Sintering of tin oxide and its applications in electronics and processing of high purity optical glasses

Sinterização de óxido de estanho e sua aplicação em eletrônica e no processamento de vidros ópticos de alta pureza

Sintering of tin oxide and its applications in electronics and processing of high purity optical glasses

(Sinterização de óxido de estanho e sua aplicação em eletrônica e no processamento de vidros ópticos de alta pureza)

J. A. Varela¹, L. A. Perazolli¹, J. A. Cerri², E. R. Leite², E. Longo2

¹LIEC, Chemistry Institute, UNESP, P.O. Box 355, Araraquara, SP, Brazil

[e-mail], [e-mail]

²LIEC, Chemistry Department, UFSCar, P.O. Box 676, S. Carlos, SP, Brazil

Abstract

Tin oxide is an n type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of aliovalent oxide doping. The sintering and grain grow of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnO₂ powder the surface state depends only in the interaction of atmosphere molecules with the SnO₂ surface. Inert atmosphere like argon promotes oxygen vacancy formation at the surface due to the reduction of SnO ₂ to SnO at surface and liberation of oxygen molecules forming an oxygen vacancy. As a consequence, surface diffusion is enhanced leading to grain coarsening, but no densification. Oxygen atmosphere inhibits the SnO₂ reduction decreasing the surface oxygen vacancy concentration. Additions of dopants with lower valence at sintering temperature create extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide. Cobalt and niobium doped SnO₂ ceramics exhibit varistor behavior, which can be applied in electronics. Moreover, SnO₂ ceramics are chemically inert and can be applied in form of crucibles to melt some optical glasses.

Keywords: Tin oxide, sintering, varistor, inert crucibles.

Resumo

Óxido de estanho é um material semicondutor do tipo n com comportamento altamente covalente. O transporte de massa neste óxido depende do estado da superfície promovido pela atmosfera ou pela solução sólida devida a dopagem de óxido aliovalente. A sinterização e o crescimento de grão deste tipo de óxido na forma de pó é então controlado pela atmosfera e pela formação de vacância de oxigênio extrínseca. Para o pó de SnO₂ puro o estado da superfície depende somente da interação das moléculas da atmosfera com a superfície do SnO₂. Atmosferas inertes como a de argônio promovem a formação de vacâncias de oxigênio na superfície devido a redução de SnO₂ para SnO na superfície e liberação de moléculas de oxigênio formando vacâncias de oxigênio. Como conseqüência, a difusão via superfície é aumentada originando crescimento de grão mas não densificação. A atmosfera de oxigênio inibe a redução de SnO₂ diminuindo a concentração de vacâncias de oxigênio na superfície. A adição de dopantes de menor valência na temperatura de sinterização cria vacâncias de oxigênio carregadas extrínsecas, que promovem transporte de massa no contorno de grão levando a densificação e crescimento de grão deste óxido policristalino. Cerâmicas de SnO₂ dopadas com cobalto e nióbio exibem comportamento varistor, que pode ser aplicado em eletrônica. Alem disso, cerâmicas de SnO₂ são quimicamente inertes e podem ser aplicadas na forma de cadinhos para a fusão de alguns vidros ópticos.

Palavras-chave: Óxido de estanho, sinterização, varistor, cadinhos inertes

INTRODUCTION

Tin oxide polycrystalline ceramics are n-type semiconductors that have been widely used as gas sensors [1, 2], as electrodes for electric glass melting furnaces and, in thin films, as electrochromic devices, crystal displays, photodetectors, solar cells and protective coatings [3-8]. However, the use of SnO₂ ceramics is limited by the low densification of this oxide during sintering due to the dominance of non-densifying mechanisms for mass transport such as surface diffusion or evaporation-condensation [9, 10]. The densification can be obtained by hot isostatic pressing (HIP) [11] or with the help of an additive like MnO₂ [12, 13], CuO [10, 13-14], Li₂CO₃ [2], ZnO [2, 3], Nb₂O₅ [6, 7], Fe₂O₃ [3] or Co₂O₃ [3]. If liquid phase formation can explain the influence of some of these additives on the sintering behavior of tin oxide (CuO or Li₂CO₃), it is not the case for most of them. So the solid state diffusion is enhanced by the formation of defects due to the dissolution of additive derived species and oxygen vacancies in the SnO₂ network [16-17]. Since tin oxide ceramics are very sensitive to the atmosphere, the interaction of gases with the SnO₂ surface promotes charge transfer and defects creation during sintering of this oxide [18].

Varistor materials with high nonlinearity in their current-voltage characteristics are used as protecting device against voltage transients in electronic and industrial equipment and as surge arrestors [19]. Varistors based on ZnO ceramics present a very high nonlinear coefficient (a = 50) [20]. Other varistor systems have been described in the literature and are based on the formation of composites of titania-doped semiconductors [20-25]. However, the nonlinearity coefficient of this material is very low (2 < a < 12) compared with that of the multicomponent ZnO varistor. Recently, nonlinear electrical properties (a = 45) were obtained for a basic ceramic system based on SnO₂ CoO Nb₂O₅ [26].

PbO-BiO_1/2-GaO_1/2 based glasses (HMO) are good candidates for optical applications due to some of their particular characteristics, such their high refraction indices and high transmission in the ultraviolet (UV), visible and infrared (IR) regions. A limiting stage of the glasses process in this system is the corrosion caused by the melting of glasses in all crucibles used, such as noble metals (platinum or gold) and alumina. The incorporation of crucible materials into the glass composition may reduce the transmission level, the cut-off in the UV - Visible and IR regions, and the thermal stability. SnO₂ crucible was tested for PbO-BiO_3/2-GaO_1/2 melted glass. Optical and thermal analyses showed, in some cases, advantages over the use of platinum and alumina crucibles [27]^.

The objective of the present work was to study the SnO₂ sintering behavior, to obtain dense SnO₂ ceramics, seeking for its potential applications in electronics and in processing of high purity optical glasses.

EXPERIMENTAL PROCEDURE

SnO₂ based powders pure and containing dopants like MnO₂, CoO, Nb₂O₅ and others were prepared by conventional mixing of SnO₂ and the commercial powders. Appropriate quantities were ball mixed for 1 h in a nylon vessel containing dense zirconia balls and ethylene glycol. The alcohol was then evaporated at 60°C for 12 h.The powder, after grinding in an agate mortar, was sifted through a 250 mm opening sieve. Them the powders were isostatically pressed at 210 MPa to form cylindrical pellets.

The pellets were sintered in an horizontal dilatometer and electrical alumina tube furnace with one closed end, where temperature was controlled by a Pt/Pt-13% thermocouple inserted into the area of the hot zone. For the purpose of an adequate gas input into the furnace, a thin alumina tube open at both ends was passed through a rubber stopper and connected by rubber tubing to the gas system. Another Pt/Pt-13%Rh thermocouple was used to measure the temperature of the sample placed inside the tube furnace. The exhausted gases were bubbled in oil after passing through an opening alumina tube.

Atmosphere control was maintained by metering the bottled argon or oxygen gases through a flow meter and then was passed through a drying column and bubbled through a gas-washer used to introduce a specific amount of water vapor to the argon. The argon that was bubbled through the gas-washer was passed through a heated rubber tube, to prevent condensation of the water vapor, into the tube furnace. The partial pressure of water vapor mixed with argon was measured at the entrance of the tube furnace by weighing the amount of water collected by a tube containing zeolite and silica gel, in a given period of time.

Bulk densities of SnO₂ compacts were measured by displacement using the Archimedes method. Pore size distribution was determined in each sample using mercury porosimeter Micromeritics model 9210. The mid pore diameter was defined as the pore corresponding to the mercury pressure where half of porosity was achieved. Surface areas of green sample and sintered samples were measured from the pore size distribution curves as described in previous work [18].

The micrographs were obtained by SEM (Scanning Electronic Microscopy) and the ceramic phases were determined by X ray diffraction and EDS. A stabilized voltage source and two digital multimeters were used for electrical characterization of the samples. The nonlinear coefficient a was obtained by linear regression of points in a log scale around 1mA.cm^-2 and the breakdown electric field E_B was obtained at this current density. Mean average grain size was measured from micrographs obtained by SEM and using the intercept method [28].

The crucibles for glasses melting resistivity were prepared by slip casting in a plaster of Paris mold, from an aqueous suspension of SnO₂ doped MnO₂. Ammonia polyacrilate was used as dispersant. After slip casting, the crucibles were dried at 100°C for 12 h and sintered at 1300 °C for 4h. After sintering the crucibles showed a density higher than 99% as reported by Cerri et al [29]. The glasses samples were characterized in relation to the UV-Visible transmission, by IR spectrum and by Differential Scanning Calorimetry (DSC). The contamination of the glasses was measured using SEM-EDS (scanning electronic microscopy with X-ray microanalyses)

RESULTS AND DISCUSSION

Influence of Atmospheres on the Coarsening and Pore Size of SnO₂

The results after characterization of SnO₂ samples sintered in dry argon and oxygen during 1 h in several temperatures are listed in Table I. As observed in this table, the pore size has increased with temperature without densification. The decrease in surface area, after sintering in dry argon, from 4.28 m²/g at 1000 °C to 0.99 m²/g at 1250 °C with no densification is related to pore growth from 0.110 µm at 1000 °C to 0.512 µm at 1250 °C.

It was determined that pore growth is due to elimination of small particles by transferring the mass to large particles, the exact mechanism of grain growth cannot be defined at this point. Both Ostwald Ripening for grain growth controlled by fluid phase (evaporation/condensation) or fast grain boundary motion can perfectly explain this pore growth behavior.

As described formally [18] for a sintering process with no densification the total pore volume (Vp) remains constant and surface area (S) is related to mean pore size (dp):

where k^* is a constant related to the pore geometry (k^* = 6 or 4 for cylindrical or spherical pores, respectively.

Fig. 1 shows the plot of surface area as function of the inverse of mid pore diameter of SnO₂ pellets, sintered in dry argon, for temperatures ranging from 900 to 1250 °C and for time ranging from 30 to 240 min. As observed in this figure the plot is a straight line that can be extrapolated to the origin with k* = 4.8. This result demonstrates that during sintering of SnO₂ the pore geometry remains constant. Moreover, the value for the geometric factor k* obtained in these conditions is intermediate between the spherical and cylindrical geometry. This is expected since the SnO₂ compacts have particles relatively uniform and equiaxial.

Fig. 2 shows two SEM micrographs obtained in SnO₂ pellets sintered in dry argon in two different sintering temperatures (900 and 1200 °C). Both micrographs show the same characteristics but in different scale. This is an indicative that the microstructure remains uniform during the process of grain growth.

Sintering atmosphere has large effect on the sintering of SnO₂ according to reaction (1). In a dry argon atmosphere one possibility is the evaporation of SnO₂ forming SnO (g) and O₂ according to:

The SnO (g) and O₂ can be combined and precipitate in large particle surface leading to grain growth (Ostwald Ripening). Other possibility is the evaporation from convex surface of two neighbor particles with precipitation of SnO₂ in the neck leading to neck growth up to critical value and then grain boundary motion. The thermodynamic calculated value for the free energy of reaction (1) at 1485 K is 67 Kcal/mol, corresponding to oxygen partial pressure of 2 x 10^-7 atm. This value is not far from that obtained experimentally by Hoening [20] at the same temperature (3.5 x 10^-7 atm), indicating that evaporation-condensation might control the coarsening of SnO₂ ceramics for temperatures higher than 1200 °C.

Mass spectrometry was used to determine desorbed species from the SnO₂ surface for temperatures up to 1500 °C in dry helium. As shown in Fig. 3, between 300 and 600 °C, CO₂, OH and H₂O are desorbed from the SnO₂ surface. Desorption of those species should result in a high oxygen vacancy concentration at SnO₂ surface. The evaporation of oxygen is effective for temperatures above 1200 °C, as shown in this figure.

The reaction (2) is effective for neutral or reducing atmosphere resulting in reduction of Sn⁴⁺ to Sn²⁺ and creation of oxygen vacancies at the SnO₂ surface. In oxygen atmosphere the rate of evaporation is reduced as observed in Table I in agreement with reactions 2.

Water vapor was found to accelerate pore growth during sintering. As observed in Table I pore growth is accelerated with water vapor partial pressures. This is due to chemical interaction of water molecules with the SnO₂ surface and formation of oxygen vacancies after desorption of species, as observed by mass spectrometry (Fig. 3).

Effect of Transition Metal Oxide Addition on the Sintering of SnO₂

Transition metal oxides such as MnO₂ and CoO form limited solid solution in SnO₂ with formation of extrinsic oxygen vacancies. The addition of 0.5 mol% of MnO₂ to SnO₂ leads to 95% of theoretical density of SnO₂ [21]. However, there is a controversy about the sintering mechanism of this system. Whether there is a formation of liquid phase is not known since there is no report on the eutectics or phase diagram of this system. However, the formation of MnSnO₃ and Mn₂SnO₄ is reported in the literature [21]. The addition of small amounts of CoO to the SnO₂ (>0.5 mol%) also leads to an increase in density of this ceramics. Fig. 4 shows the linear shrinkage as function of temperature for both systems, containing 0.5 to 2.0 mol% of MnO₂ or CoO. As observed in this figure the linear shrinkage starts at lower temperatures as the CoO or MnO₂ concentration increases. The final relative densities of samples sintered in the dilatometer, using a constant heating rate of 10 °C/min, for temperatures up to 1400 °C are higher than 99%. This indicates that these dopants are extremely effective in promoting densification of SnO₂, even for low dopant concentrations.

Although the two systems show similar densification behavior, the micrographs of Fig. 5 show the presence of trapped pores for the CoO doped SnO₂ indicating that the nondensifying mechanisms appear to be dominant at lower temperatures when compared to the system doped with MnO₂.

X ray diffractograms for compositions SnO₂ + 8 mol% MnO₂, before and after sintering at 1400 °C with a constant heating rate of 10 °C/min show that only SnO₂ phase is observed after sintering, indicating the formation of solid solution even after the cooling. Otherwise, the XRD for compositions SnO₂ + 8 mol% CoO sintered in the same conditions show two phases after cooling, SnO₂ and Co₂SnO₄. Considering that this second phase is not observed at high temperature X-ray diffraction, this phase should be formed during cooling.

Thermal analysis (DTA/TG) of the SnO₂.CoO and SnO₂.MnO₂ up to 1550 °C did not show the presence of endothermic reactions related to liquid-phase formation. Moreover, an increase in the dopant concentration does not promote a change in the microstructure and shifts the temperature at which the linear shrinkage starts, which do not characterize a eutectic system.

The absence of experimental evidence of an eutectic liquid suggests that the densification observed in the studied systems is not associated with liquid-phase sintering. Thus, the sintering of both systems containing up to 2 mol% of MnO₂ or CoO seems to be controlled by diffusion in the solid state. As is well known, the diffusion-controlled processes of oxides depend strongly on the defect chemistry and on the nature and concentration of foreign atoms. Therefore, modifying the SnO₂ crystals by adding CoO or MnO₂ could alter the sintering mechanisms and rate. SnO₂ is an n-type semiconductor with native oxygen vacancies compensated by electrons. For diffusion-controlled processes like sintering, the slowest diffusion species should determine the overall rate of the sintering. Since the oxygen is the rate controlling diffusion species [22] in SnO₂, increasing the oxygen vacancy (V_O) concentration would increase the sintering rate. Thus considering that for temperatures higher than 1100 °C Mn(IV) is reduced to Mn(II), the following reactions are proposed for both systems:

These results are in accordance with authors who showed that Li⁺ also acts as an acceptor in SnO₂, and hence improve the densification rate [17]. Contrary to these results, Nb₂O₅ doped SnO₂ does not improve the densification of the system due to the fact that Nb₂O₅ is an electron donor, which increases the electrical conductivity of SnO₂ [23]. Then the following equation controls the defect formation in SnO₂:

To rationalize the role played by the dopants and vacancies in the electrical properties, the electronic structure of doped and undoped SnO₂ was calculated as function of dopants type (niobium, cobalt and chromium) and concentration, as well as with oxygen surface concentration. The calculations were made with Gaussian 94 package at the Roothann Hartree Fock (RHF) level of theory. The results show that the energy gaps is reduced when oxygen vacancies are formed (Table II), the system becomes a semiconductor of n type and electronic conduction is therefore increased. When the dopant Nb⁵⁺ is added there is also an increase in the electrical conductivity of the SnO₂ ceramic. Otherwise the addition of Co²⁺ and Mn²⁺, although forming oxygen vacancies in the SnO₂ structure, decrease the electrical conductivity of this ceramic.

Varistor Behavior of SnO₂ Based Ceramics

Using the system SnO₂-CoO-Ta₂O₅ it was obtained densities higher than 97% and according to the XRD analysis (Fig. 6i), no other phase besides SnO₂ was observed. Fig. 6ii shows the microstructure characterized by heterogeneous grains with mean grain size of 6.9 mm

The addition of Ta₂O₅ in small amounts to the SnO₂ ceramics leads to an increase of electronic conductivity in the SnO₂ lattice due to substitution of Sn⁴⁺ for Ta⁵⁺, according to:

Fig. 7 shows the log plots of the applied electric field against current density for to system. The best nonlinear coefficient (a = 13) was obtained using Ta₂O₅ instead Nb₂O₅.Table III summarizes the results. The introduction of either Nb₂O₅ or Ta₂O₅ to the ceramics increases grain size and decreases the grain resistance. As observed in the Table III, the Ta₂O₅ doping intensified the nonlinear electrical properties. Moreover, both system present low leakage currents. The leakage current originates at the grain boundaries and determines the amount of watt loss that a varistor is expected to generate upon application of steady-state operating voltage. Thus, it is important factor to be considered in varistor fabrication. Higher breakdown electric fields E_B due to reduced grain size and higher barrier voltage per grain u_b were obtained for the SnO₂ - CoO - Ta₂O₅ system. It is also observed in Table III that u_b was influenced by the chemical composition.

Application of SnO₂ Crucibles for Processing High Purity Optical Glasses.

The system 40PbO - 35BiO_1/2 - 25GaO_1/2 (PBG) was prepared and melted in SnO₂ crucibles at 850 and 1000 °C up to 4 h. The glasses were analyzed by SEM-EDS and low contamination from the crucible was observed after melting the glasses at 850 °C for 4 h (PBG850-4), Glass contamination by Sn was detected for samples melted at 1000 °C for 2 h (PBG1000-2) as observed in Fig. 8.

The glass transparency of the samples melted in SnO₂ crucibles was evaluated through UV-Visible-IR spectrophotometry in the region between 200 nm and 100 nm. No absorption band due impurities such as manganese or tin was observed.

The experiments showed that the temperature and/or refining atmosphere have strong effect on the color of glass samples. However, melting time for a given temperature not modified either the UV-Visible neither the IR cut-off wavelength value. According to Fig. 8, for 1000 °C, the UV-Visible cut-off wavelength value shifted to a longer wavelength. Samples refined at 1000 °C present reddish color and lower transmission, while the others are clear, with a yellowish coloration. The UV-Visible cut-off shift, the decrease in the maximum transmission and the color change for glass melted at 1000 °C must be related to the tin contamination detected by EDS analysis.

CONCLUSIONS

Sintering of undoped SnO₂ compacts in inert or oxidizing atmosphere leads to coarsening without densification. Reduction of Sn⁴⁺ to Sn²⁺ by evaporation of oxygen at surface and desorption of chemically adsorbed species in the SnO₂ surface control the coarsening at temperatures above 1000 °C by surface diffusion. Evaporation-condensation controls the mass transport for temperatures above 1200 °C and is responsible for coarsening of SnO₂ ceramics.

Densifications above 99% of theoretical density were obtained during sintering of MnO₂ or CoO doped SnO₂ ceramics (up to 2 mol%). Experimental evidences indicate that these dopants act as acceptors leading to reaction of additional oxygen vacancies in the SnO₂ increasing the densification rate.

The sintered SnO₂ –CoO – Ta₂O₅ or SnO₂ –CoO – Nb₂O₅ exhibit varistor behavior with high non-linear coefficient and low current leakage. These systems can be potentially used in electronic devices.

Dense SnO₂ ceramics can be used as crucibles for melting of high metal oxide glasses such PbO-BiO-Ga₂O₃ based glasses. Experimental results are an evidence of no little contamination of Sn in those glasses melted in SnO₂ based crucibles.

ACKNOWLEDGMENTS

The authors acknowledge CNPq, FAPESP and FINEP for the financial support.

(Rec. 04/05/2001, Ac. 15/05/2001)

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Publication Dates

• Publication in this collection
  05 July 2001
• Date of issue
  June 2001

History

• Accepted
  15 May 2001
• Received
  04 May 2001