Thermal Expansion and Electro-Elastic Features of Ba2TiSi2O8 High Temperature Piezoelectric Crystal
, Fapeng Yu 1,*, Shiwei Tian 1, Xiufeng Cheng 1, Shujun Zhang 2 and Xian Zhao 1
Round 1
Reviewer 1 Report
The launch of advanced piezoelectric crystals, possessing the desired thermal and electro-elastic properties for high-temperature sensing applications, is a challenge of significant relevance to power plants, automotive and aerospace manufacturing. In order to be suitable for practical use, these crystals must be capable of withstanding high temperatures, normally above 1000 °C, without undergoing structural phase transitions or degrading their sensor performance (e.g. by unacceptable changes in the elastic, piezoelectric and thermal expansion coefficients). The present manuscript proposes a fresnoite (Ba2TiSi2O8) based crystal configuration, whose crystal cut and orientation provide a relatively constant piezoelectric (d33) and elastic (s33) coefficients over a wide temperature range (20-650 °C). I find the paper well written, organized and quite interesting, and in my opinion it merits publication in MDPI Crystals.
A few minor corrections could be applied if the authors and editorial office consider them as relevant:
1. References 4 and 5 are outdated (published before 2010), so I suggest to replace them with two relevant and recently published articles: Y. Li. et.al. Temperature Dependence of the Thermal, Electrical Resistivity, Dielectric and Piezoelectric Properties of CaYAl3O7 Crystal, (2018) MDPI Crystals; H. Zu et.al. High-Temperature Piezoelectric Crystals for Acoustic Wave Sensor Applications, IEEE TUFFC (2016).
2. “ The electrical resistivities ρ11 and ρ33 were found to be 1.9×107 and 3.6×109 Ω·cm at 600oC, respectively [15,16], three orders higher than the values of langasite type crystals.” – in my opinion this statement is incorrect, as recently published paper shows electrical resistivity of langasite-family crystals commensurable with that of fresnoite at temperatures of 600 °C. Please see Figure 1 in T. Karaki et.al. High-temperature electrical resistivity and loss tangent of langasite-family Ca3Nb(Ga,Al)3Si2O14 single crystals (2018). The latter should be cited.
3. “In addition to electrical resistivity and piezoelectric properties, the thermal expansion of the crystal is another important parameter for high temperature sensor fabrication. The thermal expansion nonlinearity plays a negative impact on the stability of piezoelectric performance over a broad temperature range, while the mismatch of thermal expansion for different components in a sensor would cause the packaged device failure under thermal cyclic loading.” – I agree, and in addition, the difference between the thermal expansion coefficients of the sensor electrodes and the crystal itself can cause severe measurement errors. If the authors deem it is appropriate, they may add some references that further elaborate and support the considerations in this paragraph, in order to make the manuscript even more attractive for a broader readership. For instance, Esmeryan et.al. Temperature behavior of solid polymer film coated quartz crystal microbalances for sensor applications, Sens. Act. B Chem. (2015); J. Li et.al. Study on Temperature and Synthetic Compensation of Piezo-Resistive Differential Pressure Sensors by Coupled Simulated Annealing and Simplex Optimized Kernel Extreme Learning Machine, MDPI Sensors (2017). Moreover, in some cases, the difference in thermal expansion coefficients of each sensor component can be used for thermal self-compensation of the sensor device (F. Ayazi et.al. Compensation, tuning and trimming of MEMS resonators, IEEE Freq. Cont. Symp. (2012) ).
4. Section 2.3 – why did you choose this specific heating rate (5 °C/min)? Don't you think that if the crystal is intended for HT piezoelectric sensors for propulsion monitoring, the temperature will change much faster?
5. Figure 2 – The authors provide in the text (Page 4, Lines 108-112) average thermal expansion coefficients within several ppm/°C, but the Y-axis in Figure 2 does not clearly support the statements. Can you convert the Y-axis to ppm/°C please?
6. “The elastic compliance constants are evaluated based on the resonance and/or anti-resonance frequencies measured by impedance method. Table 3 summarizes the elastic constants obtained by the combination of impedance and pulse-echo methods” - would you mind providing the resonance and anti-resonance curves please?
7. The appearance of Table 4 is not mentioned in the text.
8. In Figure 4, the 0 point at the X-axis of the second graph, overlaps the 700 °C of the first one, thus, imparting the impression that the temperature is 7000 °C. Please fix this.
Author Response
Response to Reviewer 1 Comments
Point 1: References 4 and 5 are outdated (published before 2010), so I suggest to replace them with two relevant and recently published articles: Y. Li. et.al. Temperature Dependence of the Thermal, Electrical Resistivity, Dielectric and Piezoelectric Properties of CaYAl3O7 Crystal, (2018) MDPI Crystals; H. Zu et.al. High-Temperature Piezoelectric Crystals for Acoustic Wave Sensor Applications, IEEE TUFFC (2016).
Response 1: Thanks for your advice, the new references have been updated.
Point 2: “The electrical resistivities ρ11 and ρ33 were found to be 1.9×107 and 3.6×109 Ω·cm at 600oC, respectively [15,16], three orders higher than the values of langasite type crystals.” – in my opinion this statement is incorrect, as recently published paper shows electrical resistivity of langasite-family crystals commensurable with that of fresnoite at temperatures of 600 °C. Please see Figure 1 in T. Karaki et.al. High-temperature electrical resistivity and loss tangent of langasite-family Ca3Nb(Ga,Al)3Si2O14 single crystals (2018). The latter should be cited.
Response 2: Thanks for your suggestion. The sentence has been revised as “The electrical resistivities ρ11 and ρ33 were found to be 1.9×107 and 3.6×109 Ω·cm at 600oC, respectively [15,16], higher than the values of the disordered langasite type crystals [17,18], while comparable to the ordered langasite crystal CNGAS [8].”. Thanks for offering the new reference about the ordered langasite crystals, we added the relevant paper in our article as the reference [8].
Point 3: “In addition to electrical resistivity and piezoelectric properties, the thermal expansion of the crystal is another important parameter for high temperature sensor fabrication. The thermal expansion nonlinearity plays a negative impact on the stability of piezoelectric performance over a broad temperature range, while the mismatch of thermal expansion for different components in a sensor would cause the packaged device failure under thermal cyclic loading.” – I agree, and in addition, the difference between the thermal expansion coefficients of the sensor electrodes and the crystal itself can cause severe measurement errors. If the authors deem it is appropriate, they may add some references that further elaborate and support the considerations in this paragraph, in order to make the manuscript even more attractive for a broader readership. For instance, Esmeryan et.al. Temperature behavior of solid polymer film coated quartz crystal microbalances for sensor applications, Sens. Act. B Chem. (2015); J. Li et.al. Study on Temperature and Synthetic Compensation of Piezo-Resistive Differential Pressure Sensors by Coupled Simulated Annealing and Simplex Optimized Kernel Extreme Learning Machine, MDPI Sensors (2017). Moreover, in some cases, the difference in thermal expansion coefficients of each sensor component can be used for thermal self-compensation of the sensor device (F. Ayazi et.al. Compensation, tuning and trimming of MEMS resonators, IEEE Freq. Cont. Symp. (2012) ).
Response 3: We are really appreciated for your guidance. All these references about thermal expansion have been added in the text.
Point 4: Section 2.3 – why did you choose this specific heating rate (5 °C/min)? Don't you think that if the crystal is intended for HT piezoelectric sensors for propulsion monitoring, the temperature will change much faster?
Response 4: Thanks for the suggestion. The purpose of measuring the thermal expansion coefficient is to understand the anisotropy of thermal expansion behaviours of BTS crystal and then design the optimum crystal cut. High heating rate is not beneficial for the measurement. Therefore, the rate of 5oC/min was used in this study. For propulsion monitoring sensing application, the property evaluation under thermal shocks are really need to be done, which will be the future research focus in our group.
Point 5: Figure 2 – The authors provide in the text (Page 4, Lines 108-112) average thermal expansion coefficients within several ppm/°C, but the Y-axis in Figure 2 does not clearly support the statements. Can you convert the Y-axis to ppm/°C please?
Response 5: Thanks for your advice. The Y-axis in Figure 2 indicates the relative variation of dL/L0, where the L0 is the initial length of the sample, and the dL is the absolute variation. The dL/L0 is a non-dimensional parameter. As to the average thermal expansion coefficient, it is a result of dL/L0/dT (unit ppm/oC and dT=T’-T0) achieved from the slope of the linear thermal expansion in a certain temperature range. If we put dL/L0/dT as the Y-axis and draw the temperature T as an X-axis (T0 refers to 25oC), Figure 2 will be presented as following where large discrepancy will occur due to the selection of T0. By comparison, we feel the original one is more reasonable and clear, so we kept the original Figure 2 in the text. Please let us know if the explanation is OK.
Point 6: “The elastic compliance constants are evaluated based on the resonance and/or anti-resonance frequencies measured by impedance method. Table 3 summarizes the elastic constants obtained by the combination of impedance and pulse-echo methods” - would you mind providing the resonance and anti-resonance curves please?
Response 6: The resonance and anti-resonance curves are shown below. Among these impedance-phase curves, the X-plate gives the k15 mode, while other crystal cuts give the vibration responses for length extensional modes. For illustration, all these figures have been added in the manuscript. Please kindly check new Figure 3.
Point 7: The appearance of Table 4 is not mentioned in the text.
Response 7: Revised, thanks.
Point 8: In Figure 4, the 0 point at the X-axis of the second graph, overlaps the 700 °C of the first one, thus, imparting the impression that the temperature is 7000 °C. Please fix this
Response 8: Revised, thanks.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The work is sound and can be accepted for publication. The reviewer has a few minor comments on the draft:
i. Details of the high purity raw materials should be included, i.e., company and purity grade.
ii. How the powders were mixed for 10 hours?
iii. Please consider adding references for the equation listed in Table I.
iv. What is the significance of FWHM of 40ʺ~79ʺ? Or Please consider adding a sentence or two why the FWHM is different at different part of the crystal?
Author Response
Response to Reviewer 2 Comments
Point 1: Details of the high purity raw materials should be included, i.e., company and purity grade.
Response 1: Added, thanks.
Point 2: How the powders were mixed for 10 hours?
Response 2: Thanks. The weighted raw materials were put in a plastic jar (no more than 2/3 of the volume) then fix the plastic jar into a mixer (YGJ-5KG, no use of ZrO2 balls) and run for 10 hours. These contents are enriched in the text.
Point 3: Please consider adding references for the equation listed in Table I.
Response 3: Revised, thanks.
Point 4: What is the significance of FWHM of 40ʺ~79ʺ? Or Please consider adding a sentence or two why the FWHM is different at different part of the crystal?
Response 4: Thanks for your suggestion. We added the following sentence in the text to explain the difference.
“the full-width half maximum (FWHM) is obtained and found to be 40ʺ~79ʺ, indicating the good structure integrity and crystal quality of the grown BTS crystal. The different FWHM values for different parts in BTS crystal might arise from the segregation of components during the crystal growth process, which affects the crystal quality to an extent, as described in Ref [16]. In addition, the possible glass phase formation [26] in local bulk crystal is deemed another factor affects the crystal quality.”
