Plasma–Chemical Low-Temperature Reduction of Aluminum with Methane Activated in Microwave Plasma Discharge

Abstract

High-purity aluminum is widely used in metallurgy, microelectronics and chemical synthesis. In this work, the method of carbothermic reduction of aluminum powder in a microwave plasma discharge with the formation of valuable organic products such as synthesis gas, acetylene and benzene was used. Al powder was studied by inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The yield of by-products was studied by gas chromatography equipped with a mass spectrometer, as well as optical emission spectroscopy of plasma discharge. High-purity aluminum powder reduced with the plasma was used to synthesize oxygen-free trimethylaluminum (TMA). For the first time, TMA was synthesized in one vacuum cycle without the system depressurizing to improve the purity of the final product. Trimethylaluminum was analyzed by gas chromatography, which confirmed that the main substance is ≥99.99% pure. Gas chromatography with a mass spectrometer was used to determine by-products and residual reaction products. Additionally, ICP-MS was used to confirm trace metal concentrations, achieving the 7N standard for ultra-high-purity materials.

1. Introduction

Powder metallurgy is currently an important area of materials science, which has advantages such as a large specific surface area, high surface activity, increased rate of chemical reaction and increased rate of dissolution of the substance. Due to these advantages, powder metallurgy has found wide application in microelectronics, chemical industry, pharmaceuticals, etc. [1,2,3]. In recent years, materials with improved electrical and magnetic properties are often required, which leads to increased requirements for precursors, which should be metal powders of high purity and homogeneity.

Metal powder material (including powders of high-purity elements and coatings composed of a high-purity element) plays an important role in modern industry and is widely used in 3D printing, 5G communication, electromagnetic shielding, aviation, navigation and electronics [4] and synthesis of metal–organic compounds. The main processes for producing high-quality metal powders include chemical vapor deposition, hydrogen flow thermal reduction deposition, electrochemical deposition [5] and carbothermic reduction [6,7,8,9,10,11,12,13]. Each process has unique advantages and disadvantages. Chemical vapor deposition is a well-known and a widely used process that requires simple equipment, but the coating often contains phosphorus, boron, carbon and other impurities, which leads to deterioration of the coating quality. Thermal reduction deposition methods in chemical metallurgy are useful for producing metal nanoparticles from solid hydroxide compounds in a hydrogen flow. This method allows materials of high purity to be obtained and enables control of the particle size [9]. Thermal reduction by precipitation is a simple process, but the resulting powder is often porous. Electrochemical deposition uses clean electrical energy as a reducing agent, which is environmentally friendly and safe. Fluidized bed galvanic coating or rolling coating is often required to achieve powder conductivity. Therefore, electrochemical deposition imposes higher performance requirements. Metal powders and composite powders can be produced using reducing gases, especially for Al. Plasma–chemical reduction of metal oxides and the processes occurring during this reduction have been widely studied previously. Earlier attempts were made to reduce TiO₂ in thermal hydrogen plasma [14]. It was shown that TiO₂ (60% Ti by weight) was reduced using thermal plasma of a 50% H₂–50% Ar mixture. A product with a Ti content of approximately 70% was obtained, while when iron oxide Fe₂O₃ was treated under the same conditions, complete reduction to metallic iron was achieved. The use of thermal plasma is a potential method for the production of Ti–O ceramics with a titanium content of 67% to 73%. The authors of the work [15] carried out experiments on the reduction of magnesium oxide to magnesium using thermal plasma treatment with methane as a reducing gas. The maximum yield of about 61% Mg was obtained using a power of 20.0 kW and a molar ratio of MgO:CH₄ = 1:1. There were also works [16] on the reduction of aluminum in thermal argon plasma in the presence of quenching gases CO, H₂ and methane. Only in the presence of methane were phases of metallic aluminum, carbide and aluminum oxide detected. The formation of aluminum carbide occurs at high temperatures [17]. The gas temperature of thermal plasma easily reaches values of 2000–6000 °C [14]. In our work, non-thermal nonequilibrium plasma at reduced pressure was used to reduce aluminum oxide. The advantage of our approach is that the gas temperature does not exceed 200–300 °C, which excludes the formation of aluminum carbide, since this reaction requires a temperature of about 1800 °C, which is easily achieved when using thermal plasma. In the MW discharge of low-pressure plasma, according to optical emission spectroscopy data, there are chemically active particles (C₂, CH, H and H₂), with the participation of which the processes of aluminum oxide reduction can occur. Some of the works are devoted to the activation of methane in the presence of oxide catalysts. It has been previously shown that the reduction of NiO with CH₄ takes place in a double dielectric barrier discharge (DBD) plasma. The resulting catalyst containing metallic nickel and carbon promoted the decomposition of methane at 330 °C to form hydrogen and carbon nanofibers [18]. In the work [19], the electro-catalytic oxidation of methane on a NiO-Cu electrode obtained by treatment in cold oxygen plasma was investigated. In the work [20], the production of hydrogen from methane in plasma discharge was investigated. It was shown that the combination of DBD plasma and a NiO/γ-Al₂O₃-based catalyst showed an obvious synergistic effect with an improved CH₄ conversion and H₂ yield, while also improving the catalytic activity of the catalyst at low temperatures and reducing the formation of liquid by-products. Methane plasma reduction of aluminum does not require the use of pressure vessels and more expensive equipment, which is also dangerous to operate. Given the pressures facing the global environment, this method is used when energy savings and emission minimization are needed, since the by-products are also valuable—synthesis gas, acetylene, benzene and toluene—and can be used in subsequent stages of production [21,22]. High-purity aluminum powder is one of the precursors for the synthesis of trimethylaluminum (TMA). One of the problems in obtaining high-purity TMA (more than 99.95% of the main substance content, 99.9999% by trace metal analysis) is the presence of the native Al₂O₃ layer on the surface of the aluminum powder. Trimethylaluminum readily reacts with aluminum oxide [23] forming oxidized forms such as dimethylaluminum methoxide and methylaluminum dimethoxide [24]. Trimethylaluminum (TMA) is used as a precursor in the production of LEDs, solar cells, phase-change memory, semiconductor lasers, infrared detectors, supercomputers and as a catalyst in the chemical and pharmaceutical industries. TMA of high purity is critical for the production of compound semiconductors such as aluminum gallium arsenide (AlGaAs) and aluminum indium gallium phosphide (AlGaInP), which are used in electronic devices, LEDs, lasers and power amplifiers, as well as Al₂O₃ insulating films for GaAs and GaN composite semiconductors. Atomic layer deposition (ALD) is widely used in the production of semiconductor components. An efficient and affordable precursor is a prerequisite for the success of ALD. Despite the fact that recently there have been works devoted to the creation of new aluminum-containing precursors for ALD and plasma-enhanced ALD (PEALD), trimethylaluminum remains the most popular due to the proven technology of its use in the production of microelectronics [25]. One of the problems in the synthesis of TMA is the existence of an oxide film on the surface of Al powder grains that significantly reduces its reactivity and conversion and causes degradation of the final product quality (reduction in the content of the main substance). The main impurity in TMA has previously been identified as dimethylaluminum methoxide and methylaluminum dimethoxide [24]. These two species are different in volatility, but not enough to allow complete separation by careful fractional distillation. Previously, a quantitative study revealed levels of these impurities with secondary ion mass spectrometry (SIMS) results on a sandwich structure of thick alternate layers of GaAs/AlAs, and photoluminescence (PL) results on GaAs/AlGaAs quantum wells. It has been shown that the oxygen content in trimethylaluminum is associated with the formation of oxygen defects in AlGaAs and GaAs. A decrease in the concentration and an increase in the mobility of charge carriers can be caused by the introduction of oxygen during crystal growth during vapor-phase epitaxy. The presence of oxygen causes a change in the surface morphology and chemical composition, which significantly reduces the electrical and optical characteristics of the AlGaAs epitaxial layers [24]. Alkoxy derivatives of organometallic precursors such as trimethylaluminum and trimethylgallium are the most important oxygen source in the deposition of semiconductor layers by metalorganic chemical vapor deposition (MOCVD, MOPVE) and atomic layer deposition ALD methods [26]. In addition, the presence of alkoxy derivatives (dimethylaluminum methoxide) in trimethylaluminum significantly increases the decomposition temperature of trimethylaluminum, which causes variations in the deposition rate and productivity of the MOCVD process [27]. Although this impurity can be removed to some extent using careful fractional distillation to give “low oxygen grade” TMA, a new chemical purification route was found to be necessary to remove the monomethoxide impurity completely and provide the next-generation purity grade material (low-oxygen-content trimethylaluminum and trimethylindium for MOVPE of light-emitting devices). Therefore, the problem of obtaining high-purity aluminum powder creates difficulties in using such a precursor in the microelectronic industry with its ever-increasing purity requirements. In this paper, a method for carbothermic reduction of high-purity aluminum powder in a methane (CH₄) flow using microwave (MW) discharge plasma is proposed. The main purpose of this study was to investigate the carbothermic reduction of aluminum powder for synthesizing high-purity trimethylaluminum (TMA) (achieving ≥ 99.99999% (7N) purity for trace metals and ≥99.99% purity for the main substance by weight). The resulting reduced aluminum was characterized by various physicochemical methods. For the first time, the synthesis of TMA was carried out in a single vacuum cycle using reconstituted aluminum powder in order to increase the yield and purity of the final product. Unlike the traditional approach, the proposed method eliminates the stage of aluminum powder activation and allows for the reduction of the oxygen content in the final product by removing aluminum oxide in the form of volatile gaseous products. The standard method of trimethylaluminum synthesis includes an induction period. From the moment of mixing the reagents (aluminum and methyl chloride) to the first signs of the onset of the reaction, several hours to several tens of hours may pass. Methods of aluminum activation, for example, by adding iodine or trimethylaluminum lead to contamination of the final product with either metallic impurities or oxidized forms such as dimethylaluminum methoxide and methylaluminum dimethoxide. In our method, aluminum powder is processed in methane plasma, freeing itself from surface aluminum oxide, and can be used without additional activation.

2. Materials and Methods

To carry out carbothermic reduction of Al powder (99.995%, “UC RUSAL”, Moscow, Russia), the experimental setup shown in Figure 1 was used. The Al powder was loaded into the reactor in the amount of 100 g. To prevent contamination by equipment materials, the reactor was made of high-purity quartz glass (“QUARZ “FACTORY” Ltd.”, Gus-Khrustalny, Russia).

The reactor consisted of a quartz tube (25 mm diameter) positioned at the center of the working chamber in a SINEO UWAVE 2000 microwave oven. Microwave radiation (2450 MHz) was supplied via the open end of a rectangular waveguide (90 × 45 mm cross-section) integrated into the chamber wall. Aluminum powder was placed in the lower section of the reactor, opposite the waveguide, and lightly compacted by tapping. A plasma-forming gas was introduced through a side inlet, while the upper end of the reactor extended beyond the oven chamber and was sealed with an optical quartz window for plasma emission diagnostics. The system was evacuated through a flange located at the reactor’s upper section.

The pressure in the system was kept constant by an oil-free scroll pump (ACTAN VACSCROLL-7) and was 10 mBar. The flow of plasma-forming gas Ar (Ar 6.0 NII KM, Moscow, Russia, O₂ < 0.2 ppmv) was 40 mL/min, while the flow of CH₄ (6.0, produced by S-Gas, Moscow, Russia, O₂ < 0.3 ppmv) was 60 mL/min. The gas supply system included a reduced pressure reducer, gas flow regulators (Bronkhorst F-201CV) and a system of shut-off valves.

The reactor was equipped with a special quartz viewing window for optical emission spectroscopy (Ocean Optics S2000-TR (UV + VIS + NIR spectrometer in the range of 180–1100 nm)) in order to determine excited particles in the discharge zone.

The plasma discharge was initiated by a microwave system (SINEO UWAVE 2000) with a power of 2000 W, which was kept constant throughout the experiment. At the reactor outlet, an IR Fourier spectrometer with a gas cell (Shimadzu IRTracer-100) was installed to determine volatile products. Gas phase sampling was carried out by condensing the products in a quartz nitrogen trap installed after the IR spectrometer. Further analysis of the sample was carried out with a chromatograph connected to a mass spectrometer (Shimadzu GCMS-QP2020 NX, GC Column 113-4362 Agilent GS-GasPro 60 m * 0.32 mm), equipped with a vacuum inlet device.

The Al powder reduced in the plasma discharge was analyzed by ICP-MS methods (Perkin Elmer Sciex ELAN 9000 ICP-MS), which confirmed the absence of additional contamination by equipment materials. The measurement was performed by dissolving the powder in 3% nitric acid, with a metal impurity content of less than 10 ppb, for which an Anton Paar Multiwave 3000 microwave decomposition system with a 16-position HF-100 rotor was used. A volume of 20 mL of nitric acid solution was added to a sample of aluminum powder weighing 8–12 mg, and the sample was processed in the microwave decomposition system. In parallel with the sample, 5 calibration solutions of the impurities at different concentrations to be determined were prepared, and a calibration graph was plotted.

X-ray diffraction analysis (XRD) was performed on a Shimadzu XRD-7000 diffractometer in the 2θ range from 20 to 70° with a scanning rate of 0.1 °/min. High-resolution SEM was used to qualitatively assess the presence of an oxide film on the surface of the Al powder (JSM IT-300LV (JEOL) scanning electron microscope with an Oxford Instruments X-MaxN 20 energy-dispersive elemental analysis detector in high vacuum and at an accelerating voltage of 20 kV).

The TMA synthesis was performed in the reactor shown in Figure 2. The reactor body was made of AISI 316L stainless steel, equipped with a flange connection to the lid and an inspection window. A liquid cooling jacket allowed precise control of the reactor wall and bottom temperatures during the synthesis.

3. Results and Discussions

3.1. Optical Emission Spectroscopy (OES)

When exposed to an external microwave (MW) field, a breakdown occurs in a gas at reduced pressure, and a nonequilibrium low-temperature plasma is formed. Chemically active components of the plasma discharge are represented by a set of molecular, atomic ions and neutrals in excited states. The lifetime of these particles in the excited state is not large, and most of them, as a result of radiative relaxation, transform into the ground state with the emission of a quantum of light. The energy of these quanta corresponds exactly to the difference in energies of the ground and excited states and uniquely characterizes the excited molecular or atomic fragment that undergoes a radiative transition.

Analysis of the emission spectra of chemically active nonequilibrium plasma is a powerful tool for studying plasma–chemical processes and allows one to determine the presence of chemically active metastable particles and make assumptions about the mechanisms and elementary stages of complex chemical processes occurring in the MW plasma discharge. The emission spectra of methane MW plasma in the range of 200–800 nm at different MW source powers are shown in Figure 3.

The emission of pure methane plasma is represented by a set of lines from atomic and molecular fragments. The spectrum clearly shows the H_α and H_β emission lines of the Balmer series for excited hydrogen atoms (656.27 and 486.127 nm) [28]. There are also broad emission bands from molecular hydrogen with maxima at 406.50, 471.90, 485.65, 487.30, 492.65 nm and a series of bands in the range of 568–624 nm [28]. The emission band with a maximum at 431.25 nm is attributed to the A²Δ → X²Π transition in the binary molecular particle CH [29,30]. In addition, the spectrum contains lines of carbon atomic particles of varying degrees of ionization C (493.20 nm) and C⁺ (391.90, 430.75, 486.26 and 512.70 nm). In addition, bands at 473.7 and 516.0 nm were detected in the spectrum, which, according to [30], belong to the diatomic neutral particle C₂.

Analysis of the emission spectra from chemically active methane plasma enables identification of elementary reactions in the discharge, including those involving metastable particles. Initially, the methane molecule is excited by the electron impact mechanism with the formation of a metastable methane molecule (Equation (1)), which, as a result of radical decay, is deactivated with the formation of radical fragments (Equation (2)):

CH₄ + e⁻ → CH₄*,

(1)

CH₄* → ·CH₃ + ·H.

(2)

Since MW energy is continuously supplied to the plasma-forming system, the generated radical fragments also undergo a cascade of transformations in the plasma discharge via the electron impact mechanism (Equations (3)–(5)) with the formation of a set of radical fragments, the emission lines of which we observe in the spectrum.

·CH₃ → :CH₂ + H,

(3)

:CH₂ → ·CH + H,

(4)

·H + ·H → H₂.

(5)

With an increase in the MW power input to the plasma discharge from 500 to 1000 W, an increase in the intensity of the emission lines in the spectrum from hydrogen atoms and molecules is observed. In addition, the relative intensity of the line and bands from C⁺ (391.9 nm) and C₂ (473.7 nm) particles increases significantly (Equations (6)–(9)).

C + e⁻ → (C⁺) + 2 e⁻,

(6)

2 ·CH → C₂H₂,

(7)

·H + C₂H₂ → ·C₂H + H₂,

(8)

·C₂H + ·H → C₂ + H₂.

(9)

The result of this cascade of transformations is the accumulation of atomic and molecular hydrogen, as well as excited carbon-containing fragments, in the plasma discharge. Atomic hydrogen, as well as highly reactive particles C, C⁺ and C₂, have strong electron-acceptor properties. It can be assumed that due to the presence of these particles in the discharge, the chemically active methane plasma will have strong reducing properties. To test this assumption, we performed experiments to reveal the effect of nonequilibrium methane plasma at low pressure on aluminum powder containing an aluminum oxide film on the surface of the particles. The emission spectra of methane plasma in the presence of oxidized aluminum in the range of 200–800 nm are shown in Figure 4.

When exposed to chemically active methane plasma at a generator power of 1000 W, in the discharge emission spectrum, in addition to the emission bands caused by metastable products of methane plasmolysis, there are lines of aluminum atoms (302.6, 308.3, 309.3 and 313.6 nm), as well as a low-intensity line of atomic oxygen at 777.2 nm. It should be noted that the intensity of the lines from excited carbon-containing fragments (C, C⁺ and CH) decreases in the presence of aluminum, which is possibly due to the consumption of these plasma components for the restoration of the oxide film on the aluminum surface. The appearance of the atomic oxygen line at a wavelength of 777.2 nm confirms its removal from the Al surface during methane plasma treatment. As we will see later, in the presence of aluminum, we observe an increase in the concentration of carbon monoxide and dioxide in the exhaust gases at the reactor outlet. The process of carbothermic reduction of the oxide film on the surface of aluminum in a plasma discharge can be described by the summary Equation (10) [9].

Al₂O₃ + 3 CH₄ → 2 Al + 3 CO + 6 H₂.

(10)

For this multi-stage thermal process, ΔH° = 1568.7 kJ/mol. The presence of plasma significantly reduces the apparent activation energy of chemical transformations by exciting molecules and facilitating the formation of highly reactive transition states. Unlike conventional chemical processes, plasma–chemical reactions occur in thermodynamically open systems, requiring external energy sources to sustain the plasma. A portion of this energy is utilized to initiate chemical reactions. The energy from the external source is first transferred to the plasma electrons, which then distribute it to heavier particles through several pathways: heating, excitation of internal molecular or atomic degrees of freedom, ionization and dissociation. This energy redistribution among heavy particles ultimately drives the creation of chemically active species.

For a better understanding of the mechanisms and elementary processes occurring in the methane plasma discharge in the presence of aluminum, we have carried out detailed studies of the solid phase (aluminum) and gaseous products at the outlet of the plasma–chemical reactor, as well as the synthesis of trimethylaluminum from plasma-treated aluminum. The results of these studies will be discussed in detail in the following sections.

3.2. X-Ray Diffraction (XRD) Study of Al Powder

To test the efficiency of the oxide film reduction on the surface of aluminum particles, we used the X-ray diffraction method. Figure 5 shows the X-ray diffraction patterns of the Al powder used in the experiment before and after the carbothermic reduction process in plasma, respectively.

As can be seen from the XRD patterns (Figure 5, bottom), the reflection at 65.3° (214) corresponds to the corundum phase of Al₂O₃ [96-900-7636], which indicates the presence of a high oxygen content on the surface of the original powder. In Figure 5 (top), this reflection is absent, which proves a decrease in the oxygen content, or its absence in the reduced Al powder within the detection limits of the method.

3.3. Scanning Electron Microscopy (SEM EDX) Study of Al

The samples were pre-dispensed in hexane using an ultrasonic bath, followed by attaching them to the sample holder with conductive carbon tape. Figure 6 shows the SEM images of the Al surface before (a) and after (b) the carbothermic reduction process, respectively. Untreated aluminum powder has a layer of oxide that completely covers the particles, explaining its low reactivity. In contrast, activated aluminum powder SEM images show disrupted oxide layers and exposed metallic aluminum.

Based on the SEM images, a change in the powder morphology is observed, which is consistent with the XRD results and may be associated with the removal of the Al₂O₃ oxide phase from the aluminum surface. The results of the EDX study of the powder composition are presented in

Table 1. EDX study of Al powder before and after carbothermic reduction in methane plasma.

Element	Before Plasma Treatment, at.%	After Plasma Treatment, at.%
Al	97	100
O	3	-

.

Based on the EDX results, it can be concluded that the oxygen content in the aluminum powder reduced by the carbothermal method is below the detection limit of the method (0.037% for oxygen) [31,32].

3.4. ICP-MS Study of the Behavior of Impurities in Aluminum During Plasma–Chemical Reduction

An important advantage of the plasma–chemical method of reducing aluminum from its oxide, in addition to the simplicity of the equipment, is the relatively low temperature of the process. In a low-pressure plasma discharge, the temperature of the reactor walls does not exceed 250 °C. It can be expected that under plasma discharge conditions, contamination of the aluminum powder being processed with impurities from the reactor material does not occur. Nevertheless, to test this assumption, we measured the content of metallic impurities and silicon in aluminum samples before and after plasma–chemical reduction. The measurements were carried out by means of a Perkin Elmer Elan 9000 ICP-MS spectrometer using the quantitative method of measuring additives, with an integration time of 10 s, using scandium internal standard.

Table 2. ICP-MS analysis of impurity composition of initial and plasma-reduced Al.

Element	Units	Before Plasma Treatment	After Plasma Treatment
Fe	ppm	8.06	8.04
Si	ppm	15.74	16.05
Cu	ppm	5.54	5.51
Mg	ppm	15.09	15.07
Ca	ppm	11.63	13.05

shows the chemical composition of the original and reduced Al.

The Si content in the final product remained at the initial level, which confirms the absence of contamination from the equipment material. According to the analysis of metal traces, the Al powder can be characterized as being of the 4.5N grade.

3.5. Study of Gaseous Products of the Plasma–Chemical Reduction Process

The gas phase was studied by directly sampling the reaction products into a cooled quartz trap for subsequent introduction into a gas chromatography mass spectrometer (GC-MS) via a special vacuum inlet (Figure 7). The main component of the carbothermic reduction process was CO, and peaks of acetylene, benzene, toluene and other hydrocarbons were also present, which is explained by the process of methane plasmolysis in the MW plasma discharge [29].

We dissolved the organic products of plasma–chemical conversion remaining in the trap after removal of the gas fraction in iso-octane. The reaction products were analyzed by gas chromatography with a mass spectrometric detector on a GC2010-Plus 9 Shimadzu gas chromatograph/mass spectrometer, Japan. An RTX-5MS column with a diameter of 0.25 mm, a length of 30 m, and a stationary phase thickness (5% diphenyl-, 95% dimethylpolysiloxane) of 0.25 μm was used to separate the components under the following conditions: a carrier gas (helium) flow of 30 cm/s and a temperature program of 40 °C for 10 min, an increase of 5 °C/min to 200 °C and remaining at 200 °C for 20 min. The spectrum was recorded in the SCAN mode in the m/z range from 10 to 400; the remaining detection parameters were in accordance with the autotuning results. A volume of 1 μL of the sample was injected at an injector temperature of 250 °C with a flow division factor of 1/10. Since some components of the analyte elute earlier than the main component (solvent), its peak was cut by a programmed switching off of the filament (cathode) of the mass detector ionizer, having previously determined the time of solvent exit with a large flow division factor (Figure 8).

According to the GC-MS analysis, in addition to the main peak of benzene, peaks of by-products (toluene, ethylbenzene, o-xylene, indan and indene) were also detected.

3.6. Synthesis of Trimethylaluminum Using Methane Plasma-Processed Aluminum

Aluminum powder activated in a methane plasma discharge was used to synthesize trimethylaluminum (TMA). At first, an ampoule with aluminum powder was sealed from the setup and placed into a 316L stainless-steel reactor (Figure 2). The reactor was purged with high-purity argon (6N), after which the ampoule with aluminum was destroyed in an inert atmosphere using a stainless-steel striker. Then, the reactor was evacuated with a vacuum pump to a residual pressure of 3 × 10⁻⁴ mBar. To remove adsorbed gases, the aluminum powder was heated in a vacuum at a temperature of 200 °C for 30 min. After cooling to room temperature, the reactor was filled with methyl chloride to an excess pressure of 4 Bar. The exothermic reaction between aluminum and methyl chloride began immediately and was accompanied by intense spontaneous heating of the reaction mass and the reactor walls (Equation (11)) [33,34].

2 Al + 3 CH₃Cl → (CH₃)₃Al₂Cl₃.

(11)

The temperature of the reaction mixture was maintained at no more than 40 °C; for this purpose, the excess heat of the exothermic reaction was removed using a liquid thermostat. After 6 h, the temperature of the reaction mixture stopped increasing. In the reaction, 680 g of methyl chloride reacted with 270 g of aluminum, representing 90% of the stoichiometrically required aluminum mass.

The reaction mixture was an opaque gray liquid. By distillation in a vacuum, 765 g of a colorless transparent liquid which may be either methylaluminum sesquichloride or dimethylaluminum chloride was isolated from the reaction mixture. The amount of methyl substituents in the product was determined by a volumetric method by measuring the volume of methane released during hydrolysis. For this purpose, a 0.510 g sample of the product was collected into a thermostat cell of the original design, where decomposition was carried out by adding a fivefold excess of isopropyl alcohol to the sample. In this case, a sharp increase in pressure and temperature was observed inside the cell due to the reactions (Equations (12) and (13)) [35,36]:

(Me)₃Al₂Cl₃ + 3 i-PrOH → iPrOAlCl₂ + (i-PrO)₂AlCl + 3 CH₄,

(12)

(Me)₂AlCl + 3 i-PrOH → (i-PrO)₂AlCl + 2 CH₄.

(13)

After cooling the cell to 20 °C, the pressure was measured. The difference in pressure values inside the cell before and after sample decomposition was used to calculate the content of methyl groups per atom of aluminum and, accordingly, the composition of the distillate. According to volumetric analysis, the product was methylaluminum sesquichloride.

To synthesize high-purity trimethylaluminum from methylaluminum sesquichloride, a dehalogenation reaction with an alkali metal was carried out. According to the classical Grosse–Mavity method [34], dehalogenation is carried out in a hydrocarbon solution at a temperature of 150–170 °C in the presence of an excess of a mixture of sodium and potassium. This method has several disadvantages. Firstly, the need to use high-boiling hydrocarbons as solvents is accompanied by an increase in the costs of additional purification from impurities, moisture removal and regeneration. In addition, an increase in the volume of the reaction mixture requires an increase in the weight and size parameters of the equipment used and additional energy costs for heating, cooling and distillation of the product from the reaction mixture. The use and storage of large volumes of flammable hydrocarbons at the production site increases the overall risks of the dangerous production of trimethylaluminum and significantly complicates the elimination of the consequences in the event of an emergency. Secondly, the necessity of using a mixture of alkali metals instead of pure sodium also significantly complicates the production operations and increases the cost of trimethylaluminum. In addition, the use of potassium introduces additional impurities into trimethylaluminum and requires additional costs for its purification.

To obtain high-purity trimethylaluminum in one step, we used a modified method of dehalogenation of methylaluminum sesquichloride with high-purity sodium in an autoclave without using any solvents. The obtained methylaluminum sesquichloride was transferred to an autoclave cooled down to a temperature of −10 °C, into which 325 g of sodium of 4N purity grade was pre-loaded. After heating to room temperature, 6N argon was added to the autoclave to a pressure of 1 Bar. The autoclave was placed in a resistive heating furnace, and its temperature was slowly increased up to 280 °C and maintained at this temperature for 1 h. High-purity trimethylaluminum was separated from the reaction mixture by vacuum distillation.

3.7. Gas Chromatography Mass Spectrometry of Trimethylaluminum

Trimethylaluminum samples obtained from untreated aluminum powder containing an oxide film and aluminum powder treated with methane plasma were tested for oxygen-containing impurities by GC-MS. For this purpose, TMA samples from the initial Al powder and from the one reduced in plasma were collected in a glove box with an inert Ar atmosphere for further introduction into GC-MS using a gas-tight chromatographic syringe. In total, two peaks were identified in TMA. The main fraction consisted of the TMA peak and a barely noticeable peak of hexane, heptane and octane isomers together with an aromatic fraction (toluene), which were also detected in the gaseous products of plasma–chemical synthesis and were probably adsorbed on the surface of aluminum powder during the treatment. The presence of dimethylaluminum methoxide was detected in TMA obtained from unreduced Al powder by GC-MS.

3.8. ICP-MS of Trimethylaluminum

The study of impurities of residual metals in trimethylaluminum was carried out by the ICP-MS method. TMA collected in a glove box with an inert atmosphere was hydrolyzed in 3% nitric acid solution, with a quantity of metallic impurities of less than 10 ppb. For this purpose, 20 mL of a nitric acid solution were poured into test tubes with a selected TMA sample by weight of 40–60 mg. The resulting solutions were analyzed using the quantitative addition method, with an integration time of 10 s, using an internal standard of indium. Calibration graphs were plotted for five points, which correspond to five calibration solutions with additives of different concentrations of the impurities under study. The content of metallic impurities in trimethylaluminum synthesized from carbothermic reduced Al in microwave discharge plasma is presented in

Table 3. Metallic impurity content in TMA according to ICP-MS data.

Substance	Unit of Measurement	Concentration
Trimethylaluminum (TMA), content of the main substance by weight	%	≥99.99
Ba	ppm	≤0.02
Ca	ppm	≤0.05
Cd	ppm	≤0.03
Cr	ppm	≤0.01
Cu	ppm	≤0.03
Fe	ppm	≤0.04
Mg	ppm	≤0,04
Mn	ppm	≤0,04
Ni	ppm	≤0,03
Pb	ppm	≤0.02
Si	ppm	≤0.02

.

Based on the ICP-MS analysis results, the obtained TMA can be characterized as 7N (99.99999%) by trace metal analysis.

4. Conclusions

Carbothermic reduction in a microwave (MW) methane plasma discharge effectively removed the oxide film from the aluminum powder surface. The presence of the oxide film reduces the TMA yield and introduces additional contamination that prevents further use of the precursor in the thin film growth in microelectronics. The obtained material was characterized by various analytical methods. The XRD, SEM and EDX methods confirmed the presence of the Al₂O₃ oxide phase on the surface of the Al powder and its absence after the carbothermic reduction process. High-purity trimethylaluminum with a low oxygen content was obtained by the interaction of plasma-treated aluminum with methyl chloride.

After 20 min of processing aluminum powder with microwave plasma at 500 W in the presence of methane, the energy-dispersive X-ray analysis did not detect any oxide on the surface of the treated powder. This is also indicated by the absence of an induction period in the reaction with chloromethane, which is a positive finding. The low processing temperature also helps to avoid contamination of the aluminum with impurities from the equipment materials, which is another advantage of this method.

A potential limitation of using this method in the aluminum industry is the production of primary raw materials. This could be due to the size of the reactor and difficulties in controlling plasma discharge when scaling up the plant to produce large amounts of Al. In the laboratory, when obtaining high-purity Al, these factors were minimized, and the resulting product was analyzed using various methods to confirm its high quality and purity.

The purity of trimethylaluminum was verified by gas chromatography (GC), confirming a primary substance purity of ≥99.99%. By-products and residual reaction products were identified using gas chromatography mass spectrometry (GC-MS). Inductively coupled plasma mass spectrometry (ICP-MS) was further utilized to confirm trace metal purity levels, achieving the 7N standard for ultra-high-purity materials. The obtained material, in terms of the content of metal impurities and oxygen, is suitable for use in the MOCVD and ALD processes of semiconductor and insulating coatings.