Research On The Self-propagating High-temperature Synthesis Mechanism Of Tin Oxide Nanowires

Self-propagating High-temperature Synthesis(SHS),namely combustion synthesis,is an important method to obtain ceramics,intermetallics and some other advanced materials by means of solid-solid reactions using an igniting system and propagation of a combustion wave.Compared with the other methods,SHS reactions have some merits,such as high purity of products,low energy requirements,simple apparatus,high production efficient,and realizing synthesis and sintering in one step.In this paper,based on the basic theory of self-propagating high-temperature synthesis,a new large-scale synthesis method of rutile SnO₂ nanowires,which is called self-propagating high -temperature synthesis jetting(SHS-J),was obtained in terms of decomposition of cuprous oxide;and a method(SHS-F) for preparation porousα-Al₂O₃/TiB₂ composite ceramics,which can control the pore size of porousα-Al₂O₃/TiB₂,was also discussed.1) The rutile phase SnO₂ nanowires have been prepared successfully with Al+Cu₂O+SnO_x(x=1,2) system by self-propagating high-temperature synthesis process.The diameters of the nanowires are in the range of 10～150nm,and mainly between 40 and 60nm.Moreover,the lengths of the nanowires are in macroscopic level.The optimum composition of raw materals for x=1 and 2 was determined.A 25.6%percent conversion was realised by combinatorial chemistry using and 19.8g of SnO₂ nanowires was obtained from 57.8g of SnO.2)The SHS forming mechanism of SnO₂ nanostructure was discussed for the Al+Cu₂O+SnO_x system.There are three fundamental processes in the experiment,including combustion reaction,jet-forming and dynamic tension.At first,the fused bath ofα-Al₂O₃,Cu₂O+SnO₂ and Cu+Sn,which were distributed hierarchically according to their density,was obtained by combustion reaction of Al+Cu₂O+SnO_x system.Then Cu₂O decomposed,O₂ was released SnO₂ was gasified and jet out through the micro-holes of theα-Al₂O₃ porous ceramics under itself gas pressure at high temperature;Because the head velocity of the SnO₂ line was far higher than its tail velocity,a dynamic tension occurred and SnO nanowires finally formed.The forming mechanism is different from the conventional mechanisms of V-L-S,V-S,and electrostatic spinning.3) The mechanism of pore formation in porous ceramics prepared by self-propagating synthesis-foaming method was studied.The results show that the oxygen from Cu₂O decomposition at high temperature was the main factor influencing the pore size of porous ceramics.The average pore size of Al₂O₃/TiB₂ increased with increasing additive,overcoming the difficulty in controlling the pore size of porous ceramics by SHS method.However,when the content of Cu₂O was above 10%,the porosity ofα-Al₂O₃/TiB₂ decreased with the increase of Cu₂O content.Moreover,its compressive strength decreased with the increase of its porosity.4) The influence of reactant composition on the structures and the properties of the porousα-Al₂O₃/TiB₂ ceramics was studied.The effective reactant composition range for preparing the porousα-Al₂O₃/TiB₂ ceramics from the system of Al+B₂O₃+TiO₂ was determined.The results show that when Al:TiO₂:B₂O₃=6:1:3,the maximum porosity reached 86%,and when Al:TiO₂:B₂O₃=7:1:2,the highest compressive strength reached 14.2 MPa,5) The hierarchical pore structure was observed in theα-Al₂O₃/TiB₂ ceramics.The pores in mm level presented irregular shapes,and their average diameter was about 200～2000μm.TiB₂ whiskers distributed on the surface of the pores,with diameter of about 1～2μm and length of about 5～10μm,their criss-cross arrangement formed the micron level pores.Furthermore,a large number of honeycombα-Al₂O₃ structures were observed on the surface of the ceramic pores.Their shapes were similar to polygon,their diameters were about 200～300nm,and the wall thickness of the pores was about 50nm.Additionally,a type of three-dimension honeycombα-Al₂O₃ structure was observed,with diameters of about 500～2000nm.It is believed that the above TiB₂ whisker and honeycombα-Al₂O₃ structures have special properties,which will be further studied in the future.