Formation Of Al4C3 Research Articles

Carbothermal reduction of alumina: Thermochemical equilibrium calculations and experimental investigation

Abstract The production of aluminum by the electrolytic Hall–Heroult process suffers from high energy requirements, the release of perfluorocarbons, and vast greenhouse gas emissions. The alternative carbothermic reduction of alumina, while significantly less energy-intensive, is complicated by the formation of aluminum carbide and oxycarbides. In the present work, the formation of Al, as well as Al2OC, Al4O4C, and Al4C3 was proven by experiments on mixtures of Al2O3 and activated carbon in an Ar atmosphere submitted to heat pulses by an induction furnace. Thermochemical equilibrium calculations indicate that the Al2O3-reduction using carbon as reducing agent is favored in the presence of limited amounts of oxygen. The temperature threshold for the onset of aluminum production is lowered, the formation of Al4C3 is decreased, and the yield of aluminum is improved. Significant further enhancement in the carbothermic reduction of Al2O3 is predicted by using CH4 as the reducing agent, again in the presence of limited amounts of oxygen. In this case, an important by-product is syngas, with a H2/CO molar ratio of about 2, suitable for methanol or Fischer–Tropsch syntheses. Under appropriate temperature and stoichiometry of reactants, the process can be designed to be thermo-neutral. Using alumina, methane, and oxygen as reagents, the co-production of aluminum with syngas, to be converted to methanol, predicts fuel savings of about 68% and CO2 emission avoidance of about 91%, vis-a-vis the conventional production of Al by electrolysis and of methanol by steam reforming of CH4. When using carbon (such as coke or petcoke) as reducing agent, fuel savings of 66% and CO2 emission avoidance of 15% are predicted. Preliminary evaluation for the proposed process indicates favorable economics, and the required high temperatures process heat is readily attainable using concentrated solar energy.

Ohmic Contacts on p-Type SiC Using Al/C Films

Al based alloys, such as Ti/Al, are commonly used for ohmic contacts on p-type SiC. The interfacial structures of a metal alloy film on SiC are very complicated after annealing. Al is considered as the key element responsible for forming ohmic contacts on p-type SiC, and reacts with C from SiC and forms Al4C3 and Si during annealing. In this study, we have investigated ohmic contact formation of a single component Al4C3 film on p-type SiC. Based on the stoichiometric formation of Al4C3 between Al and C at high temperatures, several samples with various Al/C mole ratios have been examined for ohmic contact formation after different annealing temperatures. Carbon rich and stoichiometric Al4C3 films form ohmic contacts on p-type 4H-SiC (~2.8 x1018 cm-3 ) after annealing at 800 and 900°C. X-ray diffraction (XRD) data have shown that a single component Al4C3 is formed when an ohmic contact on p-type SiC is activated. Al/SiC, as the control sample, does not form ohmic contacts under the same conditions. This study reveals that Al4C3 can be responsible for forming ohmic contacts on p-type SiC. However, its chemical instability requires that the secondary metal is necessary to form stable ohmic contacts when Albased films are used.

Alloying effects on microstructure and mechanical properties of high volume fraction SiC-particle reinforced Al-MMCs made by squeeze casting infiltration

The mechanical properties of high volume fraction SiC-particle reinforced Al-based metal matrix composites (MMCs) produced by means of pressurized liquid metal infiltration (squeeze casting) are shown to be triggered by matrix alloying and heat treatment procedures. It is distinguished between the effect of those alloying elements that only act on matrix strengthening, leaving the interface unaffected, and those alloying elements that interact with both (i.e. Mg). Among the first category a further sub-division is made between pure solid solution and precipitation hardening elements (i.e. Zn and Cu, ZnMg, respectively). In particular, this study addresses the effect of alloying and age hardening for AlCu3 and AlZn6Mg1 as well as the specific role of Mg additions to Al/SiC MMCs on interface microstructure formation, mechanical properties and fracture mode. For instance, it is shown that single additions of Mg catalyse the formation of Al4C3 whereas additions of Cu as well as (Zn+Mg) provide opportunities to enhance the composites’ strength.

Solid State Reactions in the Al-Si-C System

The study is aimed to prevent the formation of the aluminium carbide compound Al4C3 that negatively affects Al-Si-C based materials. The reaction products of elementary aluminium, silicon and graphite as well as aluminium with either β-SiC or α-SiC without and with graphite at temperatures 1200°-2500 °C under different atmospheres and reaction times were characterized using powder X-ray diffraction and scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) analysis. The results of the powder diffraction study show that under the conditions (1450 °C; 8 h; vacuum) the formation of Al4C3 could be prevented. The reaction products at those conditions consist of the ternary compound Al4SiC4 besides SiC and residual carbon. The ternary aluminium silicon carbide Al4SiC4 crystallizes in a hexagonal crystal system with unit cell dimensions a = 327.64(4) pm, b = 2171.2(6) pm and space group P63mc (no. 186). The crystal structure of Al4SiC4 is isostructural with Al5C3N and consists of layers of Al4C3 and SiC.

Stability of SiC in Al-Rich Corner of Liquid Al-Si-Mg System

The thermodynamic analysis of the SiC/Al-Si-Mg system has been performed in order to find the conditions to produce SiC/Al-Si-Mg composite materials with the stable SiC/alloy interface (for both a-SiC and b-SiC) and with the solidification of primary a-Al solid solution. The conditions to avoid the formation of Al4C3 are expressed as function of temperature, and the silicon and magnesium content of the liquid aluminium alloy. It has been shown that to ensure stabilization of (the more stable) b-SiC, lower Si-content is needed and higher working temperature is allowed, compared to the requirements to stabilize (the less stable) a-SiC.

Mechanism and rate of reaction of Al2O, Al, and CO vapors with carbon

During the production of aluminum by carbothermic reduction, large quantities of Al2O and Al vapor are generated. For the process to be economical, the aluminum and energy associated with these species must be captured and used in the process. This is accomplished by reacting them with carbon to form Al4C3. The mechanism and rate of the reactions of gas containing Al and Al2O with various forms of carbon was studied. The Al2O-Al-CO gas was generated by reacting an Al4C3-Al2O3 melt with carbon at high temperatures (2000 °C to 2050 °C). The gas then reacted with carbon at lower temperatures (1900 °C to 1950 °C). The only form of carbon that reacted extensively, forming Al4C3, was wood charcoal; with other forms of carbon, such as metallurgical coke and petroleum coke, primarily only Al2O3 condensed on the surface formed. The rate of formation of Al4C3 on wood charcoal was found to be controlled by the diffusion of Al2O and Al through the Al4C3 product layer, and their effective diffusivities were estimated to be 0.82 and 1.31 cm2/s, respectively. Over 90 pct of the carbide is formed by Al2O and only 10 pct by Al vapor. When an Al4C3-Al2O3 dense slag was formed on the charcoal at lower temperatures (1920 °C to 1930 °C) and then reacted at a higher temperature, it appears that the slag and carbon reacted to form Al4C3 relatively fast. The volume of Al4C3 formed is much greater than that of the original carbon. It is believed that this is the reason the other forms of carbon with lower porosity (25 pct vs 60 pct) did not react significantly. Any amount of Al4C3 formed would quickly fill the pores of the more dense carbon, stopping any further reaction.

Fracture Toughness of Nanocrystalline L12 (Al+X at.%Mn)3 Ti Prepared by Mechanical Alloying and Consolidated by SPS

AbstractThe effects of ternary element addition of manganese on microstructure and mechanical properties of nanocrystalline L12 (Al+X at.%Mn)3Ti (X=0 - 12) fabricated by mechanical alloying and spark plasma sintering (SPS) were investigated. The SPS method was used to consolidate nanocrystalline L12 (Al+X at.%Mn)3Ti with the pressure of 50 MPa. The heating rate was 100°C /min. The final sintering temperature was determined from the observation of the shrinkage of the specimen. In binary Al3Ti, the final sintering temperature was 864°C. The sintering temperature was reduced down to 658°C with increasing Mn concentration. After SPS, the L12 structure was maintained in the specimens which contained Mn concentration of 8- 12 at.%. The microhardness test, grain size measurements, and fracture toughness test were conducted as a function of Mn concentration. In order to investigate the effect of the grain size on the microhardness and fracture toughness of the (Al+8 at.%Mn)3Ti, heat treatments were performed in a vacuum furnace ( 10-3 torr ). With increasing heat treatment temperature, the microhardness decreased due to the formation and growth of Al4C3, Al2O3, and TiC, while fracture toughness increased. The fracture toughness of 4.12 MPa m1/2 was attained for the (Al+8 at.%Mn)3Ti specimen and it was the highest value among the specimens after 1h heat treatment at 1100°C. The grain sizes were about 60 nm and 100 nm after 1 h heat treatment at 900°C and 1100°C, respectively.

Methodology to design the interfaces in SiC/Al composites

A methodology to control interfacial microstructures, while suppressing formation of Al4C3 in wrought Al alloy composites reinforced with SiC, was demonstrated. Thermodynamic calculations were carried out to elucidate how one can select process parameters in terms of alloy composition and fabrication temperature to obtain intended interfaces. Experimental verifications were conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to validate calculated results. The reaction mechanisms for forming various interfaces were identified both theoretically and experimentally. Evaluations of the interfacial bonding strengths and interfacial stability at elevated temperatures were also carried out for various interface types.

Production of Al–Ti–C grain refiner alloys by reactive synthesis of elemental powders: Part I. Reactive synthesis and characterization of alloys

The first part of this article reports on the reactive synthesis and characterization of Al–Ti–C alloys intended as master alloys for aluminum grain refining. The alloys were produced from elemental powders by the thermal explosion mode and analyzed with x-ray diffraction, scanning electron microscopy, and differential scanning calorimetry. Parameters were the titanium concentration (15 and 30 wt%) and the Ti/C ratio (9/1, 20/1, and 120/1) in the reactant powder mixture and the cooling rate after the reactive synthesis (1 and 120 °C/min). Full conversion of titanium and carbon into Al3Ti and TiC was achieved for the 30 wt% Ti mixtures but not for the 15 wt% Ti mixtures where the reaction was not exothermic enough. The Ti/C ratio did not affect the phase composition after reactive synthesis in the 30 wt% Ti alloys and could be used to tailor the microstructure of the alloy. The formation of Al4C3 was suppressed with a high cooling rate after the exothermic formation reactions.

Modification of the interface in SiC/Al composites

Methodologies both to avoid the formation of Al4C3 and to tailor the interfacial structures in a SiC/2014 Al composite were demonstrated. Modification of the interfacial structures in the SiC/2014 Al composite was made by forming SiO2 layers on the surfaces of SiC via passive oxidation at elevated temperatures. In the 2014 Al composite reinforced with the oxidized SiC, MgAl2O4 and Si crystals were observed to be present at the interfacial region as a result of the reaction between the SiO2 layer and the matrix. On the other hand, in the case of the 2014 Al composite reinforced with unoxidized SiC, SiC was found to react with the Al matrix to form both Al4C3 and Si. Qualitative measurements of the interfacial bonding strength were carried out on composites having various types of interfaces and thicknesses. Detailed interfacial structures and phase identifications, which were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were presented.

Improvement of Interface in SiC Particle-Reinforced Al-Li Composite by Sol-Gel Coating Technique

This paper describes a study to investigate an improvement of the interface in SiC particle-reinforced Al-Li composite by coating the SiC particle with metallic oxide film. A coating method utilizing a sol-gel technique produced fine SiC powders covered with SiO2, TiO2, or ZrO2 metallic oxide film. The film thickness was not affected by the hydrolysis time of the sol solution under a dry argon atmosphere. The average thickness of the SiO2, TiO2 and ZrO2 films was about 10 nm, 8 nm and 27 nm, respectively. In metallic oxide coated SiC particle-reinforced Al-Li composite, TEM observation showed that the coating film was not damaged by fabrication processing, and that the formation of Al4C3 at the interface between the matrix and reinforcement was controlled by the coating film. The tensile strength and elongation of the extruded TiO2, coated sample increased by 12% and 24%, respectively, compared with the non-coated sample.

Interfacial reactions in aluminum/SiC fibre composite electric power cable using low oxygen SiC fibre reinforcement

We have evaluated the interfacial reactions of SiC fibre reinforced Al electrical power cable using low oxygen SiC fibre (Si : 62.4, C : 37.1, 0 : 0.5 mass%), and determined the relationship between the tensile strength and the amount of reaction products at the interface. The following are occurring at the SiC/Al interface: i) diffusion of Al atoms into the SiC fibre, ii) formation of needle–shape Al4C3 compounds, and iii) formation of Al9Si compounds. Formation of Al4C3 and Al9Si compounds at the interface causes the strength of SiC/Al composite electric power cable to deteriorate.

Scanning and transmission electron microscopy study of the microstructural changes occurring in aluminium matrix composites reinforced with SiC particles during casting and welding: interface reactions

Processing of aluminium matrix composites (AMCs), especially those constituted by a reactive system such as Al-SiC, presents great difficulties which limit their potential applications. The interface reactivity between SiC and molten Al generates an aluminium carbide which degrades the composite properties. Scanning and transmission electron microscopes equipped with energy-dispersive X-ray spectroscopes are essential tools for determining the structure and chemistry of the Al-SiC interfaces in AMCs and changes occurring during casting and arc welding. In the present work, an aluminium-copper alloy (AA2014) reinforced with three different percentages of SiC particles was subjected to controlled remelting tests, at temperatures in the range 750-900 degrees C for 10 and 30 min. Arc welding tests using a tungsten intert gas with power inputs in the range 850-2000 W were also carried out. The results of these studies showed that during remelting there is preferential SiC particle consumption with formation of Al4C3 by interface reaction between the solid SiC particle and the molten aluminium matrix. The formation of Al4C3 by the same mechanism has also been detected in molten pools of arc welded composites. However, in this case there was formation of an almost continuous layer of Al4C3, which protects the particle against further consumption, and formation of aciculate aluminium carbide on the top weld. Both are formed by fusion and dissolution of the SiC in molten aluminium followed by reaction and precipitation of the Al4C3 during cooling.

Wetting and spreading of molten aluminium against AlN surfaces

Wetting and spreading of molten aluminium against AlN substrates were investigated between 1100 and 1290°C. The contact angles decreased linearly with time under isothermal conditions between 1100 and 1200°C. The isothermal rate of spreading of molten aluminium against AlN substrates was constant between 1220 and 1290°C and the rate increased exponentially with increasing temperature. Crystals of Al4C3 nucleated and grew on the substrate surface beneath the liquid. However, the formation of Al4C3 may not be solely responsible for the changes in contact angle and spreading. It is postulated that carbon contamination from the substrate and/or experimental equipment coupled with the low oxygen partial pressure of the chamber in the presence of graphite, were primarily responsible for the observed contact angle and spreading phenomena. The activation energy for the spreading process was 448 kJ mol-1, suggesting the presence of some chemical reaction at the interface. Carbon-rich aluminium may be initiating a continuous surface reaction with the AlN substrates by reducing the native oxide layer on the substrate surface.

Interfacial reactions and ohmic contact formation in the Ni/Al–6H SiC system

The interface chemistry and the electrical properties of annealed Ni/Al–6H SiC Ohmic contacts are studied by using x-ray photoemission spectroscopy, current–voltage characteristics, and contact resistance measurements employing a four-point method. All depositions and analysis were performed on the Si face of nitrogen doped, n-type (1×1018 cm−3) wafers of 6H SiC (0001). The chemical reactions at the interface are revealed. Al at the SiC interface acts by reducing the SiOx and forming Al2O3. At elevated temperatures, SiC in the presence of Al dissociates with formation of Al4C3 which is a stable compound. At about 1000 °C the Si atoms at the interface bond to Al and Ni either in ternary compounds with different stoichiometry or in NiSi. In the subsurface region carbon is present in the graphite state of ∼5 at. %. The Ohmic behavior of the contact is related to the formation of the NiSi while the weak contact resistivity degradation is due to the low graphite precipitation. The improved temperature stability may also be due to the presence of Al2O3 on the top of the contact structure.

Metallographic investigation of TiC nucleants in the newly developed Al-Ti-C grain refiner

For grain refinement of aluminium and its alloys, a new aluminium-based master alloy containing TiC nucleants was developed a few years ago, by reacting carbon with Al-Ti binary alloy melt. However, as the melt was held at the usual melting temperatures of below 1273 K, the resultant master alloy lost its grain-refining efficiency, and so this phenomenon was called the “poisoning” effect. Nevertheless, a superheating treatment of the melt at higher temperatures (> 1523 K) rejuvenated the nucleant particles. The present investigation, dealing with electron diffraction of carbide particles extracted from the poisoned master alloy, revealed massive formation of Al4C3 and Ti3AlC, of which Al4C3 appears to account for the poisoning of TiC nucleants. On the other hand, subsequent electron diffraction studies on the rejuvenated nucleants confirmed that they were essentially composed of uncontaminated TiC.

Diamond/Al metal matrix composites formed by the pressureless metal infiltration process

Diamond particles are unique fillers for metal matrix composites because of their extremely high modulus, high thermal conductivity, and low coefficient of thermal expansion. Diamond reinforced aluminum metal matrix composites were prepared using a pressureless metal infiltration process. The diamond particulates are coated with SiC prior to infiltration to prevent the formation of Al4C3, which is a product of the reaction between aluminum and diamond. The measured thermal conductivity of these initial diamond/Al metal matrix composites is as high as 259 W/m-K. The effects of coating thickness on the physical properties of the diamond/Al metal matrix composite, including Young's modulus, 4-point bend strength, coefficient of thermal expansion, and thermal conductivity, are presented.

メカニカルアロイングによるアルミニウム-遷移金属酸化物（V<SUB>2</SUB>O<SUB>5</SUB>, Nb<SUB>2</SUB>O<SUB>5</SUB>, Ta<SUB>2</SUB>O<SUB>5</SUB>）系粉末とそのP/M材の構造と特性

In-situ particulate composites can be manufactured from aluminum-oxide system by mechanical alloying followed by the hot consolidation and heat treatment. Powders of transition metal oxides of standard free energies of formation lower than that of SiO2; V2O5, Nb2O5 and Ta2O5 were mechanically alloyed with pure aluminum powder by a high energy ball mill under an argon atmosphere. P/M materials were fabricated from mechanically alloyed powders by cold pressing, vacuum degassing and hot extrusion. Two kinds of process control agent, methanol and graphite were used for mechanical alloying. The solid state reaction of aluminum-oxide systems was studied by X-ray diffraction. In case of graphite as process control agent, formation of Al4C3 and thermite reaction occur faster than in case of methanol. It is shown by X-ray diffraction that P/M materials consist essentially of aluminum-transition metal alloy matrix with dispersion of Al2O3 after heating at 873 K for 10 h. Upon heating of P/M materials at 873 K, the hardness increased due to resulted formation of transition metal aluminides.

Preparation of ZrC/Al2O3 Fine Composite Powders by Thermite Reaction.

Preparation conditions and formation process of ZrC/Al2O3 composite powders from the ZrO2-Al-carbon black or ZrO2-CH4 system were examined. The ZrC/Al2O3 fine composite powders were prepared from the former system by heating the powder mixture of ZrO2:Al:C molar ratio=3:4:3 at 950°-1000°C for 1h. The formation process of the ZrC/Al2O3 Composite powders was composed of three main reactions: (i) the reduction of ZrO2 With Al and the formation of Al2O3, (ii) the formation of Al3Zr With Zr and Al, and (iii) the decomposition of Al3Zr with carbon black and the formation of ZrC. The process (i) was interfered by the process (ii). Al4C3 was not formed by the process (m). On the other hand, in case of the ZrO2-Al-CH4 system, preparation of ZrC/Al2O3 composite powder was difficult because of Al2C3 and ZrC were formed by reaction between Al3Zr and CH4. For the preparation of the ZrC/Al2O3 composite powder, it was most important to suppress the formation of Al4C3 by controlling the reactivity between Al and carbon source.

Discussion of “On the free energy of formation of TiC and Al4C3”

Several years ago, Banerji and Reif[1] reported some very interesting studies on a process to react Ti and C in molten Al to form particles of TiC. The process was used to prepare a master alloy with a fine dispersion of TiC to inoculate Al for grain refinement. Approximately 2 wt pct of preheated graphite particles were stirred into the Al-5 to 10 pct Ti melts. The authors explained that the melts needed to be superheated above 1000 °C to avoid the undesirable formation of A14C3 and Ti3AlC at the TiC/melt interface. Their explanation for this phenomenon was based on thermodynamics. They observed that the standard free energy of formation curves for AI4C3 and TiC cross near 1175 °C, with A14C3 having the lower free energy of formation below this temperature. There are several aspects of this work which merit further discussion.