Subsolidus phase equilibria in the PbO-poor part of RuO2–PbO–SiO2 system
Introduction
Thick film resistors materials are used for making resistors on alumina ceramics in thick-film hybrid circuits. They span very wide resistivity values from a few Ω to few MΩ, are very stable and can be laser trimmed to narrow tolerances.
In most modern thick film resistor compositions, the conductive phase is either RuO2 or ruthenates; mainly lead or bismuth ruthenates. During the firing cycle, all the constituents of the resistor paste react with each other and the melted glass also interacts with the substrate. The resistors are fired only a short time at the highest temperature, typically 10 min at 850 °C. Because of this, the reactions between the main constituents (glass and conductive phase) of the resistor material do not reach equilibrium [1], [2], [3], [4], [5].
If thick film resistors are fired at temperatures higher than the required 850 °C, and for a relatively long time, the interactions between the conductive phase and the lead–borosilicate-based glass phase approach equilibrium. This normally tends to decrease the sheet resistivities and increase the temperature coefficients of resistivity (TCRs) of these “overfired” resistors [6], [7], [8]. Adachi and Kuno [9], [10] studied high-temperature interactions between PbO–B2O3–SiO2 glasses and Pb2Ru2O6.5 or RuO2. They showed that in glasses poor in PbO, the Pb2Ru2O6.5 disappears and the RuO2 is formed, while for PbO-rich glasses, the RuO2 reacts with the PbO from the glass and forms Pb2Ru2O6.5. X-ray diffraction study of ruthenate-based “equilibrated” resistors showed that at higher firing temperatures the ruthenate decomposes forming RuO2, while the conductive phase in RuO2-based resistors stays unchanged. Presumably because of the interaction with the molten glass the lead ruthenate decompose into PbO, which is dissolved in the glass, and into RuO2 [11].
The aim of this work was to investigate subsolidus phase equilibria (in air) in the PbO-poor part of the RuO2–PbO–SiO2 system. The results would indicate possible interactions between silica-rich glass and conductive phase (either ruthenium oxide or lead ruthenate) in thick-film resistors.
Four binary compounds, PbSiO3, Pb2SiO4, Pb3SiO5 and Pb4SiO6, exist in the PbO–SiO2 system [12]. The eutectic composition on the SiO2-rich side of the system is at 62% PbO and the eutectic temperature is 739 °C. Two more eutectics are formed between PbSiO3 and Pb2SiO4, and between Pb2SiO4 and Pb3SiO5, with melting temperatures of 714 °C and 711 °C, respectively. The Pb4SiO6 compound melts incongruently at 725 °C.
Phase equilibria in the RuO2–PbO system were studied by Shaplygin et al. [13] and Hrovat et al. [14]. The pyrochlore Pb2Ru2O6.5 exists in the system. Shaplygin et al. proposed the eutectic composition around 72% PbO and the eutectic temperature 760 °C while Hrovat et al. reported the eutectic composition around 95% PbO and the eutectic temperature 875 °C. The pyrochlore series, which can be described by the general formula Pb2(Ru2−xPbx)O6.5 [15], is not a part of RuO2–PbO system because the lead ions which exchange ruthenium ions on “B” sites are Pb4+. No data relating to the RuO2–SiO2 system were found in the open literature.
Section snippets
Experimental
For the experimental work, RuO2 (Ventron, 99.9%), PbO (Johnson Matthey, 99.99%), and SiO2 (Riedel de Haen, 99.9%) were used. The oxides were mixed in isopropyl alcohol, pressed into pellets, and fired up to five times in air at 700 °C with intermediate grinding. During firing the pellets were placed on platinum foils. The compositions of the relevant samples in the PbO-poor part of the RuO2–PbO–SiO2 system are shown in Fig. 2.
The fired materials were characterised as powders by X-ray powder
Results and discussion
The nominal sheet resistivities, the conductive phase [5], [16] and the molar ratio SiO2/PbO of the glass phase of the resistors are given in Table 1. The glass compositions are rich in SiO2 with the molar ratio SiO2/PbO between 2 and 2.5.
The results of the X-ray powder analysis of the relevant samples, fired in air at 700 °C, are summarised in Table 2. The nominal compositions of the samples and the phases identified after firing are given.
There is no binary compound between RuO2 and SiO2. The
Conclusions
Subsolidus equilibria in the PbO-poor part of RuO2–PbO–SiO2 diagram were studied by X-ray powder diffraction analysis and energy-dispersive X-ray analysis. The aim was to investigate possible interactions between conductive phase (either ruthenium oxide or lead ruthenate) and silica-rich glasses (molar ratio SiO2/PbO is between 2 and 2.5) in thick-film resistors. No ternary compound was found in the system. The tie lines are between Pb2Ru2O6.5 and PbSiO3, and between RuO2 and PbSiO3. This
Acknowledgement
The authors wish to thank Mrs. Jena Cilenšek (Jožef Stefan Institute) for the preparation of samples for SEM analysis. The financial support of the Slovenian Research Agency is gratefully acknowledged.
References (17)
- et al.
The influence of firing temperature on the electrical and microstructural characteristics of thick film resistors for strain gauge applications
Sens. Actuators A, Phys.
(2003) - et al.
A characterization of thick-film PTC resistors
Sens. Actuators, A
(2005)et al.A characterization of thick-film PTC resistors
J. Mater. Sci. Lett.
(2001) - et al.
A characterisation of thick film resistors for strain gauge applications
J. Mater. Sci.
(2001) - et al.
The chemistry and stability of ruthenium based resistors
Solid State Technol.
(1982) Materials science of thick-film technology
Ceram. Bull.
(1986)- et al.
Control of electrical properties of RuO2 thick film resistors
Act. Passive Electron. Compon.
(1987) - et al.
The effect of various factors on the resistivity and TCR of RuO2 thick film resistors – relation between the electrical properties and particle size of constituents, the physical properties of glass and firing temperature
Act. Passive Electron. Compon.
(1988) - et al.
Microstructural, XRD and electrical characterization of some thick film resistors
J. Mater. Sci.: Materials in Electronics
(2000)
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