Additive-Assisted Synthesis Of Boride, Carbide, And Nitride Nanomaterials

This thesis is focused on the design and applications of a new solid-state synthesis technology-additive assisted synthesis (AAS). The contents are related to the preparation of several hydride, boride, carbide, and nitride nanocrystals through three AAS approaches:sulfur assisted synthesis, sulfur-sodium co-assisted synthesis, and magnesium-hydrolysis assisted synthesis. The reaction mechanisms and crystalline developments of these AAS processes are discussed in the thesis. We believe that these AAS results may provide a possible sample for the research of chemical reaction dynamics, and offer some new ideas for low-temperature synthesis of boride, carbide and nitride nanomaterials.(1) Sulphur assisted synthesis of nitride nanomaterials.Nitrides are important technological materials due to their specific properties including thermal, mechanical, and chemical stabilities, wide band-gap, and electrical conductivity. Thus, nitrides can be applied in many fields, for example, as ceramics (Si3N4, MgSiN2, A1N, TiN, TaN), abrasion-resistant coatings (TiN, ZrN, CrN), superhard materials (IrN2, OSN2), light semiconductors (GaN, InN), Energy storage materials (L3N, VN, CrN), catalysts (VN, Fe4N), lubricants (h-BN), superconducting films (ZrN). Traditionally, nitrides are prepared at high temperatures (>1000℃) under the protection of inert gas (N2, NH3, H2), and the products are usually mix of nano-and micro-sized powders.In this thesis, several nitride (S13N4, TiN, BN, AlN and ZrN) nanomaterials have been synthesized from single element powders, which are assisted with the exothermic reactions of sulphur (S) and sodium azide (NaN3) at 250℃. Elevating the treatment temperature could affect the phases and morphologies of the final products. For example, when the temperature rasied from 250℃to 500℃, theα/βphase ratio of Si3N4 will reduce from 9 to 5; as for TiN crystals, a shape evolution from quasi-spherical nanograin to octahedron, and finally dendrites could be found during the elevating of temperature; BN quasi-hollow spheres (400℃) with a wall thickness of～30nm can be obtained instead of BN membranes (250℃) with～2 nm thickness; AlN short-rods and ZrN octahedron will also be found at a treatment temperature above 400℃. Two reaction mechanisms are discussed in this thesis:①elf-ignite to sulfide intermediate first, following experience a solid-state metathesis reaction to nitride;②elf-ignite first, following experience a self-propagation high-temperature synthesis to nitride. Nitride nanomaterials can also be obtained at a low temperature if we use I2 or NH2CONHSNH2 instead of S, or NaNH2 instead of NaN3, or oxides with Mg powders instead of single elements.(2) Sulphur-sodium co-assisted synthesis of boride, carbide and nitride nanomaterials.Boron and carbon can form strong chemical bonds with other elements, which make the borides and carbides have stable chemical properties and interesting light-, or electro-, or magnetic-, or thermo-, or mechanical properties, along with various applications:like selective catalysts (amorphous borides like Fe-B, Co-B, Ni-B), superconductors (MgB2,NbB2, MoB, W2B), neutron absorbing material (B4C), luminescent material (SiC), superhard materials (c-BN), machining material (TiC), etc.. Similar to the preparation of nitrides, borides and carbides are often be synthesized under high temperatures and the protection of inert gas, and the products are usually have widely size and morphology dispersion, which may affect their applications in various fields.In this thesis, a new sulphur-sodium co-assisted synthesis method is developed based on the sulphur assisted synthesis (SAS). In this method, the exothermic reactions of S and Na will start at～150℃, with the released impulsion and heat energy, oxides can be deoxidized by reducing agents (Mg, or active carbon, or amorphous boron) and transformed into related niride (Si3N4, TiN, BN, AlN, VN, MgSiN2), carbide (TiC, WC, ZrC, NbC, SiC), or boride (LaB6, CeB6, PrB6, SmB6, EuB6, TiB2, ZrB2, NbB2) nanomaterials.(3) Magnesium-hydrolysis assisted reduction and conversion of oxides.Oxides are the most common chemicals in nature and they are the raw materials to produce related single elements or other compounds. Owing to their stable chemical properties, oxides always need high temperatures (>1000℃) and high-activity reducing agents (H2, CO, C, Li, Na, Mg, etc.). Thus, it is meaningful to elaborate the reduction and transformation mechanism of oxides at low temperatures.In this part, the exothermic reaction of magnesium hydrolysis is utilized to assist the reduction and transformation of oxides. The exact mechanism should be as following:the Mg hydrolysis reaction starts at 150℃, with the released H2 and heat energy, oxides can be deoxidized by reducing agents (Mg, H2, or active carbon, or amorphous boron) and transformed into related compounds, such as, nitrides (TiN,VN,BN,AlN,CrN) from ammonia; carbides (SiC,TiC,VC,WC,W2C,ZrC MoC,NbC) from active carbon; or borides (TiB2,MoB2,DyB4,ErB4,YB4,LaB6,CeB6,SmB6,EuB6) from amorphous boron. A self-made temperature-testing instrument is used to observe the reaction inside the autoclave. We found that, when the reaction happened at～150℃, the inner system will experience a temperature shock from 150℃to a temperature above 800℃. Then, it will cool to 200℃within several seconds.