IntroductionIndium tin oxide (ITO) film is an excellent transparent conductive material widely used in liquid crystal displays (LCDs), electroluminescent displays (EL/OLEDs) and various photovoltaic devices. In particular, heterojunction (HJT) solar cells have shown a broader development potential. In order to improve the photovoltaic conversion efficiency of heterojunction solar cells, transparent conductive films need to be deposited on curved or cylindrical solar collectors. The deposition of ITO conductive films on these non-planar substrates requires specially shaped ITO tubular targets as the deposition source. Therefore, the preparation of high-quality ITO tubular targets is a prerequisite for obtaining ITO films with excellent properties on curved or cylindrical substrates, and the shaping and high densification of ITO tubular targets have been a hot and difficult issue in this field. In this work, based on the theoretical model of binary particle grading and the composite spray granulation-cold isostatic pressing moulding process route, the effects of different grading ratios, moulding pressures, times and processes on the properties of powders, billets and targets were investigated.MethodsIn₂O₃ and SnO₂ powders (purity 99.99%, Hunan Ruyang Ruijin Electronic Technology Co., Ltd.) were ball-milled at a mass ratio of 9:1 to formulate indium tin oxide powders with D₅₀ (the particle size corresponding to the cumulative particle size distribution percentage of the samples reaching 50%) of 50 nm and 323 nm, respectively, and were mixed in accordance with different grade ratios. After mixing, anhydrous ethanol (analytically pure, Xilong Science Co., Ltd.) was added to disperse the powder in a ball milling tank, and the mass ratio of zirconia balls to powder in the slurry was 3:1, and the slurry was mixed and then milled in a planetary ball mill at 350 r/min for 8 h. The slurry was obtained at an inlet temperature of 190 ℃, with a slurry solids content of 60% (mass fraction) and 1.5% binder content, Spray granulation was carried out at a feed rate of 100 mL/h. The granulated powder was loaded into a cold isostatic pressing mould with an inner diameter of 6 mm and a length of 39 mm, and then fully vibrated and consolidated before bagging and sealing for vacuum treatment and cold isostatic pressing. The total holding time of the composite pressing curve was the same as that of the single-point pressing curve. The hollow tubular blanks were sintered in a tube furnace at a flow rate of 5 L/min in an oxygen atmosphere, at sintering temperature of 1550 ℃ and holding time of 8 h to produce ITO tubular targets.X-ray diffractometer (XRD, D8 Advance, Bruker) was used to study the phase structure. Scanning electron microscope (SEM, Tecnai-450, FEI) was used to observe the microstructure and elemental distribution with an operating voltage of 8 kV. The particle size distribution of the powders was tested using a laser particle size analyser model LS-609 from Omec. The loose packing density of the powders was tested using a WLD-02 Hall flow meter (funnel method). The density of ITO target was tested by Archimedes method and compared with the theoretical density to calculate the relative density of ITO target. The resistivity was measured by a four-point probe meter (MCP-T700, Mitsubishi chemical).Results and discussionBased on the tightest stacking theory model, the stacking density of ITO powder is significantly improved by binary particle grading, because a reasonable grading ratio can achieve the fine particles to fill the gaps between the larger particles, which effectively improves the loose loading density of ITO powder.The study of ITO tubular target prepared by cold isostatic pressing technology shows that the densification process of billet and target by forming pressure presents three-stage characteristics: Stage 1, the loose powder is rapidly rearranged under extrusion, with narrowing particle gaps and increasing bulk density, resulting in a lower resistivity; Stage 2, the resistance of internal friction between the particles increases, and the rate of densification tends to slow down; Stage 3, when the pressure exceeds the critical value (the limit), the particle plastic deformation or fragmentation occurs. deformation stress), the particles undergo plastic deformation or fragmentation, and the pores are further filled, but too high a pressure will form a pressure gradient due to the friction between the billet and the mould, which may, on the contrary, trigger internal slip leading to a decrease in density. The effect of holding time showed that within 5-11 min, extending the holding time significantly increased the relative density, which was attributed to the full displacement of the powder to fill the pores and the deformation of the particles, which increased the contact area and the interaction force; after more than 11 min, the pore filling tended to be saturated, and the density growth was stagnant. The composite pressing process effectively discharges the residual gas inside the billet through staged pressure release, eliminating the gas compression reaction force and achieving better densification results. The study reveals the key mechanism of cold isostatic pressure parameters on the microstructure regulation of ITO targets.ConclusionsIn this work, the effects of different grading ratios, moulding pressures, times and processes on the properties of powders, blanks and targets were investigated using a binary particle grading-spray granulation-cold isostatic pressing process. The results showed that particle grading improved the density of the powder, spray granulation improved the sphericity of the powder, and increasing the moulding pressure and prolonging the holding time improved the density of the billet. The billet with a relative density of 62.52% was prepared by the composite pressing process at a holding pressure of 200 MPa for 9 min, and the ITO tubular target with a relative density of 99.85% and a resistivity of 1.333×10^-4 Ω·cm was obtained by sintering.