Vol.5 No.3 2012

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Research paper : Development of a pressure sensor using a piezoelectric material thin film (M. Akiyama et al.)−175−Synthesiology - English edition Vol.5 No.3 (2012) durability. The authors fabricated the AlN thin film on a metal substrate to increase the durability against mechanical impact. For the metal substrate material, we selected Inconel, a nickel superalloy with excellent heat resistance. The AlN thin film was fabricated on the multicrystal Inconel substrate by the sputtering method, and we successfully fabricated the thin film with crystalline orientation (2006).[8] Figure 4(a) shows the SEM photograph of the cross section of the AlN thin film fabricated on the Inconel substrate. The XRD pattern of this thin film is shown in Fig. 4(b). From the cross section of the AlN thin film, it was observed that it was composed of fine crystal particles with fibrous structure, in which the thin film grew vertically against the substrate surface. Although the Inconel substrate is multicrystalline, the AlN thin film showed only the diffraction peak of 0002 face of wurtzite type AlN, and presented the c-axis orientation. The piezoelectric constant (d33) measured using the piezometer was 2.4 pC/N, which was about half the value of monocrystal AlN, but we were able to fabricate the AlN thin film with piezoelectric property on the Inconel substrate.Figure 5 shows the bending strength b(AlN) and the temperature dependency of in-plane stress that occurs in the AlN thin film fabricated on the ferrite stainless and Inconel substrates.With both substrates, it was found that the stress distribution range was wide, and a clear difference was seen between the two substrates. Both were fabricated at 400 °C, but after fabrication, about twice the compression stress was working on the Inconel material compared to the stainless one. Also, since the coefficient of thermal expansion of AlN was smaller than those of the two alloy substrates, the tensile stress increased by heating. As a result, stress surpassing the rupture strength of the AlN thin film might occur locally when 400 °C was surpassed in the stainless sample. On the other hand, rupture strength of the AlN thin film was not surpassed even at 800 °C in the case of the Inconel sample. From these results, it was found that the choice of the metal substrate material is important to prevent the rupture of the AlN thin film.3.2 Evaluation of the electric property in high temperature environmentThe piezoelectric property of the AlN thin film fabricated on the Inconel substrate was studied in a high temperature environment. Figure 6(a) shows the amplitude change of the charge generated by the AlN thin film when the oscillation amplitude was varied by applying pressure at a constant frequency. The AlN thin film showed linearity at a high pressure range of 10~300 MPa. It was found that the AlN thin film had sufficient pressure resistance as a combustion pressure sensor, since the maximum pressure inside the combustion chamber of a standard car is several tens of MPa.Figure 6(b) shows the frequency response of the AlN thin film at sine wave oscillation amplitude of 300 MPa for the applied pressure. Here, the gain of the AlN thin film is standardized as output value Hz. The gain was mostly flat at high frequency range (3~30 Hz) and the phase shift was small. It was found that the AlN thin film had sufficient Fig. 7 Appearance of AlN sensor and schematic diagram of interior structureFig. 6 (a) Pressure dependence of generated charge of AlN element for frequency between 0.1~30 Hz, and (b) frequency response of gain and phase of AlN element against pressure change of 150 MPa05010015020025001002003001 Hz / 10 Hz / 30 Hz0.1 Hz-20-15-10-5050.1110100050100150200(a) (b) Frequency(Hz)Pressure(MPa)Gain(dB)Generatedcharge(pC)PhasePhase(°)Gain-3dBInsulating tube(Alumina, 2 mm×1 mm)Gasket(Cu, 0.5 mmt)Insulating plate(Alumina, 1 mmt)Electrode(Inconel 600, 1 mmt)M14×1.25 AIN elementSignalPush rod

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