PROPERTIES OF POWDERS OF THE SYSTEM Sn(IV)--Sb--O AND THE RESISTIVITY OF RESISTIVE THICK FILMS BASED ON THEM
Tin dioxide powder doped with oxides of antimony are widely used as the conducting phase of high-resistance thickf i l l resistors made of composites [1]. The dispersity and structural-morphological properties of such powders determined by the method of production have a direct effect on formation of the microstructure and the mechanism of electrical conduction in the f i l l . The literature [2, p. 78] contains information only on the manner in which the resistance of such resistors is affected by the form of particles of doped tin dioxide produced by two laboratory methods. We made a study of the chemical, phase, and grannlometric composition, dispersity, and shape of particles of the system S n ( I V ) S b O obtained by four commercial methods. We also studied certain electrical properties of resistive films obtained from these particles. We used powders prepared by the following method: calcining together with hydrates of tin(IV) oxide and antimony(III) oxide precipitated from solutions of their chlorides (TU 06709-87) powder 1; calcining of powders obtained by drying (during atomization) of aqueous solutions of tin oxalate and antimony tartrate (TU 6-09-27-127-86) powder 2; calcining of products obtained by the heterogeneous oxidation of tin(II) sulfate and antimony(III) oxide by hydrogen peroxide (TU 6-09-27-127-86) powder 3; homogeneous oxidation of tin(II) sulfate and antimony(V) chloride by hydrogen peroxide (TU PG 430.411.02) powder 4. The materials were chemically analyzed by the method described in [3], which allowed us to establish the content of Sb(III) and Sb(V) ions. An x-ray diffraction analysis was performed on a DRON-0.5 diffractometer; the composition of the vapor was determined with an MI-1305 mass spectrometer. The granulometric composition of the powders was studied using an "SK Laser Micron Size PRO-7000" laser analyzer, while specific surface was determined by the method of mercury porometry. The density of the powdered materials was determined on an "Auto True Denser MAT-5000," and particle shape and size were evaluated with a "Cambridge Stereoscan $4-10" scanning electron microscope. The latter was also used to study the microstructure of longitudinal and transverse sections of resistive thick films. Thick-film elements were made by a standard technology metal-screen printing of patterns on a ceramic substrate from material VK 94-1, followed by heat treatment in air in a Pt~K-8 electric conveyor furnace. Film thickness was 25-30 /zm. The paste used in the printing process includes powders of the conducting phase, glass of grade S 279-2 [4], and an organic binder. The binder was burned off during firing of the films. In our study, the contact areas were made using a silver-palladium paste that had been pressed onto the substrate beforehand. The specimens had the form of squares with dimensions of 2 × 2 mm. The resistance of the films was measured at room temperature, while the temperature coefficient of resistance (TCR) was determined within the range 293-443 K by means of an Shch302 digital ohmmeter. It is known [5, 6] that Sn 1 _xSbxO2 solid solutions are formed in the system Sn(IV) S b O , the composition of these solutions being determined by the temperature and duration of the production process. Two-stage synthesis at 873 and 1270 K and holding for 16 and and 14 days, respectively, produces stable equilibrium phases with x _< 0.12 (9.7%).* There is as yet no consensus on the valence of the antimony in these phases. A reduction in temperature leads to the formation of solid solutions with a high concentration of antimony. Studies have shown that thick-film resistors formed with conducting-phase powders obtained at 1270-1320 K over a treatment time of 2-4 h have the optimum electrical properties. There is no advantage to a longer heat treatment. Thus, all commercial powders are produced under these condkions, regardless of the synthesis method employed. We can therefore