scholarly journals Optimization of the obtaining temperature of powder composite material B4C – ZrB2 by the boron carbide method

2021 ◽  
Vol 340 ◽  
pp. 01028
Author(s):  
Tatiana S. Gudyma ◽  
Yuriy L. Krutskii ◽  
Nikolay F. Uvarov ◽  
Aleksandr I. Aparnev
Keyword(s):  
Boron Carbide ◽  
High Hardness ◽  
Sintering Process ◽  
Self Diffusion ◽  
Operational Characteristics ◽  
Chemical Inertness ◽  
Metal Diborides ◽  
Composite Mixture ◽  
Transition Metal Diborides ◽  
Self Diffusion Coefficient

Boron carbide is characterized by a unique combination of low density (2.52 g/cm3), high hardness (up to 40 GPa), chemical inertness, the high melting point (2450 °C); for these reasons, the ceramics based on this compound have found application in a number of areas of state-of-the-art technologies. However, it is difficult to obtain dense B4C-based ceramics because of a low value of the self-diffusion coefficient, low plastic deformation of this compound, and high sliding resistance between its grains. The use of modifying additives of transition metal diborides appears to be a promising approach to improving the operational characteristics of B4C-based ceramics. They tend to activate the sintering process by means of activation energy reduction, which leads to a decrease in a grain size, an increase in density, strength, and fracture strength of sintered compositions. Zirconium diboride is often used for this purpose. The objective of the work is to study the changes occurring in the charge of boron carbide, zirconium dioxide and carbon when it is heated to determine the temperature of the complete reagents transformation into B4C –ZrB2 composite mixture.

scholarly journals Densification of ultra-refractory transition metal diboride ceramics

2020 ◽  
Vol 52 (1) ◽  
pp. 1-14
Author(s):  
W.G. Fahrenholtz ◽  
G.E. Hilmas ◽  
Ruixing Li
Keyword(s):  
Transition Metal ◽  
Hot Pressing ◽  
Full Density ◽  
Oxygen Impurity ◽  
Self Diffusion ◽  
Melting Temperatures ◽  
Plasma Sintering ◽  
Spark Plasma ◽  
Metal Diborides ◽  
Transition Metal Diborides

The densification behavior of transition metal diboride compounds was reviewed with emphasis on ZrB2 and HfB2. These compounds are considered ultra-high temperature ceramics because they have melting temperatures above 3000?C. Densification of transition metal diborides is difficult due to their strong covalent bonding, which results in extremely high melting temperatures and low self-diffusion coefficients. In addition, oxide impurities present on the surface of powder particles promotes coarsening, which further inhibits densification. Studies prior to the 1990s predominantly used hot pressing for densification. Those reports revealed densification mechanisms and identified that oxygen impurity contents below about 0.5 wt% were required for effective densification. Subsequent studies have employed advanced sintering methods such as spark plasma sintering and reactive hot pressing to produce materials with nearly full density and higher metallic purity. Further studies are needed to identify fundamental densification mechanisms and further improve the elevated temperature properties of transition metal diborides.

scholarly journals Effect of Modification on the Fluid Diffusion Coefficient in Silica Nanochannels

Molecules ◽  
2021 ◽  
Vol 26 (13) ◽  
pp. 4030
Author(s):  
Gengbiao Chen ◽  
Zhiwen Liu
Keyword(s):  
Diffusion Coefficient ◽  
Diffusion Coefficients ◽  
Surface Effect ◽  
Bulk Water ◽  
Self Diffusion ◽  
Average Diffusion Coefficient ◽  
Average Diffusion ◽  
Chain Lengths ◽  
Fluid Diffusion ◽  
Self Diffusion Coefficient

The diffusion behavior of fluid water in nanochannels with hydroxylation of silica gel and silanization of different modified chain lengths was simulated by the equilibrium molecular dynamics method. The diffusion coefficient of fluid water was calculated by the Einstein method and the Green–Kubo method, so as to analyze the change rule between the modification degree of nanochannels and the diffusion coefficient of fluid water. The results showed that the diffusion coefficient of fluid water increased with the length of the modified chain. The average diffusion coefficient of fluid water in the hydroxylated nanochannels was 8.01% of the bulk water diffusion coefficient, and the diffusion coefficients of fluid water in the –(CH2)3CH3, –(CH2)7CH3, and –(CH2)11CH3 nanochannels were 44.10%, 49.72%, and 53.80% of the diffusion coefficients of bulk water, respectively. In the above four wall characteristic models, the diffusion coefficients in the z direction were smaller than those in the other directions. However, with an increase in the silylation degree, the increased self-diffusion coefficient due to the surface effect could basically offset the decreased self-diffusion coefficient owing to the scale effect. In the four nanochannels, when the local diffusion coefficient of fluid water was in the range of 8 Å close to the wall, Dz was greater than Dxy, and beyond the range of 8 Å of the wall, the Dz was smaller than Dxy.

scholarly journals Insights into the interactions and dynamics of a DES formed by phenyl propionic acid and choline chloride

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Parisa Jahanbakhsh Bonab ◽  
Alireza Rastkar Ebrahimzadeh ◽  
Jaber Jahanbin Sardroodi
Keyword(s):  
Diffusion Coefficient ◽  
Liquid Phase ◽  
Propionic Acid ◽  
Structural Parameters ◽  
Choline Chloride ◽  
Self Diffusion ◽  
Phenyl Propionic Acid ◽  
Donor And Acceptor ◽  
Experimental Values ◽  
Self Diffusion Coefficient

AbstractDeep eutectic solvents (DESs) have received much attention in modern green chemistry as inexpensive and easy to handle analogous ionic liquids. This work employed molecular dynamics techniques to investigate the structure and dynamics of a DES system composed of choline chloride and phenyl propionic acid as a hydrogen bond donor and acceptor, respectively. Dynamical parameters such as mean square displacement, liquid phase self-diffusion coefficient and viscosity are calculated at the pressure of 0.1 MPa and temperatures 293, 321 and 400 K. The system size effect on the self-diffusion coefficient of DES species was also examined. Structural parameters such as liquid phase densities, hydrogen bonds, molecular dipole moment of species, and radial and spatial distribution functions (RDF and SDF) were investigated. The viscosity of the studied system was compared with the experimental values recently reported in the literature. A good agreement was observed between simulated and experimental values. The electrostatic and van der Waals nonbonding interaction energies between species were also evaluated and interpreted in terms of temperature. These investigations could play a vital role in the future development of these designer solvents.

Self-diffusion coefficient of lithium in lithium oxide

1979 ◽  
Vol 87 (2-3) ◽  
pp. 341-344 ◽  
Author(s):  
Y. Oishi ◽  
Y. Kamei ◽  
M. Akiyama ◽  
T. Yanagi
Keyword(s):  
Diffusion Coefficient ◽  
Lithium Oxide ◽  
Self Diffusion ◽  
Self Diffusion Coefficient

Viscosity and self-diffusion coefficient of linear polyethylene

1987 ◽  
Vol 20 (5) ◽  
pp. 1133-1141 ◽  
Author(s):  
D. S. Pearson ◽  
G. Ver Strate ◽  
E. Von Meerwall ◽  
F. C. Schilling
Keyword(s):  
Diffusion Coefficient ◽  
Linear Polyethylene ◽  
Self Diffusion ◽  
Self Diffusion Coefficient

Co2Si, CrSi2, ZrSi2 and TiSi2 Formation Studied by a Radioactive 31Si Marker Technique

1981 ◽  
Vol 10 ◽  
Author(s):  
A. P. Botha ◽  
R. Pretorius
Keyword(s):  
Diffusion Coefficient ◽  
Half Life ◽  
Vacancy Diffusion ◽  
Silicide Formation ◽  
Activity Profile ◽  
Initial Layer ◽  
Self Diffusion ◽  
Silicide Layer ◽  
Self Diffusion Coefficient

Radioactive 31Si (half-life, 2.62 h) was used as a marker to study Co2Si, CrSi2, TiSi2 and ZrSi2 formation. By marking the initial layer of silicide with radioactive silicon atoms and by measuring the activity profile in the silicide layer after further silicide formation, the dominant diffusing species and its mechanism of diffusion during the formation of these silicides could be determined. For Co2Si it was found that cobalt is the diffusing species, while disilicide formation was found to take place by silicon substitutional (vacancy) diffusion, with a high self-diffusion coefficient.

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