Figure 5
Characteristic ln E × ln J curves for the 0.05 mol% Ta 2 O 5 doped 0.99SnO 2 · 0.01CoO system sintered at 1300 ◦ C for 2 h and mea-
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The effect of Ta2O5 doping in 0.99SnO20.01CoO on the microstructure and electrical properties of this ceramic were analyzed in this study. The grain size was found to decrease from 6.87 m to 5.68 m when the Ta2O5 concentration increased from 0.050 to 0.075 mol%. DC electrical characterization showed a dramatic increase in the current loss and decre...
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... if oxygen is the controlling species for diffusion, the formation of these defects promotes the densification of SnO 2 based ceramics. The microstructures of the Ta 2 O 5 doped SnO 2 · CoO ceramics are shown in Figs 2 and 3. These micrographs show the presence of few trapped pores inside the grains or at the grain boundary. The increase of the Ta O concentration leads to the decrease in the ceramic grain size. The results of ln E versus ln J for the Ta 2 O 5 doped SnO 2 · CoO ceramics measured at room temperature are shown in Fig. 4. It is observed in this figure that the increase in the Ta 2 O 5 concentration from 0.05 to 0.075 modifies substantially the electrical behavior of the SnO 2 · CoO ceramics. Fig. 5 displays the characteristic ln E versus ln J curves, measured at different temperatures for the system containing 0.05 mol% of Ta 2 O 5 . As expected, the current leakage increase and the non linear coefficient α decrease with increasing temperature testing. The varistor electrical behavior is governed by the presence of electrical barriers at the grain boundaries of the ceramic material. Then the electric field breakdown E r depends on the average number of electrical barrier formed per unit length during sintering ( n ) and on the voltage barrier ( b ), which in ZnO based varistor is about 2 to 4 volts/barrier [16–18]. Thus the following equation relates v and E ...
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... Recientemente se ha incrementado el interés por el desarrollo de varistores con base en SnO 2 . El SnO 2 es un semiconductor tipo n, con estructura tipo rutilo, y presenta poca densificación debido a que el mecanismo de sinterización que predomina a bajas temperaturas es el de difusión superficial y evaporación-condensación a altas temperaturas, esto debido a altas presiones parciales que presenta el sistema (Antunes A. C. et al. 2000). Se ha demostrado que la adición de CoO al SnO 2 permite alcanzar densidades mayores al 97% de la densidad teórica (Brankovi G. et al. 2004 Los varistores con base en óxido de estaño presentan características eléctricas altamente no lineales, semejantes a las de los varistores comerciales de óxido de zinc (Pianaro S. 1995), con la ventaja de que la cerámica de SnO 2 es monofásica facilitando el control microestructural del material, posee elevada resistencia a la degradación (Pizarro A. R. 1996) y además, requiere de concentracio-nes bajas de dopantes para alcanzar buenas características varistoras, y alta densificación, en comparación con los varistores de ZnO (Cerri J.A. et al. 1996). ...
Los varistores, materiales que cambian su resistencia con el voltaje, son dispositivos muy utilizados en la industria para proteger equipos eléctricos y electrónicos de las sobretensiones. Dadas las exigencias actuales de la tecnología se requieren varistores que cubran un amplio rango de voltaje de ruptura, principalmente valores bajos con rápida respuesta, buen comportamiento en servicio y con microestructura sencilla que facilite su conformación. En este trabajo se indica como obtener la materia prima para fabricar varistores con base en SnO2, utilizando el método de precursor polimérico. El polvo cerámico obtenido se caracterizó utilizando difracción de rayos x (DRX) y análisis térmico (TG/ATD). Para determinar su sinterabilidad se realizaron estudios de dilatometría. Las muestras sinterizadas se caracterizaron microestructural, con MEB, y eléctricamente, determinando las curvas características corriente-voltaje de los sistemas estudiados.
... In the last two decades, a vast number of reports addressed the influence of different dopants on sinterability and related electrical properties of SnO 2 -based ceramics [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The addition of acceptor dopants, such as CoO, is essential to generate double Schottky barriers by accumulation of oxygen vacancies, V Ö, at the surface of SnO 2 grains; whereas donor dopants, such as Ta 2 O 5 [8][9][10][11][12][13][14][15][16][17], Nb 2 O 5 [11,18,19] and Sb 2 O 3 [11,18,20] improve conductivity of SnO 2 grains by creating free electrons in the lattice. ...
... In the last two decades, a vast number of reports addressed the influence of different dopants on sinterability and related electrical properties of SnO 2 -based ceramics [7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The addition of acceptor dopants, such as CoO, is essential to generate double Schottky barriers by accumulation of oxygen vacancies, V Ö, at the surface of SnO 2 grains; whereas donor dopants, such as Ta 2 O 5 [8][9][10][11][12][13][14][15][16][17], Nb 2 O 5 [11,18,19] and Sb 2 O 3 [11,18,20] improve conductivity of SnO 2 grains by creating free electrons in the lattice. Through generation of V Öb oth grain boundary mobility and densification are improved, while the addition of donor dopants was reported to play a significant role in SnO 2 grain growth [8][9][10][11][12][13][14]18,19]. ...
... The addition of acceptor dopants, such as CoO, is essential to generate double Schottky barriers by accumulation of oxygen vacancies, V Ö, at the surface of SnO 2 grains; whereas donor dopants, such as Ta 2 O 5 [8][9][10][11][12][13][14][15][16][17], Nb 2 O 5 [11,18,19] and Sb 2 O 3 [11,18,20] improve conductivity of SnO 2 grains by creating free electrons in the lattice. Through generation of V Öb oth grain boundary mobility and densification are improved, while the addition of donor dopants was reported to play a significant role in SnO 2 grain growth [8][9][10][11][12][13][14]18,19]. Therefore, a balanced addition of both donors and acceptors is required. ...
Following the 3 Sn(IV)S˟n (IV) ⇋ Co(II)Sn ̎(IV) + 2 Ta(V)˙Sn (IV) charge compensation mechanism we optimized densification and electrical properties of Ta2O5-doped SnO2–CoO ceramics. We show that incorporation of acceptor dopant Co²⁺ in SnO2 is promoted after the addition of donor dopant like Ta⁵⁺, whereas any surplus of Co would form secondary Co2SnO4 phase. A balanced addition of both dopants is needed to promote densification, and any surplus of donor dopants that remain present at the grain boundaries retard the grain growth and deteriorate electrical properties. Varistor and dielectric properties are then strongly influenced by donor doping. Optimum varistor properties (α = 40, UT = 272 V/mm, IL = 1.2 µA) were measured for the sample with 1 mol% Ta2O5 and the best dielectric properties (ε = 6525; tan(δ) = [email protected]) were measured for the sample with 0.10 mol% Ta2O5 with the largest SnO2 grain sizes.
... As both dopants are pentavalent, we could expect same charge compensation mechanism and similar tendencies in grain growth. Like in Nb 2 O 5 , already the smallest addition of Ta 2 O 5 to SnO 2 -CoO ceramics triggered exaggerated grain growth and improved densification of the ceramic, while for higher additions reduction of grain size and the increase of porosity is observed [33]. This effect has been explained by Tominc et al. ...
... However, acceptor additives lead to highly resistive material [5][6][7][8][9]. When there are no secondary phases at the GBs, defect formation by acceptor and donor dopants in the SnO 2 matrix should be responsible for the origin of the Schottky barriers at the GBs [28,30,33]. The electric conductivity of the SnO 2 -CoO-Ta 2 O 5 system thus be greatly increased with Ta 2 O 5 doping [38,39], where the amount of Co 2 SnO 4 secondary phase consistently decreased. ...
We investigated the effect of pentavalent donor dopant Ta2O5 on microstructure development, electric and dielectric characteristics of SnO2–CoO based ceramics. Already low additions of Ta2O5 (0.05 mol%) effectively reduce the porosity, improve densification and dielectric permittivity and trigger a 3–fold increase in SnO2 growth rate. Rietveld analysis shows that the amount of Co2SnO4 spinel phase drops with the addition of Ta2O5 due to incorporation of Co²⁺ and Ta⁵⁺ into SnO2 structure. With higher additions, however, Ta2O5 segregates to the grain boundaries and hinders SnO2 grain growth, which in turn improves electrical properties. TEM/EDS analysis shows that above 0.5 mol% of Ta2O5 the Co:Ta ratio in SnO2 grains is constant 1:2, which means that a twice lower amount of Ta⁵⁺ is incorporated in the SnO2 structure compared to the Nb2O5-doped SnO2–CoO system. Accordingly, the following charge compensation mechanism is proposed: 3 Sn(IV)S˟n (IV) ⇋ Co(II)Sn ̎(IV) + 2 Ta(V)˙Sn (IV).
... Secondly, Nb and Ta addition to SnO 2 is commonly used in varistor technology to control the crystal size and structure. The addition of small amounts of Ta, for example, leads to smaller crystallite sizes compared to bare SnO 2 (37). Hence, one should only draw loose conclusions from the remarkable agreement between experiment and theory shown here. ...
We find extrinsic conductivity of Ta-doped SnO2 electro-catalyst supports to be maximal at around 1% Ta-doping. Extrinsic conductivity is limited due to two effects: (i) limited dopant solubility due to thermodynamic competition with ternary oxides, and (ii) electron localization at the Ta center leading to collaborative Jahn-Teller distortions and a freezing out of the donor state with increasing dopant concentration.
For antimony-doped tin oxide (ATO), Sb content and oxidation state remarkably influence the densification, microstructure and electrical properties of ATO ceramics. In this work, ATO powders doped with Sb(III) or Sb(V) are prepared via chemical precipitation method and are then consolidated by spark plasma sintering (SPS). Results demonstrate that ATO ceramics with a small content of nanoscaled antimony oxides, either Sb(III) or Sb(V), can be densified through the rapid SPS process. The relative density of 1.5 mol% Sb-doped samples is greater than 96.5 %. However, increased Sb doping concentration, especially for Sb(V), restrains both the densification and grain growth in SPS-consolidated ATO. In contrast, Sb(V)-doped material exhibits improved electrical conductivity. The resistivity of 1.5 mol% Sb(V)-doped sample is only 0.01699 Ω cm, far lower than that of the 1.5 mol% Sb(III)-doped sample (0.1338 Ω cm). XPS results indicate a higher Sb5+/Sb3+ ratio in Sb(V)-doped ATO powder and ceramic than in Sb(III)-doped samples. The higher Sb5+/Sb3+ ratio contributes to the increase of carrier concentration and the decrease of resistivity of ATO ceramics.
The effect of MgO addition on the properties of (Co, Nb, Cr)-doped SnO2 varistors was investigated. The samples with different MgO concentrations were fabricated by the conventional ceramic method and sintered at 1,250, 1,300, 1,350 and 1,400 °C for 2 h. It was found that the nonlinear coefficient presented a peak value of 28 and lowest leakage current density of 7 μA/cm2 when 0.5 mol% MgO was added. The breakdown electrical field increased from 174 to 531 V/mm with increasing MgO from 0.0 to 2.0 mol%. The relative dielectric constant decreased with increasing MgO from 0.0 to 0.5 mol%, but increased with more MgO added. The dielectric loss decreased obviously in the case of low frequency with MgO added, and it had the lowest value when 0.5 mol% MgO added. The optimal samples were obtained by doping MgO with 0.5 mol% and sintering at 1,350 °C.
In this work the system (98.95−x)% SnO2+1%CoO+0.05%Ta2O5+x%Cr2O3 (mol %) was studied, with x=0.05 and 0.10, prepared by the conventional method of oxides mixture, and sintered at 1300 and 1350 °C for 2 h. The non-linear J versus E electrical characteristics (α=25) were obtained in the Ta2O5 and Cr2O3–CoO doped highly densified SnO2 ceramics. X-ray diffraction analysis showed that these ceramics are apparently single phase. Electrical properties and microstructure are highly dependent on the Cr2O3 concentration and on the sintering temperature. Excess of Cr2O3 leads to porous ceramics destroying the electrical characteristics of the material. Dopant solid solution formation in the SnO2 may be responsible for the formation of electrical barriers in the grain boundaries.
The effect of Cr2O3 addition on the properties of (Co, Nb)-doped SnO2 varistors were investigated. The samples with different Cr2O3 concentrations were fabricated by the conventional ceramic method and sintered at 1,250, 1,300, 1,350 and 1,400 °C for 2 h. It was found that the nonlinear coefficient present a peak value when 0.05 mol % Cr2O3 was added. The leakage current density decreases with increasing Cr2O3 from 0 to 0.05 mol %, but increases when the concentrations increase from 0.05 to 0.07 mol %. The breakdown electrical field increased from 337 to 866 V/mm with increasing Cr2O3 from 0 to 0.07 mol %. The optimal samples obtained by doping Cr2O3 with 0.05 mol % and sintered at 1,300 °C have the highest nonlinear coefficient value of 27 and lowest leakage current density of 9 μA/cm2.
This paper discusses some advances in research conducted on SnO2-based electroceramics. The addition of different dopants, as well as several thermal treatments in oxidizing and inert atmospheres, were found to influence the microstructure and electrical properties of SnO2-based varistor ceramics. Measurements taken by impedance spectroscopy revealed variations in the height and width of the potential barrier resulting from the atmosphere in which thermal treatments were performed. High nonlinear coefficient values, which are characteristic of high-voltage and commercial ZnO varistors, were obtained for these SnO2-based systems. All the systems developed here have potentially promising varistor applications.
The effects of CuO on the SnO2·Co2O3·Ta2O5 varistor system sintered at 1250°C for 80min were investigated. It was found that CuO significantly affects the grain size and electrical properties of the SnO2-based varistors. The grain size rises from 4.7 to 11.4μm, the breakdown electrical field decreases from 485 to 265Vmm−1 and relative dielectric constant (at 1kHz) increases from 970 to 2451 with an increase in CuO concentration from 0.00 to 1.00mol%. The sample with 0.25mol% CuO has the best nonlinear electrical property and the highest nonlinear coefficient (α=15.7) among all samples. The reason why grain size increases with increasing CuO concentration is explained. Also, the interpretation of the increase in nonlinearity is given.
