Method for Producing a Silicon Nitride Powder and Battery Comprising the Powder

Abstract

Method for producing a powder comprising particles (26) comprising amorphous, micro- or nano-crystalline Silicon nitride. The method comprises the steps of supplying a reactant gas (12) containing Silicon, and a reactant gas (12) containing Nitrogen, to a reaction chamber (16) of a reactor (10), and heating said reactant gases (12) to a temperature in the range of 510 C. to 1300 C. which is sufficient for thermal decomposition or reduction of the reactant gases (12) to take place inside the reaction chamber (16) to thereby produce a powder of amorphous, micro- or nano-crystalline particles (26) comprising Silicon nitride (SiNx) in which the atomic ratio of Silicon to Nitrogen is in the range 1:0.2 to 1:0.9. The produced powder of particles (26) may be used to produce a film, an electrode, such as an anode, for a battery, such as a Lithium ion battery.

Claims

1-30. (canceled)

31. A battery anode material comprising: a silicon nitride SiN.sub.X powder; where X is in a range of 0.2X<1.3; and, wherein the SiN.sub.X powder has a morphology selected from the group consisting of amorphous, micro-crystalline, and nano-crystalline.

32. The battery anode material of claim 31 wherein the SiN.sub.X powder further comprises lithium, forming Li.sub.YSiN.sub.X; where Y is greater than 0.5X and less than 2.0X.

33. The battery anode material of claim 31 wherein X is in a range of 0.4X<1.

34. (canceled)

35. The battery anode material of claim 31 further comprising: an element included in the SiN.sub.X powder selected from the group consisting of a metal, oxygen, and carbon.

36. The battery anode material of claim 31 wherein the SiN.sub.X powder comprises a plurality of SiN.sub.X cores; and, the battery anode material further comprising: a passivation material shell coating the SiN.sub.X cores.

37. The battery anode material of claim 36 wherein the passivation material is selected from the group consisting of carbon and silicon carbide.

38. The battery anode material of claim 36 wherein the SiN.sub.X powder further comprises lithium, forming Li.sub.YSiN.sub.X; and, where the lithium content is less than or equal to 350 atomic-percent of the core silicon content.

39. The battery anode material of claim 30 wherein the SiN.sub.X powder is formed of particles having a homogeneous size and spherical shape.

40. The battery anode material of claim 30 wherein the SiN.sub.X powder includes a dopant selected from the group consisting of phosphorus, boron, arsenic, gallium, and aluminum.

41. A battery comprising: a cathode; an electrolyte; an anode comprising: an active material comprising: a silicon nitride SiN.sub.X powder, where Xis in a range of 0.2X<1.3; and, wherein the SiN.sub.X powder has a morphology selected from the group consisting of amorphous, micro-crystalline, and nano-crystalline.

42. The battery of claim 41 wherein the anode active material further comprises lithium in a concentration matching a bulk irreversible bulk capacity of the SiN.sub.X powder.

43. A method for producing a battery active material, the method comprising: providing a reaction chamber; introducing reactant gases including silicon and nitrogen into the reaction chamber; heating the reaction chamber; and, chemically reducing the reactant gases to form a silicon nitride SiN.sub.X powder, where X is in a range of 0.2X<1.3, and where the SiN.sub.X powder has a morphology selected from the group consisting of amorphous, micro-crystalline, and nano-crystalline.

44. The method of claim 43 further comprising: introducing a reactant gas including lithium into the reaction chamber; and, wherein chemically reducing the reactant gases includes forming a Li.sub.YSiN.sub.X powder, where Y is greater than 0.5X and less than 2.0X.

45. (canceled)

46. The method of claim 43 further comprising: subsequent to forming the SiN.sub.X powder, coating SiN.sub.X core particles with a passivation material shell.

47. The method of claim 46 wherein coating the SiN.sub.X core particles with the passivation material includes coating with a passivation material selected from the group consisting of carbon and silicon carbide.

48. The method of claim 43 further comprising: introducing a dopant gas to the reaction chamber including an element selected from the group consisting of phosphorus, boron, arsenic, gallium, and aluminum; and, doping the SiN.sub.X powder with the selected element.

49. The method of claim 43 further comprising: subsequent to forming the SiN.sub.X powder, collecting homogeneous-sized spherically shaped SiN.sub.X particles using a method selected from the group consisting of filtering, gravitational separation, and electrostatic forces.

50. The method of claim 43 wherein providing the reaction chamber includes providing a reaction chamber comprising an outer chamber with exterior walls, separated from an inner chamber by a porous membrane; wherein introducing the reactant gases includes introducing the reactant gases into the inner chamber; the method further comprising: introducing an inert gas into the outer chamber; and, in response to the introduction of the inert gas, preventing the deposition of SiN.sub.X power on the outer chamber walls.

51. The method of claim 43 wherein forming the SiN.sub.X powder includes forming a SiN.sub.X powder where X is in a range of 0.4X<1.0.

52. The battery of claim 41 wherein X is in a range of 0.4X<1.0.

53. The battery of claim 41 wherein the battery is a lithium ion battery.

54. The battery of claim 41 wherein the electrolyte includes an additive that enhances a first cycle lithiation of the silicon nitride particles by providing a surface electrolyte interface layer capable of facilitating the lithiation of the silicon nitride particles.

55. The battery of claim 54 wherein the electrolyte additive is selected from the group consisting of Fluor Ethylene Carbonate (FEC), Vinylene Carbonate (VC), and combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended schematic figures where;

[0070] FIG. 1 shows a device for producing amorphous or nano- to micro-scale Silicon nitride particles according to a method according to an embodiment of the invention,

[0071] FIG. 2 shows a coated Silicon nitride particle,

[0072] FIG. 3 is a scanning electron microscope (SEM) image of a powder of SiN.sub.0.2 particles produced using a method according to the present invention,

[0073] FIG. 4 shows the charge capacity of one Silicon electrode and four electrodes comprising amorphous Silicon nitride with increasing Nitrogen content from x=0.36 to x=1.00,

[0074] FIG. 5 shows the differential capacity analysis of one Silicon electrode and four electrodes comprising amorphous Silicon nitride with increasing Nitrogen content from x=0.36 to x=1.00,

[0075] FIG. 6 shows the specific capacity of an electrode according to the present invention compared with the specific capacity of an electrode comprising two commercially available powders,

[0076] FIG. 7 shows the specific capacity of an electrode according to the present invention compared comprising Silicon Nitride powder having a chemical formula of SiN.sub.1.1, and

[0077] FIG. 8 is a flow chart showing the steps of a method according to an embodiment of the invention.

[0078] It should be noted that the drawings have not necessarily been drawn to scale and that the dimensions of certain features may have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF EMBODIMENTS

[0079] FIG. 1 shows a device 10 for producing a powder comprising amorphous, micro- or nano-crystalline Silicon nitride particles by homogeneous thermal decomposition or reduction of reactant gases 12 containing Silicon and Nitrogen. The device comprises a reactor 14 having a reaction chamber 16 with one or more inlets for the reactant gases 12, located at the top of the device 10 for example to obtain a descending reactant gas flow. The reactor 14 may be a Free Space Reactor having stainless steel, Silicon carbide or quartz walls for example, which is arranged to decompose the reactant gas 12 homogeneously in the gas phase and thus to grow the desired amorphous, micro- or nano-crystalline Silicon nitride particles. Volatile by-products may be removed by gas flow through the reaction chamber 16.

[0080] Contrary to the multi-stage reactor disclosed in U.S. Pat. No. 4,314,525, in the device according to the present invention, no seed particles are introduced into the reactor 14. Furthermore, particles are not grown on a substrate, such as a hot substrate or deposited on a wafer, such as a heated wafer, and no salt is used to produce the particles. In the method according to the present invention, deposition is carried out on particles floating in heated gas.

[0081] The device 10 also comprises means 18, such as heating coils, which are located around the outer wall of the reactor 14 in the illustrated embodiment, to heat the reactant gases 12 to a temperature in the range of 510 to 1300 C. which is sufficient for thermal decomposition or reduction of the reactant gases 12 to take place inside the reaction chamber 16. The reactant gases 12 are preferably pre-heated to a temperature that is just below the reaction temperature and then, when the reactant gases are in the reaction chamber 16, the temperature inside the reactor 10 provides the energy required so that the particles start forming. This produces particles with a narrow size distribution.

[0082] The reaction chamber 16 in the illustrated embodiment is constituted by a single wall constituted entirely by a porous membrane 20, such as a substantially cylindrical tube of material of suitable mechanical and chemical properties. It should be noted that the porous membrane 20 may be of any suitable shape, it may for example be in the form of an upright or inverted cone.

[0083] The device 10 may optionally comprise one or more inlets that are arranged to supply a primary gas 22, such as hydrogen or argon, through the porous membrane 20 to provide a protective inert gas boundary at the wall of the reaction chamber 16 to minimize or prevent the deposition of the material on the porous membrane 20 when the device 10 is in use. The one or more inlets may optionally also be used to supply a secondary gas through the porous membrane 20 to influence the thermal decomposition or reduction of the reactant gas 12 inside the reaction chamber 16. The expression influence the thermal decomposition or reduction of the reactant gas inside the reaction chamber as used in this document is intended to mean slow down, speed up, prevent, start, modify or change one or more chemical reactions taking place inside the reaction chamber 16. It is not however necessary for a reactor in which a method according to the present invention is carried out to comprise a porous membrane 20.

[0084] A Silicon-containing reactant gas 12, such as monosilane (SiH.sub.4), diluted in Hydrogen, may be supplied to the reaction chamber 16. Means 18 for heating the reaction chamber 16 raises the temperature of the Silicon-containing reactant gas 12 to a point of thermal decomposition whereby the following reaction takes place and elemental Silicon, is formed:

SiH.sub.4.fwdarw.Si+2H.sub.2

[0085] For monosilane this temperature is 400 C. The reactant gas 12 may also contain one or more dopant gases, such as arsine, diborane, phosphine, boron trifluoride, Boron-Il-trifluoride, trimethylboron or any other metal/organic/inorganic dopant gas, which may for example be added in the particles' nucleation and/or growth phase(s). The reactant gas 12 may for example contain a metal or Lithium-containing gas, which is supplied during the particle nucleation phase, and/or after the particle nucleation phase. Use of Lihitum containing gases for atomic layer deposition is described in e.g. Nilsen, O., Miikkulainen, V., Gandrud, K. B., streng, E., Ruud, A., & Fjellvg, H., (2014), Atomic layer deposition of functional films for Li-ion microbatteries. Physica Status Solidi (A) Applications and Materials Science, 211(2), 357-367. It should be noted that a dopant gas may additionally or alternatively be supplied through the porous membrane 20 in the illustrated embodiment.

[0086] A Nitrogen-containing reactant gas 12, such as ammonia, is also supplied to the reaction chamber 16. Means 18 for heating the reaction chamber 16 raises the temperature of the Nitrogen-containing reactant gas 12 to a point of thermal decomposition whereby the following reaction takes place and elemental Nitrogen is formed:

2NH.sub.3.fwdarw.2N+3H.sub.2

[0087] Primary gas 22, such as Hydrogen, Nitrogen or Argon is supplied to a chamber 24 outside the reaction chamber 16 that is delimited by the porous membrane wall 20. The reactor 14 is thereby divided into an outer chamber 24 for primary gas 22 and an inner reaction chamber 16 where a decomposition or reduction reaction takes places at a distance from the wall(s) of the reaction chamber 16. The primary gas 22 in the outer chamber 24 is namely arranged to pass through the porous membrane 20 from the outer chamber 24 to the near wall region of the reaction chamber 16. When the primary gas 22 enters the reaction chamber 16, the near wall region will be kept free of reactant gas 12 and thus unwanted wall depositions will be avoided.

[0088] Secondary gas may also be supplied to the chamber 24 outside the reaction chamber 16 that is delimited by the porous membrane wall 20. The secondary gas may be added in the particles' nucleation and/or growth phase(s). The secondary gas may for example contain a metal- or Lithium-containing gas, which is supplied through the porous membrane during the particle nucleation phase, and/or after the particle nucleation phase but prior to their exposure to air.

[0089] Depending on the operation temperature and requirements for the finished product, the porous membrane 20 may comprise a metal alloy such as AISI316, Inconel, 253MA or HT800. The membrane may also be produced from porous sintered silicon-nitride Si.sub.3N.sub.4, porous silica SiO.sub.2, porous alumina Al.sub.2O.sub.3, graphite or any other suitable material.

[0090] It should be noted that the reaction chamber 16 dimensions may vary from having a maximum transverse dimension of a few millimetres to a few metres.

[0091] The thermal decomposition or reduction of the reactant gases 12 inside the reaction chamber 16 is controlled so as to produce a powder of amorphous, micro- or nano-crystalline Silicon nitride particles which may subsequently form the core region 26 of a coated particle 30 as schematically shown in FIG. 2. The thermal decomposition or reduction of the reactant gases 12 inside the reaction chamber 16 may for example be controlled by adjusting the temperature, pressure, flow rate, heat capacity and/or composition, of the reactant gases 12 (and/or a reaction-influencing gas).

[0092] The outer surface of the produced coated or uncoated Silicon nitride particles, which particles may subsequently form a core region 26 of a coated particle 30, is free from irregularities, roughness and projections when viewed at a maximum resolution of a Scanning Electron Microscope (SEM).

[0093] The thermal decomposition or reduction of the reactant gases 12 inside the reaction chamber may be influenced by changing at least one of the following characteristics of the reactant gas and/or reaction-influencing gas: temperature, pressure, flow rate, heat capacity, composition, dopant type(s) and/or amount(s), catalyst type(s) and/or amount(s), and/or concentration of one or more components of said gases. By changing at least one of the characteristics of the reactant gases and/or reaction-influencing gas, the thermal decomposition or reduction of the reactant gas inside the reaction chamber, and consequently the formation and/or growth of particles, and/or their morphology and/or crystallinity, may be controlled in order to obtain a final product having the desired characteristics.

[0094] For example, the temperature of the primary gas and/or secondary gas may be increased once particles have been formed in order to produce crystalline material. Alternatively the temperature of the primary gas and/or secondary gas may be decreased to produce amorphous material. The amount of hydrogen in the primary gas and/or secondary gas may be increased to decrease the production of nuclei and thereby the total number of particles. The flow rate of the primary gas and/or secondary gas may be increased to promote turbulence inside the reaction chamber, or decreased to reduce turbulence, depending on which conditions are conducive to the production of the desired product.

[0095] The primary gas and/or secondary gas preferably has/have a high heat capacity to help provide uniform heating within the reaction chamber. This may however vary with the application since several decomposition reactions include intermediate reversible stages, whereby it may be advantageous to promote particle growth over particle formation. Such stages may be temperature dependent and in such cases a controlled uneven temperature distribution is favourable.

[0096] The secondary gas may be supplied through the porous membrane simultaneously with the primary gas, periodically, continuously, intermittently, when desired, or in any combination of these ways during the use of a reactor. The primary gas and the secondary gas may be arranged to be supplied through the same pores, or through different pores in the porous membrane.

[0097] It should also be noted that the expressions primary gas and secondary gas as used in this document need not necessarily mean that said gases comprise just one type of gas. A primary gas and/or a secondary gas may also comprise at least one catalyst gas. Furthermore, different primary gases and/or secondary gases may be used during the use of a reactor.

[0098] The actual dimensions of the components of the device 10 (such as the diameter or length of the reaction chamber tube or the shape of the reactor chamber) are not especially critical. In addition, operating parameters such as gas flow rates and operating temperatures can be established experimentally for different devices having different sizes and configurations.

[0099] Particles that descended from the bottom of the reaction chamber were collected by installing a collecting container, such as a cylinder having a diameter that was at least twice the size of the diameter of the reactor. The collecting container was located downstream of the reaction chamber and collection was facilitated by the gravitational force on the particles combined with a low gas flow in the exhaust so that the residence time of particles within the collecting container was quite long (i.e. four times the residence time of the particles in the reaction chamber tube. The fallout of the particles was assumed to be enhanced by electrostatic forces as particles piled up on the bottom and the lower walls of the collecting container. It was namely observed that the particles were charged and attracted to each other, and the more the particles that collected in the collecting container, the larger the electrostatic field seemed to be. This enabled the harvesting of particles with a narrow particle size distribution or with particular properties (charged particles).

[0100] The method according to the present invention may be used to produce a high volume of powder of particles comprising amorphous, micro- or nano-crystalline Silicon nitride. The method is easy to scale up and it is possible to achieve continuous particle production while the reactor is in use.

[0101] According to an embodiment of the invention the powder of amorphous, micro- or nano-crystalline Silicon nitride particles produced in the reactor may be coated with at least one passivating material, such as Carbon or Silicon carbide, using Chemical Vapour Deposition (CVD), such as vertical CVD, Atomic Layer Deposition (ALD), a plasma-assisted method, or a hot wire method or by immersing them in a fluid containing Lithium ions to produce a shell region 28. The shell region 28 may comprise 3-100 monolayers of passivating material so as to be thin but mechanically robust, and/or may be doped with Phosphorus, Boron, Arsenic, Gallium or Aluminium. The coating step may be carried out inside the same reactor used for the production of the core region 26 particles, or inside a different vessel.

[0102] According to an embodiment of the invention the method may comprise the step of doping the at least one passivating material with at least one element selected from the group consisting of Phosphorus, Boron, Arsenic, Gallium, and Aluminum. According to an embodiment of the invention the shell region of a coated particle may comprise 3-100 monolayers of the at least one passivating material.

[0103] The coated particles 30, which are substantially spherical, which also have a core region 26 that is substantially spherical, and which preferably have a maximum transverse dimension up to 100 m or 2 nm-10 m or 10 nm-10 m or less than 10 m or less than 1 m may be used for several applications. The coated particles 30 may for example be used to produce an anode for a Lithium ion battery. By using Silicon nitride instead of Carbon anodes in Lithium ion batteries, or at least replacing part of the Carbon with Silicon nitride, it has been shown that the storage capacity of the battery can be substantially increased.

[0104] FIG. 3 shows an SEM image of a powder of substantially spherical SiN.sub.0.2 particles 26 produced using a method according to the present invention.

[0105] FIG. 4 shows how the charge capacity of a plurality of Silicon nitride thin film electrodes with different Nitrogen content evolves during cycling. As expected, it can be seen that the initial charge capacity of the electrodes becomes lower as the Nitrogen content increases. However, at a certain threshold value, the material starts to exhibit excellent cycle stability. At this optimal composition, the material has a capacity of approximately 1500 mAh/g and an average coulombic efficiency over the first 1000 cycles of 99.7%. Differential capacity analysis further demonstrates the remarkable cycling stability of the Silicon nitrides having the higher Nitrogen contents.

[0106] Amorphous Silicon nitride films were produced made using Plasma Enhanced Chemical Vapor Deposition (PECVD) with silane (SiH.sub.4) and ammonia (NH.sub.3) as precursors. Films with thicknesses ranging from 40 nm to 200 nm were deposited on Copper foil. Different SiN.sub.x chemical compositions were achieved by varying the ratio of the precursor gases. Transmission Electron Microscopy (TEM) was used in conjunction with spectroscopic ellipsometry and Scanning Electron Microscopy (SEM) to determine the thickness and composition of the films, as well as to evaluate their evenness, coverage and interface quality. Electrochemical testing was conducted in 2032 coin cells against a Lithium metal counter electrode using a commercial separator and an electrolyte with 5% Fluoroethylene Carbonate (FEC) and 1% Vinylene Carbonate (VC).

[0107] FIG. 5 shows the charge capacity of one Silicon electrode and four electrodes comprising amorphous Silicon nitride with increasing Nitrogen content from x=0.36 to x=1.00. The composition is estimated from ellipsometry characterization of the thin films. The true Nitrogen content may be somewhat lower if the film contains other sources of band gap modulation, like nano-porosity. An increase in cycle stability between x=0.63 and x=0.88 can be seen. The plots of FIG. 5 show that the rapid degradation of the pure Silicon electrode and the lower nitrides is due to loss of active material. Neither this mode of degradation nor any other can be seen in the two electrodes that contained the most Nitrogen. The similar peak positions for all of the electrodes show that the active part of the electrodes is indeed Silicon, as hypothesized.

[0108] FIG. 6 shows the specific capacity of an electrode according to the present invention compared with the specific capacity of electrodes comprising commercially available powders which were purchased from two different vendors, Vendor 1 and Vendor 2. The electrode according to the present invention comprised a powder of Silicon nitride particles having a chemical formula of SiN.sub.ti and the commercially available electrodes comprised a powder of Silicon nitride particles having a chemical formula of Si.sub.3N.sub.4. The electrodes were only cycled for three to four cycles, but FIG. 6 clearly shows that that the specific capacity of the electrode according to the present invention is better than the specific capacity of the electrodes comprising a commercially available powder.

[0109] FIG. 7 shows the specific capacity of an electrode according to the present invention compared comprising Silicon Nitride powder having a chemical formula of SiN.sub.1.1. FIG. 7 shows the results from two electrodes that have been cycled with a different regime. The electrodes should have a specific capacity of 1000 mAh/g (of SiN.sub.x) and FIG. 7 shows that this specific capacity cannot be met in the initial cycles, indicating that a rearrangement is taking place in the electrode, and that pre-treatment might be necessary. After 60 cycles, the electrodes behave as they should and deliver the desired 1000 mAh/g (of SiN.sub.x).

[0110] FIG. 8 is a flow chart showing the steps of a method according to the present invention. The method comprises the steps of supplying reactant gases containing Silicon and Nitrogen to a reaction chamber of a reactor, heating the reactant gas to a temperature sufficient for thermal decomposition or reduction of the reactant gases to take place inside the reaction chamber, such as a temperature in the range of 510 to 1300 C., to thereby produce amorphous, micro- or nano-crystalline particles of Silicon nitride (SiN.sub.x) in which the atomic ratio of Silicon to Nitrogen is in the range 1:0.2 to 1:0.9.

[0111] Optionally, the method according to the present invention may also comprise one or more of the following steps (which have been illustrated in dashed boxes in FIG. 8): supplying at least one gas containing a metal, such as Lithium to the reaction chamber of the reactor, supplying at least one dopant gas to said reaction chamber of said reactor to dope said particles, coating the produced amorphous, micro- or nano-crystalline Silicon nitride particles with passivating material in the reaction chamber or in a different vessel, using the coated or uncoated amorphous, micro- or nano-crystalline particles comprising Silicon nitride (SiN.sub.x) to produce a film or an electrode for a battery, such as a Lithium ion battery.

[0112] It should be noted that the steps of supplying reactant gases containing Silicon and Nitrogen to a reaction chamber of a reactor, supplying at least one gas containing a metal, such as Lithium to the reaction chamber of the reactor and supplying at least one dopant gas to said reaction chamber of said reactor to dope said particles do not have to be carried out in a particular sequence. An inert gas boundary is preferably, but not necessarily established before reactant gases and/or a secondary gas are supplied to the reaction chamber.

[0113] The method according to the present invention may also comprise at least one of the optional steps of pre-heating the reactant gases to a temperature below the reaction temperature before the reactant gases are supplied to the reaction chamber of the reactor and/or heat treating the particles after their production, in an inert atmosphere or hydrogen-containing atmosphere so as not to oxidise the particles. The post-production heat treatment step may be carried out at a temperature of 600-1300 C. with a process time within 10-3600 seconds in the inert/hydrogen containing atmosphere.

Example 1

[0114] A method according to the present invention was carried out by supplying SiH.sub.4 and NH.sub.3 to a reaction chamber of a Free Space Reactor, and heating these reactant gases to a temperature of 700 C. so that thermal decomposition or reduction of the reactant gases could take place inside the reaction chamber. The effects of the following feed gas compositions were studied: 50% SiH.sub.4 and 50% NH.sub.3, and 25% SiH.sub.4 and 75% NH.sub.3. These feed gas compositions were compared with a run in which 100% SiH.sub.4 was used.

[0115] The nucleation point for 50% SiH.sub.4 and 50% NH.sub.3 was found to be just below 460 C. at a gas flow rate of 2 standard litres per minute (SLM) and with a residence time of approximately 2 seconds. An exothermal reaction occurred inside the reactor when SiH.sub.4 and NH.sub.3 reacted. This exothermic reaction occurred between a temperature between 550 and 650 C. More heat seemed to be generated by the exothermic reaction when 25% SiH.sub.4 and 75% NH.sub.3 were used than in the case when 50% SiH.sub.4 and 50% NH.sub.3 were used. More heat was generated when using a mixture of SiH.sub.4 and NH.sub.3 than when using 100% SiH.sub.4. Operation below the exothermic reaction temperature is likely to result in low Nitrogen content.

[0116] Samples of the produced Silicon nitride (SiN.sub.x) particles where taken from the floor of the reactor, the reaction chamber, from a filter inside the reactor and from a wall in the reactor exhaust, and analysed using SEM and an Energy-dispersive detector (EDS) to determine the Nitrogen content of the produced particles.

[0117] It was found that an increase in the total gas flow through the reactor decreased the growth rate of particles on the wall of the reaction chamber.

Example 2

[0118] A method according to the present invention was carried out by supplying SiH.sub.4 at a gas flow rate of 1 SLM and NH.sub.3 at a gas flow rate of 1 SLM to a reaction chamber of a Free Space Reactor, and heating these reactant gases to a temperature of 700 C. so that thermal decomposition or reduction of the reactant gases could take place inside the reaction chamber.

[0119] The total gas flow through the reaction chamber was 2 SLM and the residence time of gas in the reaction chamber was approximately 2 seconds. This gave a high yield (of about 90%) of nano-crystalline SiN.sub.0.2 particles. However, the reaction chamber tube got clogged up after 46 minutes after 78.5 litres of SiH.sub.4 had passed through the reactor Most of the powder produced was deposited on the wall in the reaction chamber and the produced powder was dark brown in colour.

Example 3

[0120] A method according to the present invention was carried out by supplying SiH.sub.4 at a gas flow rate of 1 SLM and NH.sub.3 at a gas flow rate of 3 SLM to a reaction chamber of a Free Space Reactor, and heating these reactant gases to a temperature of 690 C. so that thermal decomposition or reduction of the reactant gases could take place inside the reaction chamber. The total gas flow through the reaction chamber was 4 SLM and the residence time of gas in the reaction chamber was approximately 2 seconds. This gave a medium yield (of about 70%) of nano-crystalline SiN.sub.0.3 particles. The reaction chamber tube did not clog up even after 193 litres of SiH.sub.4 had passed through the reactor. The colour of the powder produced was similar to the colour of oxidized iron (rust).

[0121] The material growth on the wall in the reaction chamber was monitored using a charge-coupled device (CCD) camera. The reactor was flushed with Argon every 30 minutes to get a clear view of the inside of the reactor chamber. It was found that SiN.sub.0.3 particles accumulated in the lower part of the reactor's exhaust stream.

[0122] It was found that that the produced nano-crystalline SiN.sub.0.3 particles contained approximately 50 atomic-% Nitrogen. It was also found that increasing the NH.sub.3/SiH.sub.4 gas flow ratio resulted in particles containing more Nitrogen. Increasing the total gas flow rate reduced reactor wall growth.

Example 4

[0123] The residence time in the reaction chamber, the temperature, the pressure, and the gas mixture ratio between silane and ammonia is in a range which triggers a reaction between the gases to create solid silicon nitride (SiN.sub.x) where 0.2x<1.0. The temperature may be between 510 C. and 1300 C., or between 600 C. and 1000 C., the pressure may be 0.1 to 10 bar (to ensure high throughput production), the gas composition comprises at least 10% of each constituent gas (silane, ammonia) and the residence time may be from 0.1 to 10 seconds.

[0124] It has been found that most of the reactions in the reactor take place in a length that is less than the diameter of the reaction chamber tube.

Example 5

[0125] Pre-lithiating the particles may be achieved by supplying at least one gas containing Lithium to the reaction chamber. In addition to a gas flow rate of 1 SLM of SiH.sub.4 and 3 SLM of NH3, one may supply a gas flow rate of 1 SLM of lithium trimethylsilane to the chamber of a Free Space Reactor, and heat these reactant gases to a temperature so that thermal decomposition or reduction of the reactant gases could take place inside the reaction chamber. The produced nanoparticles may contain approximately 33 atomic-% Nitrogen, 33 atomic-% Silicon and 33 atomic-% Lithium.

[0126] Further modifications of the invention within the scope of the claims would be apparent to a skilled person.