The Mechanism of Mechanochemical Interaction between Amorphous Silicon Dioxide and Pyrocatechol

Silicon - The products of solid-phase mechanochemical interaction between pyrocatechol and silicon dioxide yielding a powdered composite were studied using a number of physicochemical methods. This...     

Laser-induced reaction of Si powder with a steel substrate

We have determined the phase composition and microhardness of coatings produced by the laser melting of silicon powder on the surface of steel substrates. Laser exposure leads to the formation of iron silicides on the substrate surface. The iron content of the silicides depends on the effective laser power and increases from that of FeSi₂ to Fe_0.9Si_0.1. The coatings have the highest microhardness at an effective power of ≏ 400 W s/m, which corresponds to the highest FeSi content.     

Ti3SiC2-Cu composites by mechanical milling and spark plasma sintering: Possible microstructure formation scenarios

We present several possible microstructure development scenarios in Ti₃SiC₂-Cu composites during mechanical milling and Spark Plasma Sintering (SPS). We have studied the effect of in situ consolidation during milling of Ti₃SiC₂ and Cu powders and melting of the Cu matrix during the SPS on the hardness and electrical conductivity of the sintered materials. Under low-energy milling, (3–5) vol.%Ti₃SiC₂-Cu composite particles of platelet morphology formed, which could be easily SPS-ed to 92–95% relative density. Under high-energy milling, millimeter-scale (3–5) vol.%Ti₃SiC₂-Cu granules formed as a result of in situ consolidation and presented a challenge to be sintered into a bulk fully dense sample; the corresponding SPS-ed compacts demonstrated a finer-grained Cu matrix and more significant levels of hardening compared to composites of the same composition processed by low-energy milling. The 3 vol.% Ti₃SiC₂-Cu in situ consolidated and Spark Plasma Sintered granules showed an extremely high hardness of 227 HV. High electrical conductivity of the Ti₃SiC₂-Cu composites sintered from the granules was an indication of efficient sintering of the granules to each other. Partial melting of the Cu matrix, if induced during the SPS, compromised the phase stability and uniformity of the microstructure of the Ti₃SiC₂-Cu composites and thus it is not to be suggested as a pathway to enhanced densification in this system.     

Microstructure changes in TiB2-Cu nanocomposite under sintering

Stability and growth of nanoparticulate reinforcements in metal matrix composites during heating are widely studied for dispersion-strengthened alloys, which contain several volume percent of reinforcing phase. When high volume content of nanoparticles distributed within a matrix is concerned results of particles aggregation and growth as well as crystallization mechanisms are not so evident. In this work microstructural evolution under sintering in metal-matrix composite TiB₂-Cu with high volume content (up to 57%) of titanium diboride nanoparticles 30-50 nm in size was investigated. The nanocomposite powders were produced through synthetic method combining preliminary mechanical treatment of initial powder mixtures in high-energy ball mill, self-propagating exothermic reaction and subsequent mechanical treatment of the product. We focused on microstructure changes in TiB₂-Cu nanocomposite consolidated by Spark-Plasma Sintering and conventional sintering and showed that in the former case fine-grained skeleton of titanium diboride is formed with connectivity between particles well established. In the latter case behavior of nanoparticles is surprising: at low temperatures fiber-like structures are formed while increasing temperature causes appearance of faceted crystals. These unusual results allow us to propose the direct involvement of nanoparticles in the processes of crystallization by moving as a whole in the matrix.     

Nanocomposites TiB<Subscript>2</Subscript>-Cu: Consolidation and erosion behavior

In this work we report consolidation and erosion behavior of TiB₂-Cu nanocomposites showing increased stability during electric erosion in high-current arc discharge. Composite powders containing nanoparticles of titanium diboride distributed in copper matrix were synthesized using high-energy ball milling and then shock wave consolidated to obtain fully dense compact electrode material with retention of the size of particulate inclusions. Copper weight losses by evaporation in composite electrodes were 10 times lower compared to pure copper electrodes and arc spot size was about an order of magnitude increased indicating distribution of arc on a larger surface. Porous Cu-depleted layer was formed on the surface of composite electrode and no copper melt was observed to squeeze out on the surface so that the electrode held its shape and size. The improved erosion resistance of the electrode material is believed to be due to its nanocomposite structure.     

Laser-induced reactions in layers of B4C + Fe and B + C + Fe mixtures on a steel substrate

We have studied laser-induced processes in layers of mixtures of boron carbide and iron or a stoichiometric mixture of boron with carbon (4: 1) and iron on a steel substrate. Laser processing was found to increase the microhardness of the surface owing to the formation of a coating containing iron boride phases: Fe₂B and Fe₃B. After preliminary mechanical activation of boron carbide and iron mixtures, even the lowest laser power used in the experiments was enough to initiate the formation of iron borides.     

Detonation spraying of TiO2-2.5 vol.% Ag powders in a reducing atmosphere

In the present work, rutile powders containing additions of metallic silver (2.5 vol.%) were detonation sprayed in a reducing atmosphere formed by gaseous detonation products of the C₂H₂ + 1.05O₂ mixture. The initial volume of the C₂H₂ + 1.05O₂ mixture - explosive charge - used for a detonation pulse was computer-controlled as the fraction of the barrel volume filled with the mixture. Using a previously developed model of the detonation process, the particle temperatures and velocities were calculated to explain the observed phase and microstructure development in the coatings. With increasing explosive charge, the temperature of the sprayed particles increased and rutile was partially reduced to oxygen-deficient TiO_2−x and then to Ti₃O₅. When the melting temperature of rutile was not reached, the coatings were porous; semi-molten particles formed denser coatings obtained with higher spraying efficiency. Silver inclusions in the titanium oxide matrix experienced melting and substantial overheating, but remained well preserved in the coatings.     

Phase analysis of the mechanically alloyed Fe−Si powder by differential dissolution technique

The binary Fe?Si elemental powders mixture (1∶2 in atomic proportion) has been milled for different milling times in an attrition mill. The phase characterization of mechanically alloyed powder was investigated using the chemical method of differential dissolution (DD) and the X-ray diffraction (XRD) method. In powder specimens milled for more than 15 hr, ∈-FeSi and unreacted Si were observed. The formation of a supersaturated solid solution of Si in ∈-FeSi induced by mechanical alloying (MA) was also verified. The lattice parameter of the ∈-FeSi of as-milled powders changed from 4.4876 Å to 4.4668 Å according to the increase of MA time. Based on the results of the DD analysis, unreacted Si could be classified as (1) crystalline Si, (2) Si supersaturated in ∈-FeSi, or (3) amorphous Si. Therefore formation of the β-FeSi₂ after annealing could be explained by the reaction between the ∈-FeSi and the Si classified into types (1) and (2). It seemed that the amorphous Si induced by MA did not react with the ∈-FeSi during annealing at 700°C.     

Mechanochemical synthesis and properties of thermoelectric material β-FeSi2

The mechano-chemical synthesis of thermoelectric material on the basis of -FeSi₂ has been investigated. The mixture of FeSi and amorphous Si has been shown to be a optimum precursor to produce the thermoelectric ceramics. The ceramics properties (thermoelectric power , V/K, electrical conductivity 1/*cm) have been considerably improved by means of doping with superequilibrium quantity of 12% of aluminium (substitution of silicon) or 10% of cobalt (substitution of iron). The mechanical alloying in a high energy ball mill, under the acceleration of treating balls 800 m/sec² produced homogeneous powder with a superequilibrium quantity of dopant, which conversed into thermoelectric ceramics after short annealing in vacuum at low temperature (780°C). The samples of ceramics with the maximum content of doping elements have increased thermoelectromotive force-up to 800 V/K. Mechanically alloyed ceramic is a promising material as a medium temperature thermoelectric with advanced properties for autonomous power supply units.     

Mechanochemical Transformations in Mixtures of the Silicate Phases of Granite

The effect of mechanical treatment of different degrees of action on the changes in the composition of silicate phases of natural granite is studied. Mechanochemical treatment is demonstrated to cause disordering of silicate structures. Layered hydrosilicates are most easily destroyed, which is explained by their dehydration. This destruction is accompanied by the formation and crystallization of the silicates with more stable framework structure and SiO₂. Mechanical treatment of granite leads to the transformation and absorption of bound water and to the decrease of water-loss temperature by 200–300°C. Thermal heating of mechanically activated mixture of granite particles with lime results in the formation of calcium aluminosilicates with IR absorption bands at 1020, 970, and 930 cm^–1.