Crystal Volume Fraction Research Articles

Machinability of fluormica glass-ceramics of high mica volume fraction

The high Machinability of Mica glass-ceramics is due to its unique microstructure. This study investigated barium fluormica glass-ceramics, with reference to parameters, which determine the microstructure that governs Hardness and machinability and explore the validity of Baik’s model for high mica Volume Fractions. Barium fluormica glasses were synthesized and casted into rods then heat-treated at 1175 °C and 1200 °C for various times. Glass-ceramics were characterized using DSC, SEM, Vickers microhardness and machinability test. “House of Cards” microstructure was successfully obtained upon heat treatment and was associated with an increase in Volume Fraction and Aspect Ratio of crystals. The Volume Fraction was almost identical for all the samples. However, crystal length and width increased with increasing holding time. Hardness and machinability forces decreased with increasing the Aspect Ratio and Effective Crystallinity. Hardness and Machinability data agreed well with Baik’s data and extend the validity of Baik’s model to much higher Volume Fractions.

Crystal Growth Mechanisms of BiFeO3 Nanoparticles.

A wet-chemical synthesis process was designed to obtain reproducible single-phase multiferroic BiFeO3 nanoparticles. The phase purity, single crystallinity, and size of the nanoparticles are confirmed through the analysis of X-ray diffraction patterns, Raman spectroscopy, and high resolution transmission electron microscopy experiments. Crystal nucleation happens within the amorphous-rich area in multiple seeds, leading to the formation of single crystalline nanoparticles with no preferential faceting. Crystallization mechanisms of BiFeO3 nanoparticles were investigated following the Kissinger-Akahira-Sunose approach, indicating that two crystallization steps are responsible of the complete BiFeO3 nanoparticle formation. The first crystallization step involves a maximum of 70% of the final crystal volume, arising from nanocrystal nucleation and growth. The second step occurs above this threshold crystal volume fraction, and it is related to the nanocrystallite coalescence process. Analysis of the thermodynamic process of the crystallization of BiFeO3 nanoparticles following Ostwald rules suggests a relatively low energy barrier for crystal nucleation, highlighting that phase pure, single crystalline BiFeO3 nanoparticles are obtained using the present optimized wet-chemical synthesis process, with temperatures as low as 450 °C.

Effect of cooling protocol on mechanical properties and microstructure of dental veneering ceramics

ObjectivesUnderstand how cooling protocols control the microstructure and mechanical properties of veneering porcelains. MethodsTwo porcelain powders were selected, one used to veneer metallic frameworks (VM13) and one for zirconia frameworks (VM9). After the last firing cycle, the monolithic specimens were subjected to two cooling protocols: slow and fast. Flexural strength (FS) was evaluated by three-point beam bending and fracture toughness (KIC) was evaluated by the single-edge V-notch beam (SEVNB) method. Scanning electron microscopy (SEM) was performed to determine the leucite crystal volume fraction (%), particle size, and matrix microcrack density. The results were compared by analysis of variances (ANOVA) and Tukey’s multiple comparison test. ResultsThe mechanical properties were significantly (p<0.05) higher for the VM13 porcelain (FS=111.0MPa, KIC=1.01MPa.√m) compared to VM9 (FS=79.6MPa, KIC =0.87MPa.√m) regardless of cooling protocol due to ∼250% higher volume fraction of leucite crystals. The slow cooled VM13 and fast cooled VM9 resulted in the highest and lowest mechanical properties, respectively, while the VM9 slow cooled properties were similar to the VM13 fast cooled. The SEM revealed that the slow cooling significantly increased the volume fraction of leucite crystals by 33–41 %. Across both porcelains, a significant linear correlation between both mechanical properties (strength and toughness) and leucite crystal content was found. Slow cooling was also associated with increased crystal growth resulting in more matrix microcracking. SignificanceControlled crystallization using slow cooling can be applied as a means of strengthening dental porcelains. However, the benefits of slow cooling may be partially offset by increasing the microcrack density in the glass matrix. To achieve the maximum benefit of slow cooling, it is recommending to develop heat treatments to produce porcelain with fine-grained and homogenously dispersed leucite crystals to achieve minimal glass matrix microcracking.

Residual stress effect on the fracture toughness of lithium disilicate glass‐ceramics

AbstractIn glass‐ceramics (GCs), on cooling from the crystallization temperature, internal residual stresses are generated due to the difference between the thermal expansion coefficient (TEC) of the crystal phase(s) and the residual glass. These stresses could degrade or promote their mechanical properties. In this work, we varied the magnitude of the residual stresses in lithium silicate GCs by designing their microstructures. The level of internal stresses was measured using (Synchrotron) X‐ray diffraction. The effects of anisotropy of thermal expansion, crystal shape, and intensity of the residual stresses were analyzed and compared using theoretical models. We extended the Hsueh‐Becher model to include the thermal expansion anisotropy of the orthorhombic lithium disilicate (LS2) crystals. We found that the average residual stresses within the LS2 crystals are compressive or null (−100 to ~0) and highly anisotropic. Most importantly, within the limits of this study, we found no evidence for the influence of (compressive or null) residual stresses on the fracture toughness of the studied GCs. Within the crystal size range from 1 to 5 μm, a highly crystallized volume fraction coupled to relatively large crystals (5 μm) of high elastic modulus improved the glass‐ceramic fracture toughness. This result can guide the microstructural design of novel tough GCs.

Depth-profiling of nickel nanocrystal populations in a borosilicate glass – A combined TEM and XRM study

Populations of nickel nanocrystals in a borosilicate glass were studied by TEM and XRM. It is found that XRM, which is applied for the first time to this type of material, is superior for the precise determination of the depth-dependent number densities and volume fractions of precipitated Ni crystals. Statistical precision is gained by imaging 3D data of up to 60 times larger volumes as compared to the volume of the electron transparent rim that a standard TEM wedge specimen provides. Depth-dependent particle-size distributions of XRM were in agreement with those of TEM as the mean sizes of the Ni crystal populations were considerably larger than the XRM resolution limit.

A Simple Procedure for Measuring Magma Rheology

In this study, a procedure to measure the viscosity of multi-phase magma at high temperatures (&gt;1000°C) was developed by using a simple apparatus comprising a commercially available desktop furnace and viscometer. In particular, the use of a disposable container enabled observations of the microstructure of an entire sample. The procedure was applied to basaltic andesite magma of the 1986 Izu–Oshima fissure eruption, Japan. The results show that reliable data, consistent with previous studies, were obtained and that the magma rheology became non-Newtonian with decreasing temperature, showing clear shear-thinning behavior. The rheological properties of the magma at 1180°C are quantitatively described as a function of shear rate based on three simple non-Newtonian fluid models. Sectional views of the sample confirm that plagioclase and Fe–Ti oxide crystals were nearly uniformly dispersed in the sample. The mean crystal volume fraction of 0.14 enabled crystal interactions inducing changes in crystal arrangement, affecting the rheology.

The effect of the minor Al addition on microstructure and soft magnetic properties for (Fe0.5Co0.5)73.5Si13.5Nb3Cu1B9 nanocrystalline alloy

(Fe0.5Co0.5)73.5Si13.5Nb3Cu1B9-xAlx (x = 0.2, 0.4, 0.6, 0.8) nanocrystalline alloys were investigated with the aim of improving the soft magnetic properties and keeping thermal stability of initial permeability. The temperature dependence of initial permeability (μi - T) was measured from room temperature up to 720 °C for all the alloys. The differential scanning calorimetry and X-ray diffractometry were performed for analyzing crystallization process and microstructure. Experimental results pointed out that the partial replacement of B with Al significantly decreased onset temperature of the primary crystallization (Tx1) from 498 °C to 466 °C and slightly increased the crystallized interval temperature (ΔTx) from 210 °C to 226 °C. μi of alloys annealed at 600 °C maintained a relatively stable value from ambient temperature up to 650 °C. Moreover, μi showed a slight increase with the increase of Al content from 0.2 to 0.6, which could be ascribed to the formation of the uniform nanostructure with small grain size and high volume fraction of crystals. As a result, (Fe0.5Co0.5)73.5Si13.5Nb3Cu1B8.4Al0.6 nanocrystalline alloy annealed at 600 °C exhibited excellent soft magnetic properties at high temperature.

Study of crystallization behavior and kinetics of magnetic FeCoCrNiZr high entropy amorphous alloy

A new magnetic Fe36Co22Cr17Ni15Zr10 high entropy amorphous alloy has been prepared and its crystallization behavior and kinetics been studied. The results show that, under non-isothermal conditions, the crystallization activation energy Ex was calculated by the Kissinger equation, which yield 373 kJmol−1. Relation plot of Ex vs crystallization volume fraction(x) manifests the non-isothermal crystallization process is from difficult to easy. On the other hand, base on Arrhenius relation, the crystallization activation energy Ec under isothermal conditions was calculated to be 370 kJmol−1. Relation plot of Ec vs x manifests the isothermal crystallization process is from easy to difficult. The isothermal kinetics was modeled by Johnson-Mehl-Avrami(JMA) equation, as the isothermal annealing temperature increases, the nucleus growth mechanism from diffusion-controlled 3-D growth convert into interface-controlled 3-D growth. Though proper heat treatment, the saturation magnetization Ms of this alloy can reach up to 106.8A∙m2∙kg−1, it will become promising functional materials in the future.

CRUST DEVELOPMENT INFERRED FROM NUMERICAL MODELS OF LA V A FLOW AND ITS SURFACE THERMAL MEASUREMENTS

Propagation of a lava flow is governed by slope topography, magma rheology, heat exchange with the atmosphere and the underlying terrain, and the rate of the eruption. Highly viscous crust is formed due to cooling and solidification of the uppermost layer of the flow. We consider here two numerical model problems for lava flows, both based on the fundamental physics of a hot fluid flow: a model problem, where thermal conditions (e.g. temperature and heat flow) at the lava surface are unknown a priori (a direct model problem), and a model problem, where the lava surface conditions are known and determined from observations(an inverse model problem). In both models, the lava viscosity depends on temperature and the volume fraction of crystals. By way of solving the direct model problem, we perform a parametric study of steady state lava flows to investigate the influence of the heat flux, viscosity, and effusion rate on the lava crust development. Numerical experiments show that a lava crust becomes thicker in the case of the nonlinear heat transfer compared to the case of a linear heat flow at the interface of lava with the atmosphere. Also, the crust thickens at lower lava effusion rates, while higher rates result in a rapid lava advection, slower cooling, and development of a thinner crust. Moreover, a lava crust becomes thicker with a higher coefficient of conductive heat transfer, or a higher lava viscosity, or the growth of effective emissivity of the lava surface. By way of solving the inverse model problem, we use an assimilation technique (that is, a method for an optimal combination of a numerical model of lava flows with observations) to propagate the temperature and heat flow, inferred from measurements at the interface between lava and the atmosphere, into the lava flow interior and to analyse the evolving lava crust. Results of thermal data assimilation illustrate that the physical parameters of lava flows, including the thickness of it crust, can be recovered from measured surface thermal data well enough at least for slow effusion rates.

Crystallization of hydrogenated amorphous silicon thin film on glass by using ns-pulsed fiber laser operating at 1064 nm

A nanosecond pulsed, Ytterbium doped optical fiber laser, operating at 1064 nm, has been used for laser induced crystallization (LIC) of a standard recipe-hydrogenated amorphous silicon (a-Si:H) thin film, deposited in a radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD) reactor. Despite relatively lower photon energy of 1.165 eV, the crystallization of the a-Si:H sample with the Tauc gap of 1.70 eV was succeeded. The optical energy density impinging on the sample surface was adjusted by the treatment parameters: average output power, repetition frequency and scanning speed. The laser spot energy density was varied mainly from 7.1 to 49.5 J/cm2. By using Raman scattering, scanning electron microscopy (SEM) and Fourier-Transform infrared (FTIR) spectroscopy, the LIC of the sample was investigated both quantitatively and qualitatively. Depending on the magnitude of the spot energy density, the crystal fraction of the sample could be adjusted between 16% and 99%. We have suggested a threshold spot energy density of 21.2 J/cm2, for a large crystal volume fraction of 60%. For smaller energy densities, sub-micrometer sized crystal structures were formed. The nano-scaled silicon grains of 3.05 nm has been reached by decreasing the laser energy density to 7.1 J/cm2. Since the crystallization of large fractions, as much as 99%, is possible with the increase of the laser spot energy density, the nanosecond pulsed 1064 nm laser is a possible candidate to produce crystal silicon on glass (CSG), for photovoltaic applications.

The relationship between crystallization and break temperature of mould flux

ABSTRACTMany researchers believe that crystallization is responsible for drastic viscosity changes near the break temperature of mould flux; however, there are no reasonable experimental data to support this view. In this study, the complete crystallization process is recorded in situ by inhibiting the volatilization of fluoride, and the relationship between viscosity change and crystallization near the break temperature is studied. Mould flux with a basicity of 0.65 had no break temperature, and no crystallization occurs within the temperature range of the viscosity test. The other three mould fluxes begin to crystallize before the break temperature, but the crystal volume fractions at the break temperature are different. The break temperature of mould flux is the crystallization temperature with a critical crystal volume fraction, which is related to the viscosity before the break temperature. The higher the viscosity, the smaller the critical crystal volume fraction. Therefore, break temperature ≤ initial crystallization temperature.

Crystal Plasticity Model Calibration for Ti-7Al Alloy with a Multi-fidelity Computational Scheme

The present work addresses a Gaussian process-based multi-fidelity computational scheme to enable the crystal plasticity modeling of Ti-7Al alloy. The crystal plasticity simulations are performed by using computational techniques that lead to two different solution fidelities. The first technique involves the use of a one-point probability descriptor, orientation distribution function (ODF), which measures the volume fractions of crystals in different orientations. The ODF is posed as the low-fidelity method in the multi-fidelity scheme since it neglects the effects of the microstructural correlations and grain shapes to the macro-scale stress-strain response of the material. For the high-fidelity computational technique, crystal plasticity finite element method (CPFEM) is utilized. This is because the CPFEM resolves better in grain-level microstructural features. However, the CPFEM is a computationally expensive technique and it is not feasible to be utilized for computationally costly problems. Therefore, a multi-fidelity modeling scheme that improves the low-fidelity ODF simulation data with the high-fidelity CPFEM simulations is utilized to obtain the crystal plasticity parameters. The presented approach uses more samples from the computationally less expensive low-fidelity model and less samples from the computationally expensive high-fidelity model to build a numerical framework that satisfies both accuracy and computational time expectations. The results of the presented multi-fidelity scheme show a significant improvement on the accuracy of the crystal plasticity modeling of Ti-7Al compared to the results of the previous low-fidelity solution.

Crystallization progress and soft magnetic properties of FeGaBNbCu alloys

Abstract Fe81−xGaxB15Nb3Cu1 (x = 0, 3, 6, 9, 12 at%) ribbons were fabricated by using melt-spun method and their crystallization kinetics, magnetic properties and microstructure were investigated. We found that the small quenched-in Fe–Ga clusters were formed in the as-spun ribbons with Ga addition. Besides, proper Ga addition could suppress the precipitation of Fe2B phase. During annealing, addition of Ga could provide Fe–Ga clusters to be nucleation sites for the α-Fe(Ga) phase, and could inhibit the initial grain growth. With the increase of annealing temperature above 475 °C, the crystallization was controlled by both Fe–Ga and Cu clusters, which effectively refined the grain size by facilitating the nucleation and increased the crystallization volume fraction. Consequently, in the ribbon with 6 at% Ga, the grains turned out to be the minimum in the mean size of 8 nm, and the optimal soft magnetic properties with coercivity at 6.8 A/m and saturation magnetization at 167 emu/g were obtained at annealing temperature of 490 °C for 3 min.

Structure, optical property, and reaction process of Si quantum dots embedded in an amorphous silicon carbide matrix

Silicon quantum dots (QDs) embedded in an amorphous silicon carbide matrix were prepared using silane, methane, and hydrogen as reactive gases at a low substrate temperature by radio-frequency plasma enhanced chemical vapor deposition followed by thermal annealing at 1000 °C in a N2 atmosphere in the quartz furnace. The structure, optical properties, and reaction process of the synthesized Si QDs embedded in an amorphous silicon carbide matrix under different discharge powers are investigated. With the increase in the discharge power from 40 to 100 W, the experimental result measured by X-ray diffraction, Raman spectroscopy, UV-Visible spectroscopy, and field emission scanning electron microscopy reveals that the average size and the crystal volume fraction of Si QDs decrease from 4.4 to 3.4 nm and from 61.1% to 46.3%, respectively, while the optical bandgap and the deposition rate increase from 1.75 to 1.96 eV and from 15.5 to 16.5 nm/min, respectively. Moreover, the real-time diagnosis of plasma by optical emission spectroscopy (OES) is used to determine the chemical species and excitation temperature under the glow discharge of silane, methane, and hydrogen gas mixtures. Based on the OES measurement, the chemical reactions happening in the plasma and growth mechanism for the synthesis of Si QDs embedded in an amorphous silicon carbide matrix have been proposed. This work plays a significant role in preparation of the Si QDs embedded in an amorphous silicon carbide matrix for third-generation photovoltaic solar cells.

A lattice Boltzmann study of frost growth on a cold surface

A two-dimensional Lattice Boltzmann model was established to study the dynamic physical process of frost growth on a cold flat surface. It is verified that the theoretical results are in good agreement with the experiment data with better accuracy. The frost morphology is explored by showing two-dimension density distribution with different cold surface temperatures. The results also include dynamic characteristics of frost properties such as thickness, deposition weight, volume fraction, density, temperature. According to the results, it is found that the nearer it is from the cold flat plate, the larger of frost crystal volume fraction and density are. The frost surface temperature increases rapidly at the early period of frosting and then increases gradually towards the triple point temperature of the water. The frost internal temperatures rise in a linear tendency with the frost thickness and decrease at the same frost layer location with frost growth. Super-saturation degree of moist air has non-negligible effect on frost growth parameters such as frost thickness, frost crystal deposition mass, frost deposition rate and frost crystal volume fraction.

A Numerical Investigation of Frost Growth on Cold Surfaces Based on the Lattice Boltzmann Method

A numerical investigation of frost growth on a cold flat surface was presented based on two-dimensional Lattice Boltzmann model. This model has been validated to have less prediction error by past experiments. According to the results, it is shown that average frost density appears different at an increasing rate at different frosting stages. In addition, cold surface temperature has great influence on frost growth parameters such as frost crystal deposition mass, frost deposition rate, and frost crystal volume fraction. It was found that the frost crystal deposition mass, frost crystal volume, and the deposition rate first increase rapidly, then gradually slow down, finally remaining unchanged while the cold surface temperature decreases. The further away from the cold surface, the more sparser the frost layer structure becomes due to the smaller frost crystal volume fraction.

Crystallisation in basaltic magmas revealed via in situ 4D synchrotron X-ray microtomography

Magma crystallisation is a fundamental process driving eruptions and controlling the style of volcanic activity. Crystal nucleation delay, heterogeneous and homogeneous nucleation and crystal growth are all time-dependent processes, however, there is a paucity of real-time experimental data on crystal nucleation and growth kinetics, particularly at the beginning of crystallisation when conditions are far from equilibrium. Here, we reveal the first in situ 3D time-dependent observations of crystal nucleation and growth kinetics in a natural magma, reproducing the crystallisation occurring in real-time during a lava flow, by combining a bespoke high-temperature environmental cell with fast synchrotron X-ray microtomography. We find that both crystal nucleation and growth occur in pulses, with the first crystallisation wave producing a relatively low volume fraction of crystals and hence negligible influence on magma viscosity. This result explains why some lava flows cover kilometres in a few hours from eruption inception, highlighting the hazard posed by fast-moving lava flows. We use our observations to quantify disequilibrium crystallisation in basaltic magmas using an empirical model. Our results demonstrate the potential of in situ 3D time-dependent experiments and have fundamental implications for the rheological evolution of basaltic lava flows, aiding flow modelling, eruption forecasting and hazard management.

Process‐Driven Microstructure Control in Melt‐Extrusion‐Based 3D Printing for Tailorable Mechanical Properties in a Polycaprolactone Filament

Abstract3D printing techniques are utilized to produce biomaterial scaffolds with porous architectures that enable cell attachment, biological factors, and appropriate mechanical strength. As the basic building block of a scaffold, the individual filaments should have sufficient mechanical properties, comprising high compressive loading, and fracture resistance to mimic the natural tissue organisation. In this contribution, process–structure–property relationships in melt extruded polycaprolactone filaments are investigated by considering crystalline features, tensile properties, and an array of processing parameters. The tensile properties of the filaments are improved significantly with relatively higher screw rotational speed and relatively lower processing temperature resulting in considerable increase in Young's modulus. The favorable properties are attributed to the increased crystal volume fraction and anisotropy. Thus, this study provides initial pathways for the potential control of mechanical properties of bioscaffolds via engineering crystalline structural features in printed filaments.

Nanocrystallization and magnetostriction coefficient of Fe52Co34Hf7B6Cu1 amorphous alloy treated by medium-frequency magnetic pulse

The Fe52Co34Hf7B6Cu1 amorphous alloy ribbons were prepared by single-roller-quenching method and then treated by medium-frequency magnetic pulse. The microstructural changes of specimens before and after magnetic pulse treatment were observed by TEM and Mössbauer spectroscopy techniques. The magnetostriction coefficient of specimens before and after the medium-frequency magnetic pulse was measured by homemade magnetostriction coefficient measurement system. The results showed that the specimens were nano-crystallized after treated by medium-frequency magnetic pulse. The crystalline phase was α-FeCo which dispersed in the residual amorphous matrix phase to form the two-phase nanocrystalline alloys. The magnetostriction coefficient of specimens after treated by medium-frequency magnetic pulse was composed of the positive magnetostriction coefficient of amorphous matrix phase and the negative magnetostriction coefficient of nanocrystalline phase. With the increase of the crystallization volume fraction, the magnetostriction coefficient λ decreased. And accompanied with the increase of the applied DC magnetic field intensity H, the magnetostriction coefficient λ firstly increased and then decreased.

Numerical simulation of flow and melting characteristics of seawater-ice crystals two-phase flow in inlet straight pipe of shell and tube heat exchanger of polar ship

The ice crystal particles are easy to enter into the seawater cooling system of polar ship together with seawater when it sails in the Arctic. They are easy to accumulate in the pipeline, causing serious blockage of the cooling pipe. In this study, the flow and melting characteristics of ice particles-seawater two-phase flow in inlet straight pipe of shell-and-tube heat exchanger were numerically simulated by using Eulerian-Eulerian two-fluid model coupled with the interphase heat and mass transfer model. The influences of inlet ice packing factor, ice crystal particle diameter, and inlet velocity on the distribution and melting characteristics of ice crystals were investigated. The degree of asymmetry of the distribution of ice crystals in the cross section decreases gradually when the IPF changes from 5 to 15%. The volume fractions of ice crystals near the top of the outlet cross section are 19.59, 19.51, and 22.24% respectively for ice packing factor of 5, 10 and 15%. When the particle diameter is 0.5 mm, the ice crystals are gradually stratified during the flow process. With particle diameters of 1.0 and 2.0 mm, the region with the highest volume fraction of ice crystals is a small circle and the contours in the cloud map are compact. The greater the inlet flow velocity, the less stratified the ice crystals and the more obvious the turbulence on the outlet cross section. The average volume fraction of ice crystals along the flow direction is firstly rapidly reduced and then stabilized after 300 mm.