Environmental protection and sustainable development have gradually become important, but the development inevitably brings some harmful waste, such as more and more discarded electronic products. These products contain a lot of heavy metals, which are difficult to recycle and become non-degradable, and it must be more expensive to recycle them. Measuring strain on wood generally requires an indirect measurement by pasting a metal foil gauge on its surface. This method not only has the transmission error caused by pasting, but also has the disadvantages of high cost, being difficult to recycle, and being non-degradable. Generally speaking, special instruments such as thermocouple thermometers and far-infrared thermometers are needed to measure the surface temperature, and the measurement cost is generally higher. At the same time, there are often multiple signal detection requirements in daily life, and thus the preparation of multifunctional sensors is of great significance. In this research, an integrated pressure and temperature sensor based on the laser-induced graphene (LIG) technology on wood is proposed, which can detect pressure and temperature signals at the same time and is a degradable green sensor.

Pine wood is chosen as the raw material for the experiment. The dried Pinus sylvestris wood strips are put into the vacuum chamber, and the vacuum degree in the vacuum chamber is controlled. The electronic pattern on the pine wood is processed by a fiber laser and a laser galvanometer controlled by software, and the electronic pattern is pre-designed through scanning mirror software. The defocus distance, vacuum chamber pressure, laser power, scanning speed, marking time, and laser frequency are controlled to explore the optimal parameters for preparing LIG. The scanning electron microscope (SEM), Raman spectroscopy, and infrared spectra are used to analyze the surface morphologies and chemical compositions of the samples. After the pressure and temperature sensors are successfully prepared, a constant temperature and humidity test box is used to create the constant temperature and humidity environment, and a digital multimeter and a dynamic test analyzer are used to test the performance of the pressure and temperature sensors. Finally, an integrated pressure and temperature sensor is made and its performance is tested.

The prepared LIG has good electrical conductivity, and its sheet resistance is 8 Ω·sq^-1. The thermal effect plays a key role in the formation of LIG, and the laser power density is an important factor influencing the thermal effect. With the decrease of defocus distance and the increase of laser power, the laser power density increases. With the decrease of the scanning speed and the increase of the marking time, the vacuum pressure, and the laser power density, the regional thermal effect of pine wood increases, which affects the preparation of LIG. It can be seen from the infrared spectra that in the process of laser processing, a large amount of cellulose, hemicellulose and lignin in pine are decomposed, some of which are converted into graphite microcrystals and graphene, in addition to amorphous carbon and some impurities. Some irregular porous materials are found by SEM (Fig. 3). After the fabrication of a pressure sensor with a sensitivity coefficient of 86.53 and a temperature sensor with a temperature coefficient of resistance of -0.101% based on LIG, the sensitivity, hysteresis error, nonlinear error, repeatability error, response time, recovery time, and resistance temperature coefficient are tested and calculated (Figs.46), and finally the performance of the integrated sensor is tested (Fig. 7).

We propose a method to rapidly prepare LIG-based sensors on wood using laser. The integrated pressure and temperature sensor is prepared and its performance is tested. The optimal sample is obtained by using a fiber laser with a wavelength of 1070 nm, a laser pulse frequency of 10 kHz, a laser power of 10 W, the vacuum chamber pressure of 100 Pa, a defocus distance of 4 mm, a scanning speed of 100 mm·s^-1, and marking time of 14. Its sheet resistance is 8 Ω·sq^-1. The structure and forming mechanism of laser-induced graphene are obtained by SEM, Raman spectroscopy, and infrared spectra. The porous carbon pressure sensor and temperature sensor are fabricated. The sensitivity of the pressure sensor can reach 86.53. The hysteresis error is 0.06%. The non-linear error is 15%. The repeatability error is 0.31%. The response time and recovery time are both 1 s. The temperature coefficient of resistance of the temperature sensor is -0.101%. The sensing mechanisms of the LIG-based porous carbon pressure sensor and the temperature sensor are analyzed. Finally the integrated pressure and temperature sensor is made. The above LIG-based sensors have excellent sensing performances and are degradable. They have obvious commercial advantages and research prospects in current non-degradable electronic products that pollute the environment and have important applications in wooden construction and furniture.