Local Structure And Thermal Expansion In Nanosized Metal/alloys

Thermal expansion performance decides the working environment,matching properties and serving life of materials.Effective adjustment of thermal expansion can be realized by nanosizing without the change of chemical component,which has been attracting increasing attention for the decisive contribution to improve the thermal mechanical properties and stability of nano device.In nano state of metal/alloys,the special structural feature and distribution can not be well decribed by the average unit cell at long range.Deciphering of local structure and 3D location can provide the powerful theoretical support for the optimization of thermal strain,designing of functional devices and searching for physical origin.This work focuses on several kind of nano metal/alloys.Starting from the study of real thermal expansion of unit cell,a comprehensive method combining X-ray absorption fine structure(XAFS),pair distribution function(PDF)and reverse monte carlo(RMC)was applied to analyse the local structure and spatial distribution in nanosizing metal/alloys.According to the relation among chemical bonding,local structure information and atomic distribution,the significant role of local structure was revealed.Firstly,thermal expansion properties of semimetal bismuth have been successfully tailored by introducing size effect to 13nm.A transition of the CTE(coefficient of thermal expansion)from positive to negative could be observed along c axis in nanosized bismuth.The local structural distortions of the nanosized bismuth obtained from EXAFS and PDF revealed the key role of the nearest bonding environment,especially the reduced nearest bond angle.Distortion as the decrease of nearest bond angle from size effect then would relax under the drastic thermal activation with temperature rising.Further ab initio calculations for surfaces showed the loss of covalent part for metallic-covalent bonds on the surface,which induced the weakening of orientation in chemical bonding and local structural distortion.For face-centered cubic alloy,stacking mode of atoms can play the more important role in the adjustment of thermal expansion than metallic bonds in nanosizing metal.In PtNi alloy nanoparticles,change of chemical component resulted into the evolution of particle shape and transition from positive thermal expansion to negative thermal expansion around 150 °C.Short-range coordination indicated the main distribution of local tetragonal ordering on the surface.Similarity ofstructure from EXAFS confirmed the dominated contributon of heteroatomic pair as Pt-Ni to negative thermal expansion in Pt41Ni59 nanoparticles.Layer-by-layer statistic analysis for the local atomic pairs local coordination environment as totally ordered Ll0 phase retained the key character of Pt-Ni pairs to the negative thermal expansion,which then was amplified in the nanosized PtNi alloy particles.In 3.5nm PtFe magnetic Ll0 alloy,abnormal enhanced negative thermal expansion along c axis was founded.57Fe Mossbauer spectra and EXAFS measurement revealed the existence of novel local chemical disordered structure concealed behind the ordered broadening diffraction peaks.PDF and RMC simulation revealed the surface A1-phase coating and atomic-level stretching chemical strain on the heteroatomic pair along c;axis,which made the dominant contribution to the enhanced negative thermal expansion behavior Meanwhile,around the chemical composition of Llo PtFe alloy,tiny increase of Pt resulted into the much enhanced electrocatalytic activity of hydrogen evolution reaction(HER)in Pt56Fe44.RMC on PDF data clearly demonstrated the segregation of local phase segments as disordered Pt-rich A1 and Pt3Fe L12 phases under the outermost Fe-rich shell in Pt56Fe44 nanoparticles.After the etching of outermost-surface Fe during HER,remained Pt3Fe L12 segregation provided abundant active Pt sites for discharge and electrochemical desorption reaction,and led to local-bonding Pt pairs facilitating the recombination of adsorbed hydrogen atoms,resulting in the excellent HER activity.Atomic insight into regulatory mechanism based on the local structure in nanosized metal/alloys can correlate the chemical bonding,lattice strain and vibration,which will provide a new perspective in development of nanomaterials with high thermal shock resistance and promoting the chemical design of functional properties.