energy, followed by the anti-site defects and AI interstitial. The present result shows that in all the possible chemical potential ranges, anti-site defects have relatively low defect formation energy and might exist in high concentration in YAM. Furthermore, AIy anti-site has relatively lower defect formation energy than the YAt anti- site throughout. The behaviors of defect complexes under non-stoichiometric condition, such as the AI203 or Y203 excess, are also investigated. The results provide helpful guide to optimize the experimental synthesizing of YAM.

Defect energetics and magnetic properties of 3d-transition-metal-doped topological crystalline insulator SnTe

The introduction of magnetism in SnTe-class topological crystalline insulators is a challenging subject with great importance in the quantum device applications. Based on the first-principles calculations, we have studied the defect energetics and magnetic properties of 3d transition-metal(TM)-doped SnTe. We find that the doped TM atoms prefer to stay in the neutral states and have comparatively high formation energies, suggesting that the uniform TMdoping in SnTe with a higher concentration will be difficult unless clustering. In the dilute doping regime, all the magnetic TMatoms are in the high-spin states, indicating that the spin splitting energy of 3d TM is stronger than the crystal splitting energy of the SnTe ligand. Importantly, Mn-doped SnTe has relatively low defect formation energy, largest local magnetic moment, and no defect levels in the bulk gap, suggesting that Mn is a promising magnetic dopant to realize the magnetic order for the theoretically-proposed large-Chern-number quantum anomalous Hall effect(QAHE) in SnTe.

Vacancy formation enthalpies of high-entropy FeCoCrNi alloy via first-principles calculations and possible implications to its superior radiation tolerance

Because atoms in high-entropy alloys （HEAs） coordinate in very different and distorted local environ- ments in the lattice sites, even for the same type of constituent, their point defects could highly vary. Therefore, theoretical determination of the thermodynamic quantities （i.e., defect formation enthalpies） of various point defects is rather challenging because each corresponding thermodynamic quantity of all involve constituents is not unique. The knowledge of these thermodynamic quantities is prerequisite for designing novel HEAs and understanding the mechanical and physical behaviors of HEAs. However, to date there has not been a good method to theoretically derive the defect formation enthalpies of HEAs. Here, using first-principles calculations within the density functional theory （DFT） in combina- tion of special quasi-random structure models （SQSs）, we have developed a general method to derive corresponding formation enthalpies of point defects in HEAs, using vacancy formation enthalpies of a four-component equiatomic fcc-type FeCoCrNi HEA as prototypical and benchmark examples. In difference from traditional ordered alloys, the vacancy formation enthalpies of FeCoCrNi HEA vary in a highly wide range from 0.72 to 2.89 eV for Fe, 0.88-2.90 eV for Co, 0.78-3.09 eV for Cr, and 0.91-2.95 eV for Ni due to high-level site-to-site lattice distortions and compositional complexities. On average, the vacancy formation enthalpies of 1.58 eV for Fe, 1.61 eV for Cr, 1.70 eV for Co and 1.89 eV for Ni are all larger than that （1.41 eV） of pure fcc nickel. This fact implies that the vacancies are much more difficult to be created than in nickel, indicating a reasonable agreement with the recent experimental observation that FeCoCrNi exhibits two orders of amplitudes enhancement of radiation tolerance with the suppression of void formation at elevated temperatures than in pure nickel.

Application of Synchrotron X-Ray Imaging and Diffraction in Additive Manufacturing:A Review

Additive manufacturing(AM)is a rapid prototyping technology based on the idea of discrete accumulation which off ers an advantage of economically fabricating a component with complex geometries in a rapid design-to-manufacture cycle.However,various internal defects,such as balling,cracks,residual stress and porosity,are inevitably occurred during AM due to the complexity of laser/electron beam-powder interaction,rapid melting and solidification process,and microstructure evolution.The existence of porosity defects can potentially deteriorate the mechanical properties of selective laser melting(SLM)components,such as material stiff ness,hardness,tensile strength,and fatigue resistance performance.Synchrotron X-ray imaging and diffraction are important non-destructive means to elaborately characterize the internal defect characteristics and mechanical properties of AM parts.This paper presents a review on the application of synchrotron X-ray in identifying and verifying the quality and requirement of AM parts.Defects,microstructures and mechanical properties of printed components characterized by synchrotron X-ray imaging and diffraction are summarized in this review.Subsequently,this paper also elaborates on the online characterization of the evolution of the microstructure during AM using synchrotron X-ray imaging,and introduces the method for measuring AM stress by X-ray diffraction(XRD).Finally,the future application of synchrotron X-ray characterization in the AM is prospected.