INTRODUCTION

In the silicon–nitrogen system, there is only one chemical compound—silicon nitride of the composition Si3N4. The current state of development of materials based on silicon nitride is described in detail in [1–18]. Si3N4 is a refractory compound with a predominantly covalent type of interatomic bond. The melting point of silicon nitride is about 1900°C, at which decomposition takes place into Si and N2. At 1800°С, dissociation starts, and at 1900°С, silicon nitride completely dissociates into silicon and nitrogen in a wide pressure range [2, 19–21]. Three crystalline polymorphic modifications of Si3N4 are known: α-, β-, and γ-Si3N4. The alpha- and beta-silicon nitride crystallize in trigonal (space group P31c) and hexagonal (space group P63/m) syngonies and have the same phenacite structural type [22–25]. The gamma-silicon nitride crystallizes in cubic syngony [26–28] and is of the spinel structural type (space group Fd3m). The main structural element of α- and β-Si3N4 is SiN4 tetrahedra bonded by common nitrogen atoms and located at a relative angle of 120°. The chain arrangement of tetrahedra makes silicon nitride isomorphic to many silicate minerals, which determines the possibility of formation of complex phases in the Si–Al–O–N system (for example, sialons).

There are several methods for the synthesis of silicon nitride: furnace—direct nitriding of silicon at 1000–1500°C; nitriding in plasma flow; reduction of SiO in the presence of nitrogen; reaction of SiH4 with nitrogen in the presence of ammonia; gase phase deposition in SiCl4 + NH3 + C3H8 + H2, SiH4 + NH3 + CH4 + H2 systems, etc.; thermal degradation of silazanes RnSi–NH–SiRn, alkylaminoboranes R3NBH, methacarborane C2B10H12, decaborane B10H14 in the presence of ammonia, hydrogen, and nitrogen. In particular, there are reports on the use of hexamethyldi-silazane (HMDS) for the preparation of silicon oxycarbonitride films of variable composition SiCxNyOz:H by plasma-chemical decomposition [29], amorphous Si/C/N powders by laser spray pyrolysis [30], and SiBN ceramics by pyrolysis of polyborosilazane [31].

The purpose of this work is the synthesis of silicon nitride and oxonitride powders using hexamethyldisilazane (HMDS, C6H19NSi2, structural formula (CH3)3–Si–NH–Si–(CH3)3, boiling point of 125°С, density of 0.75 g/cm3) by gas phase pyrolysis.

EXPERIMENTAL

Experiments on the preparation of silicon nitride and oxonitride were carried out by the method of pyrolysis of organometallic compounds (OMC) in a quartz reactor. The installation includes the following main units: quartz pyrolytic reactor 1 m in length and 70 mm in diameter; a system for drying, dosing, and feeding nitrogen and ammonia into the reactor; a noncontact type feedstock evaporator; a feed unit for feeding the reactor; and a quenching and capture system for the finished product—fine powders of silicon nitride and oxonitride on a bag filter. To compensate the increasing pressure in the system caused by an increase in gas-dynamic resistance of the bag filter as it operates, a vacuum pump is provided at the outlet. Two different methods of introducing raw materials were used in the experiments—introduction of a vapor–gas mixture (a bubbler feeder with heating to supply HMDS vapor in a carrier gas stream) and introduction in the form of a gas-droplet stream (pneumatic nozzle). This is due to the fact that, upon obtaining composite materials, the initial reactants often have very different boiling points, which makes impossible their combined supply to the reactor in the vapor–gas form. Introduction in the form of a gas-droplet stream allows one to use combined mixtures of organic and inorganic substances that have different physical properties (boiling point, vapor pressure, solubility, etc).

The installation works as follows. A temperature close to the boiling point of HMDS (125°C) was set in the bubbler feeder, and the flow was fed into the reactor through the central channel of the input device. The amount of raw material fed to the reactor was controlled by temperature in the evaporator and the flow rate and temperature of the carrier gas. At the same time, a nitrogen–ammonia mixture preheated to 125°C was fed coaxially into the reactor through a ring channel of a larger diameter. The volumetric ratio of nitrogen/ammonia was varied from 1/3 to 1/10 with a total flow rate of 2 to 8 L/min. Owing to the tangentially located inlet of the mixture of nitrogen and ammonia, a rotational movement is imparted. Such a device made it possible to stabilize the feed stream, preventing it from dispersing prematurely in the zone of relatively low temperatures at the inlet of the reactor; to shield the near-wall regions of the reactor from excessive subsidence of the skull layer of the target product near the feed inlet; and also to improve mixing conditions of the reagents in the zone of working temperatures by increasing the flow turbulence. The resulting vapor–gas mixture was sent to a pyrolytic reactor, in which the temperature of 800–1100°С was maintained. Silicon nitride (or oxonitride) resulting from pyrolysis was collected on a filter.

In the case of the gas–droplet method, the feedstock was preliminarily prepared for introduction into the pyrolytic reactor. For this, the corresponding amounts of hexamethyldisilazane (HMDS) and tetraethoxysilane (TEOS) were mixed. Separately, with vigorous stirring at the rate of not more than 20 mL/min, a 5% solution of anhydrous lanthanum formate in butanol-1 was introduced into aluminum sec-butoxide (ASB). Immediately before synthesis, the prepared solutions were mixed. The lanthanum formate was introduced from the calculation of the final oxide content in silicon oxonitride at the level of 0.2–1.0 wt %. Before use, REE formate was dried in vacuum at a temperature of not more than 180—200°C until complete removal of traces of moisture. The temperature in the reactor ranged from 600 to 1150°C. Solid pyrolysis products were captured using a bag filter made of quartz fabric. To feed raw materials into the reactor, a specially developed three-channel pneumatic nozzle was used. Raw materials were introduced through a tube of the central channel of the nozzle with a diameter of 1 mm under excess pressure of 1–1.5 atm. Through a coaxially located annular channel under pressure of 3–3.5 atm, a nebulizer gas (carrier) was supplied—nitrogen, the flow of which undergoes rotational motion as described previously. The channel gap is about 0.5 mm. The shape of the spray torch, i.e., its width and length, was controlled by the position of the cutoff of the central channel relative to the annular channel and the flow rate of the gas spray, so that it would minimize the reflux of raw materials onto the walls of the reactor near the input point, since this effect is very critical in the implementation of the process. According to the results of experiments on a model solution, the viscosity and density of which is similar to the actual working composition, the optimal values of the carrier gas flow rate and the cutoff values of the central feed channel are found, which provide a spray cone width of no more than the inner diameter of the reactor, which allows minimizing the casting of raw materials to the walls of the reactor.

Regardless of the composition of the initial mixture, the behavior of individual droplets during their passage along the axis of the reactor can be represented as several successive stages. The first stage is rapid heating of the droplet; the second stage is evaporation of volatile components of the initial mixture, i.e., organometallic compounds (HMDS, TEOS) and organic solvent. At the same time, as temperature rises, thermal decomposition of nonvolatile components of the mixture (REE formate) proceeds and they mix with gaseous nitrogen and ammonia. The third stage is the actual pyrolysis of components, which follows the general mechanism—evolution of gaseous products—methane, ethane, saturated and unsaturated hydrocarbons of the C2–C4 series, and hydrogen; and the formation of a solid phase—nitride, oxide, oxonitride, partially silicon carbide, REE oxide, and carbon black.

The heating of the droplet stream and its evaporation cause a noticeable drop in temperature in the reactor. With a decrease in the droplet size and, correspondingly, with an increase in their total surface, the heating efficiency and evaporation rate increase. The best conditions for spraying raw materials are achieved with a carrier gas flow rate of 2–3 L/min. The second and third stages are accompanied by a significant increase in the volume of the gas phase because of the release of gaseous pyrolysis products. In this case, the average gas flow rate increases, and the residence time of the components of the mixture in the reaction zone decreases, which can lead to a slippage of raw material and reduction of the degree of its conversion. A change in the volume of the components of the gas mixture (ammonia and nitrogen) also affects this parameter. To study the influence of these effects on the pyrolysis process, measurements of the temperature and composition of the gas phase in the axial section of the reactor were carried out, and the degree of conversion of the feedstock was studied depending on the gas-dynamic mixing conditions of the reactants, the contact time of the reacting components, and the state of aggregation of the products of pyrolysis. It is experimentally established that, in the synthesis mode, there is a decrease in temperature (to 400°C) at a distance of 2.5 calibers of the reactor (about 200 mm) from the nozzle cut. The effect is associated with a sharp absorption of heat during the evaporation of droplets of raw material (first stage). Next, the temperature profile enters the operating mode. The analysis of the composition of the gas phase was carried out using a water-cooled quartz tube probe, which was introduced from the end of the reactor and moved in the axial direction. Scanning at the sampling points was carried out with a step of 70 mm (d of the reactor). Analysis of samples was performed with an ALTAIR 5X gas analyzer. At a distance of 250 mm from the nozzle cut in the gas samples, traces of pyrolysis products—carbon monoxide, methane, ethane and ethylene, hydrogen—were detected, and then their concentration uniformly increased along the length of the reactor and became maximal. The content of gaseous pyrolysis products in the gas samples along the axis of the reactor for different values of the carrier gas flow rate at the pyrolysis temperature of 1100°C is shown in Fig. 1.

Fig. 1.
figure 1

The change in the content of pyrolysis products in the gas phase along the length reactor depending on the flow rate of the carrier gas, L/min: (1) 2, (2) 3, (3) 4.

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At the flow rate of the carrier gas about 2 L/min, the spraying conditions reduce the number of droplets with a size of more than 0.06–0.08 mm to 25%; the reaction under these conditions ends at 8–10 calibers of the reactor. At the flow rate of the carrier gas of 4 L/min and more, despite high-quality atomization (the content of droplets with a size of more than 0.08 mm was less than 5%), because of the high flow rate, the pyrolysis reaction within the reactor length does not end; there is a slippage of unreacted raw material. The dependence of the degree of conversion of raw material on the gas flow rate is shown in Fig. 2.

Fig. 2.
figure 2

Dependence of the degree of conversion of the feedstock on the gas flow rate.

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Under the best conditions of dispersion (consumption of the carrier gas is 2–2.5 L/min), one should consider the optimal flow rate to be no more than 0.8–1 m/s, while conversion is about 100%.

The pyrolysis of OMC (apparently, both oxygen-containing and oxygen-free) results from the elimination of the radicals RO and R from the initial molecules. Considering this mechanism of thermal decomposition, one can propose a scheme of radical interaction reactions that describe the formation of all experimentally detected pyrolysis products. At temperatures of 600–700°C, reactions of the interaction of radicals with the formation of alcohols, saturated and unsaturated hydrocarbons, and water dominate. At temperatures above 1000°C, the primary process of decomposition of primary radicals (R and RO) occurs with the formation of the simplest compounds—carbon monoxide, methane, ethane, and elemental carbon. Under the same temperature conditions, ammonia also undergoes partial decomposition with the formation of NH and NH2 radicals. Ammonia is a component of the mixture, which, on one hand, promotes a shift in the equilibrium of the pyrolysis reaction toward the formation of nitrides and, on the other hand, acts as an inhibitor of the formation of soot carbon. The latter effect is explained by the formation of alkyl amines Rn-NH3 – n, which helps to reduce the concentration of the carbon-forming component of the gas phase—ethylene.

As a result, when ammonia is introduced into the system, an increase in the degree of conversion of raw material is observed. Complete conversion is achieved when the ammonia content is not less than 0.5–0.6 mol per 1 mol of the feedstock. Figure 3 shows the general dependence of the degree of conversion of raw material, calculated on the basis of the gas phase composition, depending on the ammonia content in the gas phase.

Fig. 3.
figure 3

Dependence of the degree of conversion on the concentration of ammonia in the gas phase.

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To identify the phase and chemical composition of the samples of the obtained powders, the methods of X-ray diffraction (XRD, XRD 6000 Shimadzu diffractometer, CuKα radiation, graphite monochromator, λ = 1.540598 Å) and IR spectroscopy (Nicolet iS5 IR spectrometer) were used. The microstructure and morphological features of the samples were studied using scanning electron microscopy (SEM) (LEO 1420 electron microscope, JEOL).

RESULTS AND DISCUSSION

The determining parameter of the gas-phase pyrolysis process is the degree of conversion, which depends on the process temperature, gas-dynamic mixing conditions of the reactants, contact time of the reacting components, and the aggregation state of the pyrolysis products. The study of the pyrolysis patterns was carried out in the range of operating temperatures of 950–1050°С. It is found that, at the average gas flow rate in the reactor of less than 1 m/s (the residence time of the reactants in the reactor is about 1 s), the degree of conversion is 100%. At high rates, this indicator decreases rapidly, since the vapor–gas mixture with decrease in the residence time in the reaction zone does not have time to warm up sufficiently—a breakthrough of the unreacted raw material to the filter occurs. At the optimal gas flow rate (less than 1 m/s), the fullness of the conversion is achieved only at the ammonia content of at least 0.5–0.6 mol per 1 mol of HMDS. Increasing the volume of ammonia above this value does not cause any changes. At the same time, the worsening of the conditions for mixing of gaseous reactants caused by the decrease in the total volume of the nitrogen–ammonia mixture leads to a decrease in the degree of conversion even under conditions of a sufficient amount of the supplied ammonia. With an increase in the flow rate of the nitrogen–ammonia mixture, a slip is observed. The best mixing conditions are achieved when the volume of the nitrogen–ammonia mixture exceeds the volume of the vapor–gas mixture (feed + carrier gas) by 8–12 times.

Thus, the optimal conditions of the synthesis of silicon nitride for the dimensions of the pyrolytic reactor used in the experiments at a temperature of 1000°C are determined: the consumption of HMDS is not more than 200 mL/h, the carrier gas consumption flow rate is about 3 L/min, and the flow rate of the nitrogen–ammonia mixture should exceed the volume of the vapor–gas mixture (raw material + carrier gas) by 8–12 times at the ammonia content of at least 0.5–0.6 mol per 1 mol of HMDS feed to the reactor.

Experiments on the use of the gas-droplet method of injection by means of a nozzle showed similar results. However, the optimal residence time of the reagents providing 100% conversion should be at least 1.5–1.7 s. This, apparently, is due to significant heat losses associated with the need for heating and evaporation of the feedstock.

The dispersed Si2N2O powder obtained by the pyrolysis of HMDS according to the XRD data is X-ray amorphous with the specific surface area of about 15 m2/g and bulk density of 0.2–0.25 g/cm3. According to the SEM images (Fig. 4), the synthesized powder has spherical particles with an average size of 10–40 nm and, according to the particle size analysis (Fig. 5), is prone to agglomeration. The pore size calculated by the Barrett–Joyner–Halenda (BJH) method is 2 nm (i.e., micropores predominate). According to the results of the chemical analysis, the content of elements in the samples of powders is 50 wt % silicon, 24 wt % nitrogen, and 28–32 wt % oxygen. Concentration of the doping oxide (lanthanum oxide) is 0.2 wt %. These data are in complete agreement with the composition of silicon oxonitride Si2N2O with an excess in the oxygen content, which is associated with the presence of silicon oxide impurities in the material.

Fig. 4.
figure 4

SEM image of silicon oxonitride particles Si2N2O.

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Fig. 5.
figure 5

Particle size distribution of silicon oxonitride powder.

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The IR absorption spectrum of Si2N2O powders obtained in the experiments confirms the presence of chemical bonds of the main elements. The most intense absorption bands observed are ascribed as follows: at 948 cm–1, to the stretching vibrations of the Si–N bonds (950 cm–1) in the Si3N group; a wide band with a maximum at 1074 cm–1, to stretching vibrations of the Si–O bond in Si–O–Si fragments. A less intense band with a maximum at 473.9 cm–1 is due to vibrations of Si–N bonds. A wide band with a maximum at 3330 cm–1 can be attributed to stretching vibrations of N–H bonds (3390 cm–1) [32–34].

Thus, the preparation of doped silicon oxonitride by pyrolysis using gas-droplet input is possible under the following conditions: the size of the drops of raw material does not exceed 0.05 mm; the maximum diameter of the cone of the spray torch should not be more than 0.8 of the inner diameter of the reactor; the gas flow rate should provide the residence time of the reagents in the reactor of at least 0.8–1 s with a ratio of its diameter to the length of 1 : 12—1 : 14; the process temperature is 1000–1100°С; complete conversion is achieved when the ammonia content in the gas phase is 0.5–0.6 mol per 1 mol of the initial HMDS. The introduction of alloying additives (REE oxides) to the composite does not have a significant effect on the pyrolysis process in a fairly wide concentration range.

In addition to the Si2N2O powders considered above, single-phase powders of alpha-modification of silicon nitride Si3N4 are obtained in the form of fibrous crystals with a particle diameter of 50–200 nm, length from tens to hundreds of microns, and specific surface area of up to 11 m2/g (Fig. 6) by gas-phase pyrolysis of hexamethyldisilazane.

Fig. 6.
figure 6

Si3N4 whiskers obtained by gas-phase pyrolysis of hexamethyldisilazane.

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CONCLUSIONS

The conditions for the preparation of amorphous powders of silicon nitride Si3N4 and silicon oxonitride Si2N2O by gas-phase pyrolysis of hexamethyldisilazane are experimentally studied.

The technique is developed and the installation is done for the implementation of pyrolysis of organometallic compounds in nitrogen–ammonia mixture at temperatures up to 1100°C.

The influence of gas-dynamic conditions of synthesis is studied on the properties of nanopowder silicon oxonitride. The optimal conditions for pyrolysis process are found: temperature, ratio of gas mixture components, mixing conditions, and phase contact times.

X-ray amorphous Si3N4 and Si2N2O powders are obtained with particle sizes of 50–200 nm and specific surface up to 15 m2/g.