Enhancing the Electrochemical Performance of High Voltage LiNi0.5Mn1.5O4 Cathode Materials by Surface Modification with Li1.3Al0.3Ti1.7(PO4)3/C.

A novel method for surface modification of LiNi0.5Mn1.5O4 (LNMO) was proposed, in which a hybrid layer combined by Li1.3Al0.3Ti1.7(PO4)3 (LATP) and carbon (C) composite on LNMO material were connected by lithium iodide. Structure and morphology analyses illustrated that a higher contact area of active substances was achieved by the LATP/C composite layer without changing the original crystal structure of LNMO. XPS analysis proved that I- promoted the reduction of trace Mn4+, resulting in a higher ion conductivity. Galvanostatic charge-discharge tests exhibited the capacity of the LNMO with 5% LATP/C improved with 35.83% at 25 °C and 95.77% at 50 °C, respectively, compared with the bare after 100 cycles, implying the modification of high-temperature deterioration. EIS results demonstrated that one order of magnitude of improvement of the lithium-ion diffusion coefficient of LATP/C-LNMO was achieved (3.04 × 10-11 S cm-1). In conclusion, the effective low-temperature modification strategy improved the ionic and electronic conductivities of the cathode and suppressed the side reactions of high-temperature treatment.

Experimental Characterization of an Embossed Capacitive Micromachined Ultrasonic Transducer Cell.

Capacitive Micromachined Ultrasonic Transducer (CMUT) is a promising ultrasonic transducer in medical diagnosis and therapeutic applications that demand a high output pressure. The concept of a CMUT with an annular embossed pattern on a membrane working in collapse mode is proposed to further improve the output pressure. To evaluate the performance of an embossed CMUT cell, both the embossed and uniform membrane CMUT cells were fabricated in the same die with a customized six-mask sacrificial release process. An annular nickel pattern with the dimension of 3 µm × 2 µm (width × height) was formed on a full top electrode CMUT to realize an embossed CMUT cell. Experimental characterization was carried out with optical, electrical, and acoustic instruments on the embossed and uniform CMUT cells. The embossed CMUT cell achieved 27.1% improvement of output pressure in comparison to the uniform CMUT cell biased at 170 V voltage. The fractional bandwidths of the embossed and uniform CMUT cells were 52.5% and 41.8%, respectively. It substantiated that the embossed pattern should be placed at the vibrating center of the membrane for achieving a higher output pressure. The experimental characterization indicated that the embossed CMUT cell has better operational performance than the uniform CMUT cell in collapse region.

Aqueous Phase Synthesis of Cu2-x S Nanostructures and Their Photothermal Generation Study.

Size- and shape-dependent features of plasmonic nanocrystals govern the development of their applications. In the past decades, gold nanostructures, such as gold nanorods and nanoshells, have been well studied and applied for sensing, bioimaging, and photothermal generation. However, knowledge of copper chalcogenide, a new generation of plasmonic nanomaterials, is limited, especially about their preparation and size- and shape-dependent photothermal properties. In this work, controllable size and shape Cu2-x S nanocrystals (NCs) are synthesized by a facile aqueous route. Using low-molecular-weight polyethylenimine (PEI) as the reducing and capping agents, the size and shape of Cu2-x S NCs can be controlled with lengths from 6.5 to 46.5 nm and the aspect ratio from 2.2 to 7.5 by adjusting the concentration of PEI. The plasmonic peak of Cu2-x S experiences a redshift (from 1145 to 1369 nm) when the length increases from 6.5 to 44.5 nm. Under the irradiation of 1064 nm laser with 1.33 W/cm2, an excellent photothermal conversion rate (from 34.9 to 49.0%) is obtained. The characterization of Cu2-x S NCs is conducted with a UV-vis spectrometer, transmission electron microscopy, powder X-ray diffraction measurements, and 1064 nm laser.

Monolithic Multiband CMUTs for Photoacoustic Computed Tomography With In Vivo Biological Tissue Imaging.

Among the biomedical imaging modalities, photoacoustic computed tomography (PACT) was one of the emerging hybrid techniques in recent years. In designing the PACT imaging system, a finite-bandwidth transducer is one of the limited factors for the overall performance. As the target size is inversely proportional to the dominant frequency components of the generated photoacoustic (PA) signal, a broad bandwidth transducer is desired for different scales' imaging. In this paper, a monolithic multiband capacitive micromachined ultrasonic transducer (CMUT) array was designed and fabricated for the reception of the wideband PA signals so as to provide high-resolution images with high-frequency CMUT arrays and present the high signal-to-noise-ratio major structure with low-frequency CMUT arrays. To demonstrate its performance, a phantom experiment was conducted to show and evaluate the various qualities of multiresolution images. In addition, an in vivo mouse model experiment was also carried out for revealing the multiscale PA imaging capability with the multiband CMUTs on biological tissues. From the obtained results, the images from different CMUT arrays could show the structures of the mouse brain in different scales. In addition, the images from the high-frequency CMUT arrays were able to reveal the major blood vasculatures, whereas the images from low-frequency CMUT arrays showed the gross macroscopic anatomy of the brain with higher contrast.

Development of a multi-band photoacoustic tomography imaging system based on a capacitive micromachined ultrasonic transducer array.

Photoacoustic tomography (PAT) as a hybrid technology combines the high optical contrast and high acoustic resolution in a single imaging modality. However, most of the available PAT systems cannot comprehensively or accurately characterize biological systems at multiple length scales due to the use of narrow bandwidth commercial ultrasonic transducers. In this study, we fabricated a novel multi-band capacitive micromachined ultrasonic transducer (CMUT) array, and first developed a CMUT-based multi-band photoacoustic tomography (MBPAT) imaging system. The MBPAT imaging system was examined by the phantom experiment, and then was successfully applied to image the zebrafish in vivo. The imaging results indicated that CMUT-array-based MBPAT can provide a more comprehensive and accurate characterization of biological tissues, which exhibit the potential of MBPAT/CMUT in various areas of biomedical imaging.

Design of a Collapse-Mode CMUT With an Embossed Membrane for Improving Output Pressure.

Capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a competitive alternative to piezoelectric ultrasonic transducers, especially in medical ultrasound imaging and therapeutic ultrasound applications, which require high output pressure. However, as compared with piezoelectric ultrasonic transducers, the output pressure capability of CMUTs remains to be improved. In this paper, a novel structure is proposed by forming an embossed vibrating membrane on a CMUT cell operating in the collapse mode to increase the maximum output pressure. By using a beam model in undamped conditions and finite-element analysis simulations, the proposed embossed structure showed improvement on the maximum output pressure of the CMUT cell when the embossed pattern was placed on the estimated location of the peak deflection. As compared with a uniform membrane CMUT cell worked in the collapse mode, the proposed CMUT cell can yield the maximum output pressure by 51.1% and 88.1% enhancement with a single embossed pattern made of Si3N4 and nickel, respectively. The maximum output pressures were improved by 34.9% (a single Si3N4 embossed pattern) and 46.7% (a single nickel embossed pattern) with the uniform membrane when the center frequencies of both original and embossed CMUT designs were similar.

Estimation of measurement uncertainty caused by surface gradient for a white light interferometer.

Although the scanning white light interferometer can provide measurement results with subnanometer resolution, the measurement accuracy is far from perfect. The surface roughness and surface gradient have significant influence on the measurement uncertainty since the corresponding height differences within a single CCD pixel cannot be resolved. This paper presents an uncertainty estimation method for estimating the measurement uncertainty due to the surface gradient of the workpiece. The method is developed based on the mathematical expression of an uncertainty estimation model which is derived and verified through a series of experiments. The results show that there is a notable similarity between the predicted uncertainty from the uncertainty estimation model and the experimental measurement uncertainty, which demonstrates the effectiveness of the method. With the establishment of the proposed uncertainty estimation method, the uncertainty associated with the measurement result can be determined conveniently.

Capacitive micromachined ultrasonic transducers with piston-shaped membranes: fabrication and experimental characterization.

Capacitive micromachined ultrasonic transducers (CMUTs) featuring piston-shaped membranes (piston CMUTs) were developed to improve device performance in terms of transmission efficiency, reception sensitivity, and fractional bandwidth (FBW). A piston CMUT has a relatively flat active moving surface whose membrane motion is closer to ideal piston-type motion compared with a CMUT with uniformly thick membranes (classical CMUT). Piston CMUTs with a more uniform surface displacement profile can achieve high output pressure with a relatively small electrode separation. The improved device capacitance and gap uniformity also enhance detection sensitivity. By adding a center mass to the membrane, a large ratio of second-order resonant frequency to first-order resonant frequency was achieved. This improved the FBW. Piston CMUTs featuring membranes of different geometric shapes were designed and fabricated using wafer bonding. Fabricating piston CMUTs is a more complex process than fabricating CMUTs with uniformly thick membranes. However, no yield loss was observed. These devices achieved ~100% improvement in transduction performance (transmission and reception) over classical CMUTs. For CMUTs with square and rectangular membranes, the FBW increased from ~110% to ~150% and from ~140% to ~175%, respectively, compared with classical CMUTs. The new devices produced a maximum output pressure exceeding 1 MPa at the transducer surface. Performance optimization using geometric membrane shape configurations was the same in both piston CMUTs and classical CMUTs.

Capacitive micromachined ultrasonic transducers (CMUTs) with isolation posts.

In this paper, an improved design of a capacitive micromachined ultrasonic transducer (CMUT) is presented. The design improvement aims to address the reliability issues of a CMUT and to extend the device operation beyond the contact (collapse) voltage. The major design novelty is the isolation posts in the vacuum cavities of the CMUT cells instead of full-coverage insulation layers in conventional CMUTs. This eliminates the contact voltage drifting due to charging caused by the insulation layer, and enables repeatable CMUT operation in the post-contact regime. Ultrasonic tests of the CMUTs with isolation posts (PostCMUTs) in air (electrical input impedance and capacitance vs. bias voltage) and immersion (transmission and reception) indicate acoustic performance similar to that obtained from conventional CMUTs while no undesired side effects of this new design is observed.

Comparison of conventional and collapsed region operation of capacitive micromachined ultrasonic transducers.

We report experimental results from a comparative study on collapsed region and conventional region operation of capacitive micromachined ultrasonic transducers (CMUTs) fabricated with a wafer bonding technique. Using ultrasonic pulse-echo and pitch-catch measurements, we characterized single elements of 1-D CMUT arrays operating in oil. The experimental results from this study agreed with the simulation results: a CMUT operating in the collapsed region produced a higher maximum output pressure than a CMUT operated in the conventional region at 90% of its collapse voltage (3 kPa/V vs. 16.1 kPa/V at 2.3 MHz). While the pulse-echo fractional bandwidth (126%) was higher in the collapsed region operation than in the conventional operation (117%), the pulse-echo amplitude in collapsed region operation was 11 dB higher than in conventional region operation. Furthermore, within the range of tested bias voltages, the output pressure monotonously increased with increased bias during collapsed region operation. It was also found that in the conventional mode, short AC pulses (larger than the collapse voltage) could be applied without collapsing the membranes. Finally, while no significant difference was observed in reflectivity of the CMUT face between the two regions of operation, hysteretic behavior of the devices was identified in the collapsed region operation.

Volumetric ultrasound imaging using 2-D CMUT arrays.

Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128 x 128-element 2-D CMUT array with through-wafer via interconnects and a 420-microm element pitch. As an experimental prototype, a 32 x 64-element portion of the 128 x 128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16 x 16-element portion of the array to surrounding bondpads. An 8 x 16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4 x 4 group of elements in the middle of the 8 x 16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.