# Research on Key Technologies of 260 \({}^{\circ}\)C/210 MPa Ultra-Deep Logging Equipment
## 3 Research on Key Technologies of Ultra-High-Temperature and High-Pressure Logging  
### 260 \({}^{\circ}\)C/210 MPa Ultra-High Temperature and Pressure Logging Sensors  
#### 3.1.1 Acoustic Transducer  
Given the urgent need for high-temperature piezoelectric acoustic logging transducers for ultra-deep oil and gas exploration in China, it is necessary to develop BS-PT-based piezoelectric ceramic materials and transducers capable of withstanding operating temperatures up to 260 \({}^{\circ}\)C. This development aims to provide the core components for high-end acoustic deep well detection instruments that are independently controllable. BS-PT-based piezoelectric ceramics possess piezoelectric properties comparable to PZT-based piezoelectric ceramics, with a Curie temperature nearly 100 \({}^{\circ}\)C higher than that of PZT. Therefore, the safe operating temperature limit of BS-PT-based piezoelectric ceramics is above 300 \({}^{\circ}\)C, meeting the high-temperature environmental requirements for the current and future ultra-deep well exploration [7]. (1) BS-PT-Based High-Temperature Piezoelectric Ceramic Formulation  
Research on the effects of doping with transition elements or rare earth elements on the dielectric, ferroelectric, and piezoelectric properties, and their stability at high temperatures, is necessary. This research aims to clarify the relationship between the compositional modification, phase transition behavior, and temperature stability of high-temperature BS-PT piezoelectric ceramics [8; 9]. As shown in Figure 1a, at 260 \({}^{\circ}\)C, the piezoelectric constant d31 of BS-PT is approximately 320 pC/N, which is greater than the piezoelectric constant d31 of PZT-5 (approximately 205 pC/N) at the same temperature, indicating that BS-PT material exhibits higher temperature stability. The value of d31 varies with temperature, and piezoelectric ceramic materials undergo depolarization at temperatures above their Curie temperature, resulting in a decrease in the piezoelectric constant d31 and a significant reduction in their material performance. The depolarization temperature Td of BS-PT ceramics is approximately 400 \({}^{\circ}\)C, which is much higher than that of PZT-4, PZT-5A, and PZT-8 ceramics, with Td values of 180 \({}^{\circ}\)C, 360 \({}^{\circ}\)C, and 275 \({}^{\circ}\)C, respectively (test standards refer to \"Static Test Method for the Measurement of Piezoelectric Charge Constant d33 of Piezo-Ceramic Materials\" [10]). Compared to the temperature resistance of traditional PZT, BS-PT material can maintain both high temperature resistance and high piezoelectric performance in environments of 260 \({}^{\circ}\)C.  
Figure 1b illustrates the temperature dependence of the electromechanical coupling coefficient k\({}_{31}\). The results show that the k\({}_{31}\) of BS-PT ceramics remains stable over a wide temperature range of 30 to 400 \({}^{\circ}\)C, whereas the k\({}_{31}\) of all PZT ceramics gradually decreases  
Figure 1: (**a**). The trend of the piezoelectric strain constant (d\({}_{31}\)) of BS-PT and PZT-based piezoelectric ceramics with temperature. (**b**). The trend of the electromechanical coupling coefficient (k\({}_{31}\)) of BS-PT and PZT-based piezoelectric ceramics with temperature. with increasing temperature. This indicates the better stability of BS-PT ceramics under high-temperature conditions. (2) Production Process of BS-PT-Based Piezoelectric Ceramics  
To meet the demand for ultrasonic transducers in deep well detection, especially large-sized high-temperature piezoelectric ceramics, it is crucial to develop the engineering and production processes for BS-PT-based piezoelectric ceramics. This includes addressing the challenges related to the forming and sintering processes of large-sized BS-PT-based piezoelectric ceramics and ensuring performance consistency.