Numerical modeling of SiC by low-pressure chemical vapor deposition from methyltrichlorosilane

The development of functional relationships between the observed deposition rate and the experimental conditions is an important step toward understanding and optimizing low-pressure chemical vapor deposition(LPCVD)or low-pressure chemical vapor infiltration(LPCVI).In the field of ceramic matrix composites(CMCs),methyltrichlorosilane(CH3 SiCl3,MTS)is the most widely used source gas system for SiC,because stoichiometric SiC deposit can be facilitated at 900°C–1300°C.However,the reliability and accuracy of existing numerical models for these processing conditions are rarely reported.In this study,a comprehensive transport model was coupled with gas-phase and surface kinetics.The resulting gas-phase kinetics was confirmed via the measured concentration of gaseous species.The relationship between deposition rate and 24 gaseous species has been effectively evaluated by combining the special superiority of the novel extreme machine learning method and the conventional sticking coefficient method.Surface kinetics were then proposed and shown to reproduce the experimental results.The proposed simulation strategy can be used for different material systems.

Sintering reaction and microstructure of MAl(M=Ni,Fe,and Mg)nanoparticles through molecular dynamics simulation

The sintering-alloying processes of nickel(Ni),iron(Fe),and magnesium(Mg) with aluminum(Al) nanoparticles were studied by molecular dynamics simulation with the analytic embedded-atom model(AEAM) potential.Potential energy,mean heterogeneous coordination number NAB,and surface atomic number Nsurf-A were used to monitor the sintering-reaction processes.The effects of surface segregation,heat of formation,and melting point on the sinteringalloying processes were discussed.Results revealed that sintering proceeded in two stages.First,atoms with low surface energy diffused onto the surface of atoms with high surface energy;second,metal atoms diffused with one another with increased system temperature to a threshold value.Under the same initial conditions,the sintering reaction rate of the three systems increased in the order MgAl <FeAl <NiAl.Depending on the initial reaction temperature,the final core-shell(FeAl and MgAl) and alloyed(NiAl and FeAl) nanoconfigurations can be observed.

Preparation and Characterization of 3D Printed Hydroxyapatite-Whisker-Strengthened Hydroxyapatite Scaffold Coated with Biphasic Calcium Phosphate

This study investigates the in vitro degradation of calcium-deficient hydroxyapatite powder after heat treatment at different temperatures and analyzes the calculated phase composition,particle size distribution,degradation rate,and bioactivity of the powder after heat treatment.A mixture of hydroxyapatite and𝛽-tricalcium phosphate(BCP)coatings was prepared on the surface of a 3D-printed hydroxyapatite-whisker-strengthened hydroxyapatite scaf-fold(HAw/HA)by vacuum impregnation and ultraviolet light curing combined with an optimized heat treatment process.The performance of the coatings under different methods was characterized.The composite scaffolds with highly interconnected pores and excellent mechanical properties were prepared,and their biodegradation performance,bioactivity,osteoconductivity,and osteoinductivity of the scaffolds were improved.The results showed that calcium-deficient hydroxyapatite began to transform into BCP between 600℃and 800℃,and the powder treated at 800℃has better bioactivity.The BCP coating prepared by light curing was more uniform,resulting in a higher interfacial bonding strength,and has better osteoconductivity and osteoinductivity than that prepared by vacuum impregnation.

First-principles study of structure,mechanical and optical properties of La-and Sc-doped Y2O3

The electronic,mechanical and optical properties of La-and Sc-doped Y2O3 were investigated using firstprinciples calculations.Two doping sites of Sc and La in Y2O3 were modeled.The calculated values of the energy of formation show that the most energetically favorable site for a La atom in Y2O3 is a d-site Y atom,while for Sc a b-site Y atom is the more stable position.The calculated band gap shows a slight decrease with increasing La or Sc concentration.The calculated results for the mechanical and optical properties of Y(2-x)RxO3(R=Sc or La,0<x≤0.1875)show that La-or Sc-doped Y2O3 would have enhanced strength,and thus an ability of resisting external shocks,and increased hardness and mechanical toughness.These improved mechanical properties are achieved without sacrificing the optical properties of the doped compounds.So the doping of La or Sc in Y2O3 is permissible in the preparation of Y2O3 transparent ceramics,of course,doping of La or Sc will benefit the sintering of transparent ceramics.

Effect of sintering temperature in argon atmosphere on microstructure and properties of 3D printed alumina ceramic cores

Alumina ceramics with different sintering temperatures in argon atmosphere were obtained using stereolithography-based 3D printing.The effects of sintering temperature on microstructure and physical and mechanical properties were investigated.The results show that the average particle size,shrinkage,bulk density,crystallite size,flexural strength,Vickers hardness,and nanoindentation hardness increased with the increase in sintering temperature,whereas the open porosity decreased with increasing sintering temperature.No change was observed in phase composition,chemical bond,atomic ratio,and surface roughness.For the sintered samples,the shrinkage in Z direction is much greater than that in X or Y direction.The optimum sintering temperature in argon atmosphere is 1350℃with a shrinkage of 3.0%,3.2%,and 5.5%in X,Y,and Z directions,respectively,flexural strength of 26.7 MPa,Vickers hardness of 198.5 HV,nanoindentation hardness of 33.1 GPa,bulk density of 2.5 g/cm^3,and open porosity of 33.8%.The optimum sintering temperature was 70℃higher than that sintering in air atmosphere when achieved the similar properties.

Microstructure-based multiphysics modeling for semiconductor integration and packaging

Semiconductor technology and packaging is advancing rapidly toward system integration where the packaging is co-designed and co-manufactured along with the wafer fabrication.However,materials issues,in particular the mesoscale microstructure,have to date been excluded from the integrated product design cycle of electronic packaging due to the myriad of materials used and the complex nature of the material phenomena that require a multiphysics approach to describe.In the context of the materials genome initiative,we present an overview of a series of studies that aim to establish the linkages between the material microstructure and its responses by considering the multiple perspectives of the various multiphysics fields.The microstructure was predicted using thermodynamic calculations,sharp interface kinetic models,phase field,and phase field crystal modelingtechniques.Based on the predicted mesoscale microstructure,linear elastic mechanical analyses and electromigration simulations on the ultrafine interconnects were performed.The microstructural index extracted by a method based on singular value decomposition exhibits a monotonous decrease with an increase in the interconnect size.An artificial neural network-based fitting revealed a nonlinear relationship between the microstructure index and the average von Mises stress in the ultrafine interconnects.Future work to address the randomness of microstructure and the resulting scatter in the reliability is discussed in this study.

First-principles study of electronic structure and optical properties of Er:Lu_(2)O_(3)

In the present computational study,we found that Er:Lu_(2)O_(3)materials have promise for application in laser applications.The crystal structure and the electronic and optical properties of Er:Lu_(2)O_(3)materials were studied using first-principle calculations under the framework of density functional theory.Based on the experimental and calculated results,the structure of Lu_(2)O_(3)was established.The calculated results show that doping by Er^(3+)can effectively improve its absorption coefficient in the ultraviolet region and improve the static dielectric constant of Lu_(2)O_(3).As the doping concentration of Er^(3+)increases,the energy of the valence band electrons excited to the conduction band decreases,and the transition is more likely to occur.The absorption coefficient,reflectance,and electron energy loss spectroscopy are bathochromic shifted.The Lu_(2-x)Er_(x)O_(3)(0<x<0.09375)system still retains a low absorption coefficient reflectance in the mid-infrared and visible regions.Our calculations therefore show that rare earth doping can effectively regulate the electronic structure and optical properties of Lu_(2)O_(3).

Machine learning and a computational fluid dynamic approach to estimate phase composition of chemical vapor deposition boron carbide

Chemical vapor deposition is an important method for the preparation of boron carbide.Knowledge of the correlation between the phase composition of the deposit and the deposition conditions (temperature,inlet gas composition,total pressure,reactor configuration,and total flow rate) has not been completely determined.In this work,a novel approach to identify the kinetic mechanisms for the deposit composition is presented.Machine leaning (ML) and computational fluid dynamic (CFD) techniques are utilized to identify core factors that influence the deposit composition.It has been shown that ML,combined with CFD,can reduce the prediction error from about 25% to 7%,compared with the ML approach alone.The sensitivity coefficient study shows that BHCl_(2 )and BCl_(3) produce the most boron atoms,while C_(2)H_(4) and CH_(4) are the main sources of carbon atoms.The new approach can accurately predict the deposited boron-carbon ratio and provide a new design solution for other multi-element systems.

Degradation modeling of degradable copolymers for biomimetic scaffolds

Biomimetic scaffolds provide a suitable growth environment for tissue engineering and demonstrate good potential for application in biomedical fields.Different-sized copolymerized biomimetic scaffolds degrade differently,and the degradation rate is affected by the copolymerization ratio.The study of the degradation property is the foundational research necessary for realizing individualized biomimetic scaffold design.The degradation performance of polyesters with different copolymerization ratios has been widely reported;however,the modeling of this performance has been rarely reported.In this research,the degradation of copolymers was studied with multi-scale modeling,in which the copolymers were dispersed in a cellular manner,the chain break time was simulated,and the chain selection was based on the Monte Carlo(MC)algorithm.The probability model of the copolymer's chain break position was established as a//roulette,/model,whose probability values were estimated by the calculation of the potential energy difference at different chain break positions by molecular dynamics that determined the position of chain shear,thereby fully realizing the simulation of the chain micro-break process.The diffusion of the oligomers was then calculated using the macro diffusion equation,and the degradation process of the copolymer was simulated by three-scale coupling calculations.The calculation results were in good agreement with the experimental data,demonstrating the effectiveness of the proposed method.

High-throughput systematic topological generation of low-energy carbon allotropes

The search for new materials requires effective methods for scanning the space of atomic configurations,in which the number is infinite.Here we present an extensive application of a topological network model of solid-state transformations,which enables one to reduce this infinite number to a countable number of the regions corresponding to topologically different crystalline phases.We have used this model to successfully generate carbon allotropes starting from a very restricted set of initial structures;the generation procedure has required only three steps to scan the configuration space around the parents.As a result,we have obtained all known carbon structures within the specified set of restrictions and discovered 224 allotropes with lattice energy ranging in 0.16–1.76 eV atom^(−1) above diamond including a phase,which is denser and probably harder than diamond.We have shown that this phase has a quite different topological structure compared to the hard allotropes from the diamond polytypic series.We have applied the tiling approach to explore the topology of the generated phases in more detail and found that many phases possessing high hardness are built from the tiles confined by six-membered rings.We have computed the mechanical properties for the generated allotropes and found simple dependences between their density,bulk,and shear moduli.

Multi-Scale Modeling and Numerical Simulation for CVI Process

We consider the multi-scale modeling of the isothermal chemical vapor infiltration (CVI) process for the fabrication of C/SiC composites. We first present a microscopic model in which the preform is regarded as a two-phase porous media describedby a dynamic pore-scale node-bond network during the fabrication process. We thendevelop a macroscopic model by a upscaling procedure based on the homogenizationtheory.