Cement is widely used in wellbores to stabilize the steel casing used in wellbore operations for oil and gas production, enhanced geothermal systems and carbon sequestration, and to limit fluid movement between sub-surface strata. Flaws such as microcracks in wellbore cement can lead to leakage along the wellbore compromising wellbore integrity. There is an increasing need for methods to monitor cement crack propagation in wellbore environments. In this study, we develop and report the first cementitious sensors capable of exhibiting high frequency acoustic bandgaps (ABGs) using carbon nanotubes (CNTs). Computational simulations of a sensor unit cell are used to design cement-multi walled carbon nanotubes (MWCNTs) sensors that show a wide bandgap. When the cement-MWCNTs sensors is embedded in cement specimens, bandgaps were measured experimentally under 300 kHz and under 600 kHz, consistent with the computationally predicted bandgaps in the range of 290–360 kHz, 410–460 kHz and 515–585 kHz. These bandgap features were absent in homogeneous cement specimens. X-ray tomographic reconstructions showed microscopic debonding at cement-MWCNTs sensor interface. Frequency response analysis of a three-dimensional computational model indicated a shift of frequency of minimum transmission due to the interface debonding, but no perturbation of bandgap response was observed. The cement-MWCNTs sensors developed in this study show the potential of a packed CNT inclusion material in cementitious matrix to create ABGs in a cement matrix.

Cement sensors with acoustic bandgaps using carbon nanotubes

In this work, the effects of physical dimensions such as length, width and thickness as well as the mode of vibration and the number of anchors on the performance of longitudinal thin film piezoelectric on silicon (TPoS) MEMS resonators have been studied. TPoS resonators, designed for a resonant frequency of around 1 GHz, were fabricated with a 4 mask CMOS compatible process. A $225~\mu \text{m}$ wide resonator excited in its $23^{rd}$ order had an unloaded quality factor of 9453 (in vacuum), which is the highest value reported so far for similar resonators, motional resistance of $107~\Omega $ and linear thermal coefficient of frequency of -28.4 ppm. We have also studied and modeled the different loss mechanisms in these devices. The model matches well with measured results for resonators of different geometries, modes of vibrations and number of anchors. [2020-0397]

Design, Modeling and Fabrication of TPoS MEMS Resonators With Improved Performance at 1 GHz

Anchor quality factor improvement of a piezoelectrically-excited MEMS resonator using window-like phononic crystal strip

This paper reports and validates the effect of additional loss due to the isotropic etch used to release the extensional mode thin piezoelectric on silicon (TPoS) resonators. Devices fabricated to operate at a resonant frequency around 1 GHz are kept at varying times in vapor HF to remove the buried oxide, thereby changing the additional area etched. When devices designed with different geometries are present in the same wafer, the etching times differ, causing more undercuts than required in some of the devices. The Quality factor (Q) calculated from the measured transmission characteristics is used to derive a semi-analytical model for the additional loss due to undercut. The total quality factor of these devices is estimated after adding other loss mechanisms with the help of simulations and is validated with measured experimental data from our group as well as those found in the literature. The Q predicted using this model matches the measured data for state-of-the-art devices with different frequencies, dimensions, and materials. 

Effect of Undercut due to Isotropic Etch while Releasing on the Performance of TPoS Resonators

This paper discusses applicability of two dimensional phononic crystals (PnC) structure for suppression of energy side leakage in the A 1 mode Lamb wave resonator with the broadband piston mode structure employing rotated Y-cut LiNbO 3 thin plate. Three types of PnC structures are compared, and it is shown that a cross hole-shaped PnC structure offers good energy leakage suppression in addition to producibility. In addition, discussions are extended to adding non-PnC areas to the tether region. They are effective for further suppression of the energy leakage.

Application of Phononic Crystal Structure for Side Leakage Suppression in A 1 -Mode Lamb Wave Resonators

Design and simulation of Phononic Crystals (PnCs) with a wide Acoustic Band Gap (ABG) around 1 GHz is presented. A new PnC unit cell topology is designed, which has a very wide ABG of 138 MHz with center frequency around 1 GHz and minimum feature size of 0.6 μm. Wider ABG allows better enhancement of the Quality factor (Q) by reducing the anchor loss. Effect of geometrical variation and the periodicity of PnC are discussed. ABG for 1D and 2D periodicity is simulated and geometrical dimensions to obtain similar ABG in both cases are compared.

1D and 2D wide acoustic bandgap Phononic Crystal structures for performance improvement of AlN-on-Si resonators operating in GHz range

In this paper, a semi-analytical model is derived to calculate the anchor loss of TPoS resonators operating in higher-order length extensional modes. This model provides valuable insights on the effect of order, device dimensions, position and number of anchors on the quality factor (Q) of these resonators. Results from this model are in close agreement with the FEM simulations. The optimum position, dimension, and the number of anchors to deliver the highest Q are also discussed. Predictions of the model are validated through characterization of TPoS resonators fabricated to operate at a resonant frequency of around 1 GHz with different number of anchor pairs and position. A $23^{rd}$ order resonator with a W/L ratio of 1.5 with three pairs of anchors placed at positions suggested by the model gave a loaded Q of 4431, which is 65% more than the conventional case where anchors are uniformly placed towards the edge, and also an improvement in the insertion loss by 5.5 dB compared to the latter. For resonators where the anchor loss is predominant, our results show that the performance can be improved by placing the anchors at the optimum position, closer to the middle of the resonator than the conventional scheme where anchors are uniformly placed towards the edge.

Optimization of Anchor Placement in TPoS MEMS Resonators: Modeling and Experimental Validation

This paper reports and validates the effect of additional loss due to the isotropic etch used to release the extensional mode thin piezoelectric on silicon (TPoS) resonators. Devices fabricated to operate at a resonant frequency around 1 GHz are kept at varying times in vapor HF to remove the buried oxide, thereby changing the additional area etched. When devices designed with different geometries are present in the same wafer, the etching times differ, causing more undercuts than required in some of the devices. The Quality factor (Q) calculated from the measured transmission characteristics is used to derive a semi-analytical model for the additional loss due to undercut. The total quality factor of these devices is estimated after adding other loss mechanisms with the help of simulations and is validated with measured experimental data from our group as well as those found in the literature. The Q predicted using this model matches the measured data for state-of-the-art devices with different frequencies, dimensions, and materials.

J Bijay

Papers

1D and 2D wide acoustic bandgap Phononic Crystal structures for performance improvement of AlN-on-Si resonators operating in GHz range

Optimization of Anchor Placement in TPoS MEMS Resonators: Modeling and Experimental Validation

Effect of Undercut due to Isotropic Etch while Releasing on the Performance of TPoS Resonators