Plasma-chemical synthesis of highly dispersed core–shell structures from a mechanical mixture of titanium carbide and titanium nickelide

Introduction

At present, the nanocrystalline state of matter is extensively investigated [1–5] as it has a number of unique physicochemical and physicomechanical properties determined by the high dispersion of particles. For example, the most productive methods for the formation of nanocrystalline materials include plasma-chemical synthesis in a low-temperature gas plasma [6]. From the standpoint of fundamental research [7], “quasi-equilibrium” processes occurring during plasma-chemical synthesis in a low-temperature gas plasma enable one to use the laws of equilibrium thermodynamics to calculate the final state of the reacting system.

The formation of core–shell structures of a given composition during synthesis of ultra- and nanodisperse materials based on refractory compounds of IV–VIA subgroups of the periodic table, with the participation of metals such as Ni and Co, makes it possible to synthesize composite powder products suitable for direct use. One of the technological examples is the use of nanomaterials, obtained during plasma-chemical synthesis in a low-temperature nitrogen plasma and based on refractory compounds of titanium, vanadium, zirconium and other elements of IV–VIA subgroups, as modifiers for casting steels and non-ferrous alloys, as described in [8–10]. During extra-furnace steel processing, nanocrystalline materials are placed into a ladle using different methods and are rather evenly distributed throughout the molten steel or non-ferrous alloy, acting as artificial nuclei during crystallization. The metal components of composite nanocrystalline particles, therefore, serve as a buffer layer between the melt and the refractory core, protecting the latter from early solid-phase dissolution. The microquantities of such modifiers improve the physical and mechanical properties of cast materials while maintaining their specified chemical composition.

On the other hand, refractory compounds based on elements of IV–VIA subgroups of the periodic table, with high hardness values, are used as the basis for tool materials [11]. The binding phases are metals and their intermetallic compounds, which allow metal ceramic compositions to be formed, where the matrix in the form of grains of refractory compounds is impregnated with a metal melt during high-temperature sintering in vacuum with the participation of a liquid phase. The patterns of such processes for various powder compositions based on titanium carbonitride TiC_0.5N_0.5 were earlier described in [12–16].

The main objective of this paper is to study the patterns for the formation of ultradisperse and nanocrystalline particles with a core–shell structure during plasma-chemical synthesis of a mechanical mixture of TiC and TiNi (1:1) in a low-temperature nitrogen plasma.

Methods

Microcrystalline powders of titanium carbide (50 μm) and titanium nickelide (40 μm) were used as the initial components of the charge for plasma-chemical synthesis. The industrial plasma-chemical plant described in[6] was used for plasma-chemical synthesis. The plant productivity can reach 1 t/h, which confirms a quite reasonable cost of this technology.

The capacity of the plasma chemical plant (FSUE SRIOCCT, Saratov) was 25 kW, voltage – 200–220 V, current – 100–110 A, plasma flow speed – 55 m/s, gaseous nitrogen flow rate in the plasma reactor – 25–30 m³/h (of which plasma formation accounted for 6 m³/h and stabilization and hardening – 19–24 m³/h). The initial mechanical mixture consumption was 200 g/h.

The pneumatic transport transferred the processed ultrafine and nanocrystalline powder into a vortex-type cyclone and a bag-type fabric filter for separation. Nitrogen was used as a transport gas. After cooling, air was slowly introduced into the separation units to form a thin passivating oxide film. At the next passivation stage, the materials were encapsulated in a specialized unit of a plasma-chemical plant (capsulator), which ensures long-term storage of highly dispersed materials under normal conditions. The technique of plasma-chemical synthesis in a low-temperature nitrogen plasma based on the plasma recondensation scheme is described in more detail in [6].

The core–shell structures processed in the form of ultrafine and nanocrystalline powders were studied by X-ray diffraction (SHIMADZU XRD 7000 X-ray diffractometer, CuK_α-cathode, Japan) and high-resolution transmission electron microscopy (HR TEM) (JEOL JEM 2100 transmission electron microscope, Japan). The X-ray investigation results were processed using the WinXPOW software (ICDD database) to determine the phase composition of the resulting core–shell structures. The crystallographic parameters of the phase components were specified in the PowderCell 2.3 software package using the ICSD file located on the Springer Materials e-platform. The electron microscope images were processed to measure particle sizes in Measurer software and then in standard mathematical editors to construct distribution histograms and determine the average particle size. High-resolution images were processed in the DigitalMicrograph 7.0 software. The results of interplanar spacing measurements were compared with the ICDD database file to specify the phase composition and determine the local states of additionally detected phases.

The density of the final synthesis products was assessed using a helium pycnometer (AccuPyc II 1340 V1.09, Micromeritics, USA). The specific surface area was measured on a specific surface area analyzer (Gemini VII 2390 V1.03 (V1.03 t), USA) using the BET method. The average particle size was determined for each of the processed fractions based on the values of density and specific surface area [17].

Results and discussion

The results of X-ray investigations of fractions of core-shell structures obtained during plasma-chemical synthesis in a low-temperature nitrogen plasma of a mechanical mixture of TiC and TiNi are presented in Fig. 1 and in the table. The refractory phase in both fractions of dispersed materials is represented by cubic compounds.

While specifying the unit cell parameters, we revealed that during crystallization in a quenching chamber at a rate of 10⁵ °C/s, oxycarbide and carbonitride phases of various compositions are formed in each fraction, as indicated in the table. The carbonitride composition is formed with a predominant amount of nitrogen in the non-metallic sublattice.

According to X-ray diffraction results, the cubic Ni phase (Fm-3m space group) is observed only in the fabric filter fraction, where its amount is determined to be 5 wt. % (see the Table). At the same time, the X-ray diffraction pattern of the powder composition from the filter contains complex titanium-nickel nitride Ti_0.7Ni_0.3N in the amount of 5 wt. %. According to the X-ray diffraction patterns (see Fig. 1), titanium-nickel nitride Ti_0.7Ni_0.3N, the visualization of which was presented in [18], is in a highly deformed state causing changes in intensities [19]. The resulting preferential orientation of the crystal lattice, in accordance with [20], can be ensured by the high rate of crystallization of the obtained powders. The issues related to formation and identification of Ti_0.7Ni_0.3N using the example of the TiN–Ni core–shell structure, studied within the framework of high-resolution transmission electron microscopy, are discussed in [21]. The rutile modification of TiO₂ is formed in the course of forced acidification by slow inflow of air into classifiers 1 and 2, its share being 2 wt. %.

The measurements of pycnometric density and specific surface area by the BET method, presented in the table, revealed that the core–shell structures formed differ in density values. This effect can be attributed to the quantitative content of the processed compositions.

Visualization of the core–shell structure using the example of a nanocrystalline fraction from classifier 2 – a bag-type fabric filter – was confirmed by high-resolution transmission electron microscopy (Fig. 2).

Figure 2, a, b shows the general picture, which demonstrates that the fraction from the filter is really nanodispersed, since the average particle size based on the results of 767 measurements is 18.9 ± 0.2 nm; the histogram of particle size distribution is shown in Fig. 2, c.

The presence of a core–shell structure is determined by the presence of high-contrast areas at the periphery of the grains, and the grains themselves have both a round and faceted shape, as shown in Fig. 3.

Figure 3 shows the results of fixation of metallic nickel (plot 1) and titanium carbide TiC (plot 2). According to the results of measurements of interplanar spacing in plot 1, cubic metallic Ni (Fm-3m space group) is characterized by interplanar spacings d₂₀₀ = 0.1797 nm, d₁₁₁ = 0.2054 nm, and d_–111 = 0.2087 nm. In plot 2, the identified planes belong to TiC (Fm-3m space group), d₁₁₁ = 0.2533 nm.

The hexagonal Ni is present in the form of a (002) plane with interplanar spacing d₀₀₂ = 0.2189 nm in the section of the electron microscope image shown in Fig. 4.

The presence of titanium-nickel nitride Ti_0.7Ni_0.3N of the hexagonal modification (P-6m2 space group, d₁₀₀ = 0.2543 nm) along with hexagonal Ni (space group P6₃/mmc, d₁₀₀ = 0.2250 nm) and cubic TiC (Fm-3m space group, d₁₁₁ = 0.2568 nm) is shown in Fig. 5.

Based on the TEM studies, Fig. 6 shows an example of the presence of faceted particles of cubic titanium carbide TiC – the composition of the presented faceted particle is interpreted by the (200) TiC plane (Fm-3m space group), d₂₀₀ = 0.2150 nm.

Summarizing the findings of X-ray diffraction and high-resolution transmission electron microscopy, we can formulate a chemical model of the core–shell structures formed during plasma-chemical synthesis in a low-temperature nitrogen plasma followed by crystallization in an intensely swirling nitrogen flow (Fig. 7). The model is based on the physicochemical properties of all interpreted phase components, which include boiling, condensation and crystallization temperatures [22; 23], as well as functional dependences ΔG(t) under equilibrium conditions [24]. Additionally, information on the wettability of refractory compounds [22] by molten nickel metal was utilized to justify the metallic shell on the periphery of nanocrystalline refractory particles.

Within the framework of this model, the flow of low-temperature plasma with a mechanical mixture of TiC and TiNi is separated by temperature barriers as it enters the quenching chamber filled with nitrogen. The boiling or crystallization temperatures of all phase components determined by X-ray diffraction are selected as temperature barriers.

Keeping in mind that low-temperature plasma can exist only in the temperature range of 4000–6000 °C, the first temperature barrier responsible for the crystallization of the refractory components of the emerging core–shell structures can be identified as the transition of titanium carbide from a gaseous state to a solid state described in [25–28], its temperature is 4300 °C [23]. Considering the significant excess of gaseous nitrogen in the entire volume of the quenching chamber, it can be stated that under these conditions it interacts with solid-phase TiC with the subsequent formation of carbonitride TiC_xN_z (see the Table). The functional dependences ΔG(t) for these processes [24] and the data on phase formation in the Ti–C–N system [29] confirm that mutual solid solutions with wide homogeneity regions can be formed. At the same time, during the crystallization of refractory components in the quenching chamber, TiN_y titanium nitride nanoparticles, isomorphic to Ti–C–N compounds, can be formed on the surface.

Separately, it should be mentioned that passing the temperature range of 4300–3930 °C nickel remains in a gaseous state up to the temperature of 2730 °C [30], at which it passes from a gaseous to a liquid state and which is the second temperature barrier. Passing the boiling point, liquid nickel actively interacts with refractory grains. Under these conditions, titanium-nickel nitride Ti_0.7Ni_0.3N is formed [21] according to the theory of heterogeneous nucleation by B. Chalmers [31], some provisions of which are given in [8] by the reaction equation

It should be noted that the complex nitride Ti_0.7Ni_0.3N and its analogue Ti_0.7Co_0.3N were earlier detected by X-ray diffraction and transmission electron microscopy and described in [21; 32–34].

Reaction (1) occurs in the temperature range from 1600 °C [18] to 1455 °C, which corresponds to the crystallization temperature of metallic nickel and constitutes the third temperature barrier in the presented model. As nickel crystallizes, no chemical interactions occur in the formed core–shell structures, and all the resulting compositions can only cool down at this stage. Next, the mixture of processed fractions is transported to classifiers 1 and 2 for separation.

Conclusion

As a result of plasma-chemical synthesis in a low-temperature nitrogen plasma, we obtained ultrafine and nanocrystalline fractions of particles with a core–shell structure from a mechanical mixture of titanium carbide TiC and titanium nickelide TiNi in the ratio of 1:1.

All the resulting powder compositions were studied by X-ray diffraction and helium pycnometry. The specific surface area was determined using the BET method. The nanocrystalline fraction was thoroughly investigated using high-resolution transmission electron microscopy.

Based on the research findings, the following conclusions can be drawn:

1. The ultra- and nanodispersed compositions formed during plasma-chemical synthesis have a core–shell structure. According to X-ray phase analysis confirmed by the results of high-resolution transmission electron microscopy, the refractory core is the TiC/TiC_xN_y /TiC_xO_z compound coated with a metallic Ni shell; the complex titanium-nickel nitride Ti_0.7Ni_0.3N acts as an interphase layer.

2. Based on X-ray diffraction and transmission electron microscopy data, taking into account the physicochemical features of the detected phase components, we formulated the chemical mechanism for the formation of ultradisperse and nanocrystalline particles with a core–shell structure under crystallization conditions at a rate of 10⁵ °C/s in a tangential nitrogen flow in a quenching chamber of the plasmatron.

3. The chemical mechanism of the formation of nanocrystalline particles is the overcoming of temperature barriers by the plasma flow, with elements evaporated in it that are a part of the charge. The crystallization temperatures of phase components that are present, according to X-ray diffraction, in ultradisperse and nanocrystalline particles act as temperature barriers.