Preparation And Sensing Performance Of Thick-film Resistor Strain Sensors

Thick-film resistors are electrical composites containing particles of ruthenate/ruthenium oxide conductor distributed in a highly modified silicate glass.Strain sensors based on the piezoresistive effect of thick-film resistors are known for their good strain sensitivity,reliability,and durability.This fact makes thick-film technology a good candidate for civil engineering applications.However,the usual alumina substrate with a large elastic modulus?300?400 GPa?will cause a sensitivity decrease,and strain mismatch between sensors and structures,is not suitable for strain monitoring,especially for civil engineering structures.It is of great significance to find a suitable substrate material,study the compatibility between substrate and resistor paste,and determine the firing process.A persistent shortcoming of thick-film strain sensors is their thermal sensitivity.It is necessary to reduce the cross-sensitivity between strain and temperature,improve the accuracy and convenience of the application of such sensors.Because of the above background,firstly,the influence mechanism of firing conditions on strain sensitivity of thick-film resistors was studied from the perspective of the conduction mechanism.Secondly,based on the detailed study on the resistance-temperature characteristics of the thick-film resistors,the feature that the absolute value of temperature coefficients of resistance reaches the minimum at the lowest point of the resistancetemperature curve was used to prepare thick film resistor strain sensor with low temperature-sensitivity.Then,fluorophlogopite glass-ceramic?FGC?with low elastic modulus and good processability was used as a substrate,and the compatibility with resistor pastes was studied.Finally,a thick-film resistor smart aggregate sensor based on fluorophlogopite glass-ceramic was designed and fabricated,and its stability and compatibility with mortar specimens were evaluated.The main research works are as follows:?1?The influence of firing condition and composition of resistor paste on the sheet resistivity and gauge factor?GF?of thick-film resistors was studied.Through microscopic analysis,combined with tunneling conduction theory,it is revealed that the main factor determining the strain sensitivity of thick-film resistors is the average distance between conductive particles.It is pointed out that that the distribution uniformity of conductive particles is an important factor affecting the gauge factor of thick-film resistor.The results show that the GF and the sheet resistivity of the thick-film resistor decreased with the increase of RuO₂ concentration,the particle size of glass powder,peak firing temperature,and dwell time.The sheet resistivity and GF decrease simultaneously with the increase of dwell time and move along the straight line.When resistors of the same sheet resistivity are considered,the increase of the firing temperature leads to a decrease of GF.According to the research results,in the case of a certain sheet resistivity,the purpose of increasing GF can be achieved by decreasing the firing temperature or increasing the particle size of the conductive phase.?2?The resistance-temperature characteristics of thick film resistors prepared with different paste components and under different firing conditions were studied.The idea of preparing a strain sensor with low temperature-sensitivity is proposed based on the feature that the absolute value of temperature coefficients of resistance reaches the minimum at the lowest point of the resistance-temperature curve.The relationships between sheet resistivity,TCR,and GF were studied.The results show that hot TCR increases with increasing RuO₂ concentration,firing temperature,and dwell time.It is found that both GF and hot TCR are strongly dependent on sheet resistivity of thickfilm resistors.A thick-film resistor with a small temperature-sensitivity but large strain sensitivity at room temperature was realized by adjusting the sheet resistivity.?3?Compatibility between resistor pastes and fluorophlogopite glass-ceramic?FGC?with low elastic modulus and good processability were studied.Under the same firing condition,it is found that TFRs on FGC substrates show higher hot TCRs,lower sheet resistivities,and gauge factors than that of Al2O3 substrates,and the difference decreases with the increase of RuO₂ concentration of resistor paste.Microstructural analyses of the interface between resistor layer and substrate indicate that the properties change of TFRs on FGC substrates are mainly due to the change in the RuO₂ concentration and the composition of the glass phase,which were caused by the interaction between the resistor layer and the FGC substrate.The interaction between the FGC substrate and the resistor layer can be greatly reduced by lowering the firing temperature to 750°C,resulting in TFRs on FGC substrates with properties similar to TFRs on alumina substrates.The influence of CTE of substrates on TCR can be eliminated by calculating the temperature coefficient of resistivity?TC??of the resistor layer.The degree of interaction between the FGC substrate and the resistor layer can be analyzed by comparing the relationship between temperature coefficient of resistivity of the resistor layer?TC??/GF and sheet resistivity of TFRs on FGC and Al2O3 substrate.?4?Smart aggregates made of thick film resistor was designed and manufactured.The matching performance of smart aggregates based on FGC substrate and alumina substrate between mortar in was studied and compared.Based on the theory of the geotechnical pressure sensor,the matching performance between the sensor and the mortar was analyzed.By comparing with the experimental results,the applicability of the theory in this experiment is verified.The sensing performances of thick-film resistors on FGC substrate under monotonic compression load and cyclic compression load was tested.The results show that thick-film resistor on FGC substrate has good sensitivity,excellent linearity and repeatability,and small hysteresis.In the range of 0.1Hz-5Hz,the sensor can maintain a good dynamic response.Compared to the alumina ceramic substrate,the TFRs on FGC show better compatibility and matching performance.