Preparation And Properties Of Bi/PET Layered Composite Shielding Material

In X-ray photography,the grid can effectively absorb the scattered radiation in the non-initial X-ray direction,filter out the influence of scattered radiation on the image,and improve the image clarity.The appearance of the grid is a flat plate with a thickness of 4?8 mm.The internal structure is made up of many ultra-thin shielding strips arranged in the same direction.The two shielding strips are filled and positioned with materials that are easily transparent to X-rays,and the upper and lower sides are packaged with thin X-ray transparent plates to form a grid plate.Seen from the cross section,the arrangement direction of the shielding strips will all converge to one point.Seen from the entire grid,there is a line of convergence.At present,there are still many problems in the manufacture and application of grid.First of all,lead foil is commonly used as shielding material in the existing grid.While lead will seriously endanger human health and cause serious environmental pollution in the process of smelting,processing,using and discarding.Secondly,the thinner the lead foil,the more difficult the processing and the higher the cost.Moreover,the commonly used interspace material is aluminum foil.When the effective X-ray passes along the plane direction of aluminum foil,it will still produce a large degree of attenuation,resulting in the decrease of image brightness and the loss of information.With the development of digital X-ray photography,the shielding layer and interspace layer in the grid will be thinner and thinner.Therefore,it is necessary to explore the preparation of the grid with non-toxic,ultra-thin shielding layer and low absorption coefficient transmission layer.In order to solve the above problems,bismuth,which is nontoxic and has equivalent shielding performance to lead,was selected as shielding material,and PET with low absorption coefficient was selected as interspace material.Bismuth thin film was deposited on PET by vacuum evaporation and magnetron sputtering,respectively.Then,the grid samples were fabricated by laminated object manufacturing technology.Finally,the microstructure,morphology and structure of those samples were analyzed.Using bismuth as the shielding material and PET as the interspace material,the VE-Bi/PET layered composite shielding material was prepared by the vacuum evaporation method and studied.The results showed that the crystal form of the VE-Bi film was rhombohedral,and its microstructure close to PET is equiaxed crystals,which gradually become columnar crystals growing along the evaporation direction as the thickness increased.The surface of the VE-Bi film was relatively flat,and no obvious cracks and holes were found.The Ra of the surface was 152 nm,Rq was 202 nm,and the roughness was low.The thickness of the VE-Bi film was about 20 ?m,and the thickness deviation was within 5%.The combination of Bi and PET was weak,and the bonding strength was ISO 5,or ASTM OB,and Bi and PET were prone to peeling off.Due to the pure bismuth film was relatively brittle,this article chose the tinbismuth alloys Bi99Sn1,Bi95Sn5 and Bi90Sn10 with better toughness as the candidate materials for the shielding layer without significantly reducing its density and ensuring that the material is environmentally friendly and non-toxic.PET was the interspace material.The Bi/PET layered composite shielding material was prepared by vacuum evaporation method and studied.The results showed that when the photon energy was 40-100 keV,the mass attenuation coefficient of the three zinc-bismuth alloys calculated by XmuDat and Win XCOM was close to that of lead,that is,the shielding performance was equivalent to that of lead.The surface of the deposited tin-bismuth film was relatively flat,no obvious cracks and holes were found,and the film thickness was about 20 ?m.The cross-sectional microstructure shape was similar to that of pure bismuth film,and the addition of tin was beneficial to the growth of equiaxed crystals and the formation of columnar crystals.The bonding between the tin-bismuth alloy film and PET was weak,and the bonding strength was ISO 5,or ASTM 0B,and it was easy to peel off.In order to enhance the bond between Bi and PET,using bismuth as the shielding material and PET as the interspace material,the MS-Bi/PET layered composite shielding material was prepared by magnetron sputtering and studied.The results showed that the crystal form of the MS-Bi film is rhombohedral,and the cross-sectional metallographic structure was columnar crystals growing along the sputtering direction.The surface of the MS-Bi film was relatively flat,and no obvious cracks and holes were found.The Ra of the surface was 238 nm,and the Rq was 291 nm.The thickness of the MS-Bi film was about 20 ?m,and the thickness deviation was within 5%.The bonding strength of MS-Bi and PET was ISO 1,or ASTM 4B.Both vacuum evaporation and magnetron sputtering can deposit dense and continuous bismuth film.Compared with VE-Bi film,MS-Bi film had slightly higher roughness,but the bonding strength of MSBi and PET was much higher than that of VE-Bi,which contributed to the preparation of grid.Therefore,in this paper,the MS-Bi/PET composite material was selected as the raw material for subsequent laminated object manufacturing.In order to explore the application of MS-Bi/PET composite materials in CT and PET-CT ASG in the future,lasers were used to cut them into the same porous parts,and laminated them to form a pattern.The average tensile shear strength of the MSBi/PET layered material after PES bonding was 0.952 MPa,the average peeling strength of the T type was 0.34 N/mm,and the MS-Bi/PET layered material had a higher strength after bonding.The cross-sections of the prepared ASG sample molds were in a periodic and repeated arrangement of Bi/PET/PES,and the layers were straight and neat,which provided a reference for the design and application of the new generation of CT and PET-CT ASG.