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Based on the laser flash method, the selection of experimental parameters on the accuracy of measurement results for measuring thermal diffusivity is investigated in this paper. High purity graphite is employed as the experimental material. Three experimental parameters are taken into consideration, including specimen thickness, laser pulse power, and laser pulse width. Firstly, the principle of the laser flash method is introduced. Then, a numerical simulation model is established and independence tests are performed. In order to investigate the impact of different experimental parameters to the precision of the measurement results using the laser flash method, six thicknesses of the specimens, five laser pulse powers, and five laser pulse widths are selected for numerical simulation and LFA 427 measurement experiments. Finally, an orthogonal design method with three-factor and three-level is constructed to investigate the influence degree of these three factors on the measurement results. It is found that the laser pulse width has the most significant influence, while the laser pulse power has the least impact. For high purity graphite specimens, it is required to choose a thicker specimen, a lower laser pulse power, and a smaller laser pulse width to ensure better measurement accuracy. [ABSTRACT FROM AUTHOR]

Background and objective: Femtosecond laser (fs-laser) is a novel tooth preparation tool. After cavities were prepared by fs-laser, their axial-wall taper is closely related to the retention force and success of restorations, while the taper after preparation by fs-laser is not clear yet. This study aimed to explore the value of the taper after fs-laser tooth preparation and the effect of laser fluence, scanning line spacing, and scanning layers on taper. Materials and methods: Twenty cavities with a width of 0.5mm were prepared by an fs-laser both in enamel and dentin to obtain the axial-wall taper (No. PKUSSIRB-201949124). To study the effects of three parameters of fs-laser, five different laser fluence were set as 1.56, 3.13, 4.69, 6.25, and 7.81 J/cm² of five cavities. Scanning line spacing was set as 6, 12, 25, 30, 40, and 50 lm of six cavities and the scanning layers were 5, 10, 25, 50, 75, 100, 200, 300, and 400, respectively, of nine cavities. Laser pulses irradiated perpendicular to the tooth and the scanning path was controlled via a vibratory mirror. The taper was measured with a laser confocal microscope. Curves of the taper versus different parameters were drawn. Results: There was a significant difference in taper between enamel and dentin. Overall, the taper decreased gradually with the increase of laser fluence. The optimal taper obtained was 20.53°- 0.28° in dentin and 38.71°- 0.46° in enamel when the corresponding fluence was 6.25 J/cm². In comparison, the scanning line spacing has little effect on the taper. The scanning layers significantly affect the taper, which decreased dramatically as scanning layers increased within 75 layers and does not change considerably as it was beyond 100 layers. Conclusions: Reasonable selection of laser fluence, scanning line spacing, and the number of scanning layers can optimize the cavity taper by fs-laser. [ABSTRACT FROM AUTHOR]