Nb/Sn Liquid-Solid Reactive Diffusion Couples and Their Application to Determination of Phase Equilibria and Interdiffusion Coefficients of Nb-Sn Binary System

Nb3Sn plays an irreplaceable role in superconducting parts due to its stable performance under high field conditions. Accurate phase equilibria and interdiffusion coefficients are of great significance for designing novel Nb3Sn superconductors. However, the related experimental information is still in a state of scarcity because of the difficulty in fabrication of Nb-Sn alloys caused by the large difference in melting points of Nb and Sn. In this paper, a simple but pragmatic approach was first proposed to prepare the Nb/Sn liquid-solid reactive diffusion couples (LSDCs) at 1100 °C and 1200 °C, of which the phase identification of the formed layer and the measurement of composition-distance profiles were conducted. The formed layer in Nb/Sn LSDCs was confirmed to be Nb3Sn compound. While the measured composition profiles were employed to determine the phase equilibria according to the local equilibrium hypothesis and the interdiffusion coefficients with an aid of the latest version of HitDIC software. The determined phase equilibria of Nb3Sn, (Nb) and liquid show good agreement with the assessed phase diagram. While the calculated interdiffusion coefficients and activation energy for diffusion in Nb3Sn are consistent with both experimental and theoretical data in the literature. Moreover, the growth of the formed Nb3Sn layer in Nb/Sn LSDCs was also found to be diffusion controlled. All the obtained phase equilibria and interdiffusion coefficients are of great value for further thermodynamic and kinetic modeling of the Nb-Sn system. Furthermore, it is anticipated that the presently proposed approach of fabricating liquid-solid reactive diffusion couple should serve as a general one for various alloy systems with large differences in melting points.

Effect of Initial Microstructure on Soft Annealing of a Low-Carbon Bainitic Steel

A low-carbon bainitic tool steel exhibiting high hardness after hot rolling typically has poor machinability. To soften this type of steel and to accelerate the soft annealing process, an austenitizing step was designed based on thermodynamic calculations of phase stability and introduced prior to the annealing step. Different initial microstructures were prepared by three austenitizing temperatures (680 °C, 850 °C, 1000 °C) and three cooling methods (water quenching, oil quenching, and air cooling). The effect of initial microstructure on microstructures and hardness was studied. Softening equations, a function of annealing temperature and time, were established for different initial microstructures, and the relationships between annealing temperature, annealing time, activation energy, and hardness were explored. The predicted hardness was consistent with the measured values. Martensitic structure has a low activation energy for diffusion and a higher softening rate compared to that of the bainitic structure. In addition, the higher the carbide content in the bainitic structure, the smaller the activation energy tended to be.

PENGARUH KONSENTRASI DAN SUHU LARUTAN GULA PADA PROSES DEHIDRASI OSMOTIK BUAH NAGA (Hylocereus sp.)

ABSTRACTOsmotic dehydration is a water removing process that carried out on a object by immersing the object into an hyper-tonic (osmotic) solution. The process is commonly applied on pre-drying of fruit. The purpose of this research is to investigate the effect of concentration and temperature of osmotic solution on moisture content change, total dissolved solids change and to determine of water diffusion coefficient and solid diffusion coefficient during the process of osmotic dehydration of dragon fruit. Factorial design was used with 2 factors, each consisting of 3 levels with 3 replication. The treatment on this research are : solution consentration of 30 °Brix, 50 °Brix, and 70 °Brix and solution temperature of 30 °C, 40 °C, and 50 °C were applied to this research. The osmotic dehydration process lasts for 8 hours. Initial moisture content of dragon fruit used for the research between 511.17-665.97 (% db). Moisture and solid diffusivities were in the range of 2.810 x 10-8 m²/s - 7.003 x 10-8 m²/s and 0.973 x 10-8 m²/s until 4.734 x 10-8 m²/s. The activation energy for diffusion of water to the concentration of 30 °Brix, 50 °Brix, and 70 °Brix respectively is 9.963 kJ/mol, 3.249 kJ/mol, and 5.372 kJ/mol. While the activation energy for diffusion of solids is 24.946 kJ/mol, 8.908 kJ/mol, and 27.343 kJ/mol. Keywords: dragon fruit, moisture diffusivity, osmotic dehydration, solid diffusivity ABSTRAK Dehidrasi osmotik adalah suatu proses pengeluaran air yang dilakukan terhadap suatu bahan dengan cara merendam bahan tersebut ke dalam suatu larutan hipertonik. Proses ini pada umumnya diaplikasikan untuk pra-pengeringan buah-buahan. Tujuan penelitian ini adalah untuk mengkaji pengaruh perlakuan konsentrasi dan suhu terhadap perubahan kadar air, perubahan total padatan terlarut dan untuk menentukan nilai koefisien difusi air dan difusi padatan selama proses dehidrasi osmotik pada buah naga. Digunakan rancangan faktorial dengan 2 faktor yang masing-masing terdiri atas 3 taraf dengan 3 kali ulangan. Perlakuan yang digunakan pada penelitian ini adalah variasi konsentrasi larutan 30 °Brix, 50 °Brix, dan 70 °Brix dan suhu larutan 30 °C, 40 °C, dan 50 °C. Proses dehidrasi osmotik berlangsung selama 8 jam. Kadar air awal buah naga yang digunakan untuk penelitian berkisar antara 511,17-665,97 (%db). Nilai difusivitas air antara 2,810 x 10-8 m²/s – 7,003 x 10-8 m²/s dan difusivitas padatan antara 0,973 x 10-8 m²/s - 4,734 x 10-8 m²/s. Energi aktivasi untuk difusi air untuk konsentrasi 30 °Brix, 50 °Brix, dan 70 °Brix secara berurutan adalah 9,963 kJ/mol, 3,249 kJ/mol, dan 5,372 kJ/mol. Energi aktivasi untuk difusi padatan adalah 24,946 kJ/mol, 8,908 kJ/mol, dan 27,343 kJ/mol. Kata kunci: buah naga, dehidrasi osmotik, difusivitas air, difusivitas padatan

Inspection for supercritical CO2 dyeing of poly(m-phenylene isophthalamide) by kinetics and thermodynamics analysis

Eco-friendly dyeing by using supercritical carbon dioxide as a medium has already been investigated worldwide due to the advantages of dyeing without water and recyclability of dyes and carbon dioxide. In this article, dyeing mechanism of poly(m-phenylene isophthalamide) was investigated in supercritical carbon dioxide. The obtained results showed that the dye uptake of Disperse Red 60 increased moderately with the temperature raising at constant pressure and achieved dyeing equilibrium after 70 min. By adding the carrier, diffusion coefficients of Disperse Red 60 in the polymer increased significantly in supercritical carbon dioxide. The activation energy for diffusion of Disperse Red 60 with and without carrier was 1165.91 and 1050.66 kJ mol−1, respectively. Moreover, the distribution coefficient, the standard affinity, the standard enthalpy, and the standard entropy of dyeing were also determined in supercritical carbon dioxide. These fundamental data are of vital importance on the green dyeing production of poly(m-phenylene isophthalamide).

Diffusion Coefficient of Nickel in Ni2Al3 Intermetallic Compound

Nickel alloys have been widely used for high temperature applications such as gas turbine engine, turbine blade and many high temperature resistance components. Aluminizing is one of effective to increase oxidation resistant of nickel alloys by forming nickel aluminide compounds on nickel surface. Nickel aluminide is formed by the diffusion mechanism. This research aims to study the diffusion behavior of nickel in Ni2Al3compound. The diffusion coefficient is determined using Ni/Al diffusion couple forming Ni2Al3and NiAl3phases. The temperatures under study are 873, 973, and 1073 K, which are at above and below melting point of aluminum. Determination of diffusion coefficient of Ni in Ni2Al3is from mass balance concept: flux difference at interphase causes accumulation of atoms in compound layer, which as be derived as: dxαβ/dt = [1/(nβ-nα)] * [Dα*(dnα/dx) - Dβ*(dnβ/dx)] From this equation, diffusion coefficient of Ni in Ni2Al3at 873 and 1073 K are calculated as 6.243×10-11and 6.82×10-9m2/s, respectively. From Arrhenius equation of diffusivity, D = Doexp (-Q/RT), the activation energy for diffusion of nickel in Ni2Al3is found to be 183 kJ/mol. The result obtained in this research is of great use in controlling aluminizing process.

Determination of Impurity Diffusion Coefficients in a Single Experimental Cycle

An experimental approach employing temperature and concentration gradients is presented that is suitable for determining impurity diffusion coefficients in a single experimental cycle. The Al-Cu system is used to illustrate the feasibility of the method. In a single phase α-Al solid solution, concentration gradients are generated by exposing a cylindrical sample to steep temperature gradients and by annealing until the initially formed mushy zone is re-solidified. The annealing is performed such that a symmetric, ramp shaped profile in the form of a roof is generated. The sample is then again exposed to a temperature gradient at somewhat lower temperatures for an extended time period. The symmetric profile then becomes asymmetric due to the varying diffusion coefficient along the sample. Information on the pre-exponential factor D0 and the activation energy for diffusion QD is retrieved from the asymmetry of the resulting concentration profile. The asymmetry becomes increasingly pronounced with longer diffusion times, yielding an increasing accuracy of the diffusion coefficients. The experimental approach is generally applicable to alloy systems with finite solubility.

Assessment of Mobilities of the B2 Phase in the Nickel-Chromium-Aluminum System

A model for diffusion is applied to the nickel-chromium-aluminum (Ni-Cr-Al) system to optimize the mobility parameters for the B2 phase by fitting the experimentally obtained inter-diffusivities. In the model, the activation energy for diffusion is expressed as a function of the degree of ordering which is evaluated from thermodynamic data. These data are used to evaluate the thermodynamic factors of the B2 phase in the Ni-Cr-Al ternary system. The interdiffusivities from the model are used to simulate diffusion experiments. The results are generally agreed with the experimental data.

Microstructural Characterisation of Devitrite, Na2Ca3Si6O16

On suitably heat treated samples of float glass, devitrite, Na2Ca3Si6O16, nucleates heterogeneously on the glass surface and grows as thin needles with a distinct [100] growth axis. The needles group together as spherulites and show evidence of low-angle branching in order to fill space. Analysis of the rate of needle growth along [100] between 680°C and 950°C suggests that the activation energy for diffusion governing the transport across the devitrite–glass interface is 260 kJ mol–1, somewhat higher than previous analyses have suggested. The devitrite needles are often twinned, with the twins related to one another by a Type II twinning rotation of 180° about the [100] growth axis direction. The formally irrational twin plane is very close to (010). Molecular dynamics simulations have been undertaken to establish the form of these twin interfaces to understand how and why devitrite twins in this manner.