| A new method is applied to prepare silver-tin oxide composite powders making the best of properties of raw materials and hydrothermal method. Two experimental programs are designed using AgNO3 and Na2SnO3 as raw materials: 1) Na2SO3 is used as the complex agent and reductant of Ag+. 2) H2C2O4 is used as the reductant of Ag+ and NH3 as the complex agent of Ag+. Intensive studies are conducted for the hydrothermal preparation process and structure and properties of silver-tin oxide composite materials of the two programs.Hydrothermal systems of two experimental programs, including Na2SnO3 solution system, Ag-SO32--H2O system, Ag-NH3-H2O system and Ag-NH3-C2O42--H2O system, are subjected to thermodynamic research by MATLAB calculating and mapping software. In the Na2SnO3 solution system, when pH<0, dominant species is free Sn4+. When 04+, Sn(OH)3+, Sn(OH)22+, Sn(OH)3+ and Sn(OH)4 complex ions. When 24 complex ion. When 74, Sn(OH)5- and Sn(OH)62- complex ions. When pH> 12, dominant species is Sn(OH)62- complex ion. Sn(OH)4 precipitate can be formed by partial hydrolysis of Na2SnO3 solution. When pH value ranges from 1.2 to 8.4, [Sn]T in the Na2SnO3 solution drop to 1×10-3mol·L-1. Under hydrothermal conditions, SnO2 crystals are formed by dehydration and crystallization of Sn(OH)4 precipitate, which increase greatly deposition ratio of tin in the Na2SnO3 solution and [Sn]T in the solution is desrease to 1×10-1mol·L-1. In the Ag-SO32--H2O system, when [SO32-] is less or larger than 0.5mol·L-1 dominant species is Ag(SO3)23- and Ag(SO3)35- complex ions, respectively. Free Ag+ in the Ag-SO32--H2O system can form complex ions and also can form Ag2SO3 precipitate with SO32- at the same time. [Ag]T in the Ag-SO32--H2O system vary with pH value and [SO32-]T. When [SO3(2-0]T and pH value respectively equals to 1.8mol·L-1 and 10.0, theoretical [Ag]T is 0.84mol·L-1. In the Ag+-NH3-H2O system, as long as [NH3] is larger than 1× 10-3mol·L-1, dominant species is Ag(NH3)2+. Free Ag+ can form complex ions with NH3 and also can form AgOH with OH- in the Ag+-NH3-H2O system. [Ag]T in the Ag+-NH3-H2O system vary from [NH3]T and pH value. When [NH3]T equals to 2mol·L-1 and pH value equals to 9, theoretical [Ag]T is 1.00mol·L-1. When oxalic acid solution is added into the Ag+-NH3-H2O system, Ag+ in the Ag+-NH3-H2O system may form Ag2C2O4 precipitate with C2O42-. [NH3]t equals to 2mol·L-1 and pH value is larger than 4, [Ag]T in the Ag+-NH3-H2O system keep with relatively low concentration. When [C2O42-]T equals to 0.5mol·L-1 and pH value equals to 8, theoretical [Ag]T is 6.24×10-5mol·L-1.In the system of silver tin oxide composite powders obtained by hydrothermal reduction of sodium sulfite, reducing reaction of complexes of Ag+ and SO32- and crystalling reaction of Sn(OH)4 respectively produce H+ and OH- under hydrothermal condition, which accelerate the two reactions and make silver and tin oxide copercipitate ultimately. The obtained composite powders are mainly flake about 300nm thickness and there are a small amount sphere particles. The powders have homogeneous distribution of tin and good crystal. It conduct contrasting experiments of silver powders and silver tin oxide composite powders prepared by hrydtothermal reduction of sodium sulfite and silver tin composite powders at different pH value conditions. The result shows that there are different structure and surface topography between silver powders and silver tin oxide composite powders. Silver powders are similar sphere with 300nm diameter, however silver tin composite powders mainly are irregularly flake with 300nm thickness and obviously show preferred orientation in the (111) crystallographic face diretion. There are also differient structure and surface topography among silver tin oxide composite powders at different pH value conditions. When initiative pH value of precursor of hydrothermal reaction is 10.00, obtained powders mainly are irregularly flake with 300nm thickness. When initiative pH value of the precursor is 8.49, obtained powders consist of flake and sphere particles. When initiative pH value of the precursor is 6.51, obtained powders are sphere with 200nm diameter. Moreover, oriented index of silver crystal in (111) crystallographic face direction of silver tin oxide composite powders at different pH value conditions decrese with initiative pH value decreasing, is 2.58, 1.63 and 1.12, respectively. Therefore, its oriented index is closely related to flake structure of the composite powders, which silver crystal of flake particles show preferred orientation and sphere particles don't show preferred orientation. Formation mechanism of flake silver tin oxide composite powders due to role of hydrogen bond between Sn(OH)62- and Ag(SO3)23- complex ions, which change crystallization behavior of silver and get flake silver tin oxide composite powders. Therefore, beside interaction of reducing reaction of Ag+ and crystaling reaction of Sn(OH)4 is multual promotion for producing and consuming of H+ or OH-, but also there are inner factors changing microstruction of silver tin oxide composite powder.In the system of silver tin oxide composite powders obtained by hydrothermal reduction of oxalic acid, H2C2O4 is used as a coprecipitate of Ag2C2O4 and Sn(OH)4. H2C2O4 solution is added into mixed solution of Ag(NH3)2+ and Sn(OH)62- adjusting pH value of the solution to 4.40-9.15, which can realize coprecipitation of Ag2C2O4 and Sn(OH)4. Under hydrothermal conditions, reducing reaction of Ag2C2O4 and crystalling reaction of Sn(OH)4 respectively produce H+ and OH-, which promote each other and realize copercipitation of silver and tin oxide. The obtained powders are regular sphere with 200nm diameter and have homogeneous distribution of tin and good crystal.It is ascertained the experimental conditions of hydrothermal reduction of sodium sulfite: Silver concentration is 0.74mol·L-1. pH value is more than 9.2. Hydrothermal temperature and time is 160℃and 4h, respectively. It is ascertained the experimental conditions of hydrothermal reduction of oxalic acid: Molar ratio of oxalic acid and silver is 1 - 1.5. pH value is 8-8.4. Hydrothermal temperature and time is 160℃and 4h, respectively.Tin oxide crystals in silver tin oxide sinter samples obtained by hydrothermal reduction of oxalic acid are preferentially oriented in the (110) crystallographic face direction and its oriented index reach 5.0 when sintering 4 hours at 750℃. The oriented index of silver crystals in silver tin oxide sinter samples obtained by hydrothermal reduction of sodium sulfite is higger than in silver tin oxide composite powders obtained by hydrothermal reduction of sodium sulfite and up to 4. In silver tin oxide sinter samples obtained by hydrothermal reduction of oxalic acid, sinter temperatures and time impact density, hardnessthe and metallographic phase.The results of the metallographic analysis show that tin oxide particles of sinter samples set into silver matrix and its distribution in silver matrix is homogeneous. The best sinter condition: the sinter temperature and time is 750℃and 3h, respectively. In silver tin oxide sinter samples obtained by hydrothermal reduction of sodium sulfite, there are reticulated conglomeration of tin oxide particles in silver matrix. The density and hardness of silver tin oxide sinter samples obtained by hydrothermal reduction of sodium sulfite are less than and its resistance ratio is more than of silver tin oxide sinter samples obtained by hydrothermal reduction of oxalic acid. Therefore, performance of silver tin oxide sinter samples obtained by hydrothermal reduction of oxialic acid is better than of silver tin oxide sinter samples obtained by hydrothermal reduction of sodium sulfite.
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