Ultrasonic thickness measurement technology
1.Needs for lithium battery electrode net coating measurement
Lithium battery electrode is composed of collector, coating on the surface A and B. Thickness uniformity of coating is the core control parameter of lithium battery electrode, which has a critical impact on safety, performance and cost of lithium battery. Therefore, there are high requirements for testing equipment during lithium battery production process.
2.X-ray transmission method meeting the limit capacity
Dacheng Precision is a leading international systematic electrode measurement solution provider. With more than 10 years of research and development, it has a series of high-precision and high-stability measurement equipment, such as X/β-ray areal density gauge, laser thickness gauge, CDM thickness and areal density integrated gauge, etc., which are capable of achieving online monitoring of lithium-ion battery electrode’s core indexes, including the net coating amount, thickness, thickness of thinning area, and areal density.
Besides, Dacheng Precision is also carrying out changes in non-destructive testing technology, and has launched Super X-Ray areal density gauge based on solid-state semiconductor detectors and infrared thickness gauge based on infrared spectral absorption principle. Thickness of organic materials can be accurately measured, and the accuracy is better than imported equipment.
Figure 1 Super X-Ray areal density gauge
3.Ultrasonic thickness measurement technology
Dacheng Precision has always been committed to research and development of innovative technologies. In addition to the above non-destructive testing solutions, it is also developing ultrasonic thickness measurement technology. Compared with other inspection solutions, ultrasonic thickness measurement has the following characteristics.
3.1 Ultrasonic thickness measurement principle
Ultrasonic thickness gauge measures the thickness based on the principle of ultrasonic pulse reflection method. When the ultrasonic pulse emitted by the probe passes through the measured object to reach the material interfaces, the pulse wave is reflected back to the probe. The thickness of measured object can be determined by accurately measuring the ultrasonic propagation time.
Almost all products made of metal, plastic, composite materials, ceramics, glass, glass fibre or rubber can be measured in this way, and it can be widely used in petroleum, chemical, metallurgy, shipbuilding, aviation, aerospace and other fields.
3.2 Advantages of ultrasonic thickness measurement
The traditional solution adopts ray transmission method to measure the total coating amount and then use subtraction to calculate the value of lithium battery electrode net coating amount. While ultrasonic thickness gauge can directly measure the value due to the different measurement principle.
①Ultrasonic wave has strong penetrability due to its shorter wavelength, and it is applicable to a wide range of materials.
② Ultrasonic sound beam can be concentrated in a specific direction, and it travels in a straight line through the medium, with a good directivity.
③ There is no need to worry about the safety issue because it doesn’t have radiation.
However, despite the fact that ultrasonic thickness measurement has such advantages, compared with several thickness measurement technologies that Dacheng Precision has already brought to the market, the application of ultrasonic thickness measurement has some limitations as follows.
3.3 Application limitations of ultrasonic thickness measurement
①Ultrasonic transducer: ultrasonic transducer, that is, the ultrasonic probe mentioned above, is the core component of ultrasonic testing gauges, which is capable of transmitting and receiving pulse waves. Its core indicators of working frequency and timing accuracy determine the accuracy of thickness measurement. The current high-end ultrasonic transducer is still dependent on imports from abroad, whose price are expensive.
②Material uniformity: as mentioned in the basic principles, ultrasonic will be reflected back on the material interfaces. The reflection is caused by sudden changes in acoustic impedance, and the uniformity of the acoustic impedance is determined by the material uniformity. If the material to be measured is not uniform, the echo signal will produce a lot of noise, affecting the measurement results.
③ Roughness: the surface roughness of measured object will cause low reflected echo, or even unable to receive the echo signal;
④Temperature: the essence of ultrasonic is that the mechanical vibration of medium particles is propagated in the form of waves, which cannot be separated from the interaction of medium particles. The macroscopic manifestation of thermal motion of medium particles themselves is temperature, and thermal motion will naturally affect the interaction between medium particles. So temperature has great impacts on the measurement results.
For conventional ultrasonic thickness measurement based on pulse echo principle, people’s hand temperature will affect the probe temperature, thus leading to the drift of zero point of the gauge.
⑤Stability: the sound wave is the mechanical vibration of medium particles in the form of wave propagation. It is susceptible to external interference, and the collected signal is not stable.
⑥Coupling medium: ultrasonic will attenuate in the air, while it can be well propagated in liquids and solids. In order to better receive the echo signal, a liquid coupling medium is usually added between ultrasonic probe and measured object, which is not conducive to the development of on-line automated inspection program.
Other factors, such as ultrasonic phase reversal or distortion, the curvature, taper or eccentricity of the surface of measured object will influence the measurement results.
It can be seen that ultrasonic thickness measurement has many advantages. However, it currently can not be compared with other thickness measurement methods because of its limitations.
3.4 Ultrasonic thickness measurement research progress of Dacheng Precision
Dacheng Precision has always been committed to research and development. In the field of ultrasonic thickness measurement, it has also made some progress. Some of the research results are shown as follows.
3.4.1 Experimental conditions
The anode is fixed on the worktable, and the self-developed high-frequency ultrasonic probe is used for fixed-point measurement.
Figure 2 Ultrasonic thickness measurement
3.4.2 Experimental data
The experimental data are presented in the form of A-scan and B-scan. In the A-scan, X-axis, represents the ultrasonic transmission time and Y-axis represents the reflected wave intensity. B-scan displays a two-dimensional image of the profile parallel to the direction of sound velocity propagation and perpendicular to the measured surface of the object under test.
From the A-scan, it can be seen that the amplitude of returned pulse wave at the junction of graphite and copper foil is significantly higher than that of other waveforms. The thickness of graphite coating can be obtained by calculating the acoustic-path of ultrasonic wave in the graphite medium.
A total of 5 times of data were tested at two positions, Point1 and Point2, and the acoustic-path of graphite at Point1 was 0.0340 us, and the acoustic-path of graphite at Point2 was 0.0300 us, with a high repeatability precision.
Figure 3 A-scan signal
Figure 4 B-scan image
Fig.1 X=450, YZ plane B-scan image
Point1 X=450 Y=110
Acoustic-path: 0.0340 us
Point2 X=450 Y=145
Figure 5 Two-point test image
4. Summary of lithium battery electrode net coating measurement technology
Ultrasonic testing technology, as one of the important means of non-destructive testing technology, provides an effective and universal method for evaluating the microstructure and mechanical properties of solid materials, and detecting their micro- and macro-discontinuities. Faced with the demand for on-line automated measurement of net coating amount of lithium battery electrode, the ray transmission method still has a greater advantage at present due to the characteristics of ultrasonic itself and the technical problems to be solved.
Dacheng Precision, as an expert in electrode measurement, will continue to carry out in-depth research and development of innovative technologies including ultrasonic thickness measurement technology, making contribution to the development and breakthroughs of non-destructive testing!
Post time: Sep-21-2023