High Pressure Study On The Phase Transitions Of Boron Nitride Nanomaterials

Hexagonal boron nitride is a kind of layered material,which is very similar to the structure of graphite.It has a plane network composed of B and N alternately arranged together and the atoms in the layer are combined with stronger covalent bonds,the interlayer is connected by relatively weak Van Der Waals force with a layer spacing of3.4?.It is an excellent semiconductor material.h-BN is one of the most widely studied crystalline forms in BN.It is a sp² hybrid structure with many excellent physical and chemical properties,such as high temperature resistance,oxidation resistance,chemical corrosion resistance,high thermal conductivity,good neutron absorption and wave permeability,etc.It has a wide range of applications and is active in many fields,such as mechanical,metallurgical,aerospace,and electronics.High pressure,which is an extreme experimental condition,can effectively change the distance and interaction between molecules or atoms through the application of pressure,and it could lead to the change in the structure of a substance and the formation of new materials.At present,there have been a lot of studies on the high-pressure phase transition of bulk boron nitride materials.However,there are few studies on the phase transition of high pressure structures of nanostructured BN materials under high pressure.In order to investigate the influence of morphology,size and other parameters of BN nanomaterials on their pressure-induced phase transition,we have conducted systematic high pressure studies on BN nanoparticles,nanosheets and nanofibers.1.The boron nitride nanoparticles,nanofibers and nanosheets at atmospheric pressure were characterized by scanning electron microscopy,transmission electron microscopy,Raman spectroscopy and X ray diffraction.we observed that the boron nitride nanoparticles have an average size of about 20-80nm,which are well crystallized in a hexagonal structure.Note that such BN nanoparticles with high crystallinity are the first example that are synthesized by freeze-drying method.The crystallinity of the nanofibers is lower and while the diameter distribution is relatively uniform,all are about 100nm.2.In situ Raman spectroscopy and electron microscopy observation were used to investigate the structure transition of BN nanosheets and nanoparticles under high pressures and the released samples from high pressure.We firstly analyse the high pressure behavior of the E_2g Raman vibration peak and find out the reason for its structural phase transition,and also study the structure after compression.The results show that nanosheets undergo structural phase change at 11Gpa from hexagonal structure to wurtzite structure,and upon decompression this transition is reversible.And we found that the phase transition pressure point is lower than that of 13Gpa for the bulk;While small-sized nanoparticles exhibit completely different high-pressure behavior from the bulk materials and nanoplatelets,that is,no phase transition happen to the BN nanoparticles under high pressure,in which the peak position and half peak width of the Raman vibration peak are almost linearly upshifted as pressure increases.The material becomes amorphous after compression.We further study of the decompressed BN nanoparticle samples by high-resolution electron microscopy and found that its shape changed to a near-circular shape.Such amorphization-induced changes in the morphology of nanomaterials are observed for the first time in BN materials.The results show that there are also significant size-dependent phase transition behaviors in BN nanomaterials under highpressure.Decreasing the BN grain size can cause the decrease of the phase change pressure,while further reducing the size,BN may undergo an amorphization transformation.The underlying mechanism for the amorphization in BN nanocrystals under pressure requires further investigation.3.The structural transformations of BN nanofibers and nanosheets under high pressure and high temperature?HPHT?were studied by using large volume press.After HPHT treatments at 15GPa,1800?,the nanofibers transforms into c-BN dominant product with a small amount of w-BN structure.Treating the nanosheets at18GPa,2000?,10 min conditions results in a c-BN polycrystalline sample with very small amount of w-BN.It should be noted that the polycrystalline c-BN samples prepared by us mainly contain c-BN nanocrystals with size of about dozens of nanometers,which are well connected with each other.Our results might be useful for the synthesis of c-BN polycrystalline sample at more gentle HPHT conditions.