Record-Low Thermal Boundary Resistance between Diamond and GaN-on-SiC for Enabling Radiofrequency Device Cooling.

The implementation of 5G-and-beyond networks requires faster, high-performance, and power-efficient semiconductor devices, which are only possible with materials that can support higher frequencies. Gallium nitride (GaN) power amplifiers are essential for 5G-and-beyond technologies since they provide the desired combination of high frequency and high power. These applications along with terrestrial hub and backhaul communications at high power output can present severe heat removal challenges. The cooling of GaN devices with diamond as the heat spreader has gained significant momentum since device self-heating limits GaN's performance. However, one of the significant challenges in integrating polycrystalline diamond on GaN devices is maintaining the device performance while achieving a low diamond/GaN channel thermal boundary resistance. In this study, we achieved a record-low thermal boundary resistance of around 3.1 ± 0.7 m2 K/GW at the diamond/Si3N4/GaN interface, which is the closest to theoretical prediction to date. The diamond was integrated within ∼1 nm of the GaN channel layer without degrading the channel's electrical behavior. Furthermore, we successfully minimized the residual stress in the diamond layer, enabling more isotropic polycrystalline diamond growth on GaN with thicknesses >2 µm and a ∼1.9 µm lateral grain size. More isotropic grains can spread the heat in both vertical and lateral directions efficiently. Using transient thermoreflectance, the thermal conductivity of the grains was measured to be 638 ± 48 W/m K, which when combined with the record-low thermal boundary resistance makes it a leading-edge achievement.

Vibrational and sonochemical characterization of ultrasonic endodontic activating devices for translation to clinical efficacy.

Passive activation of endodontic irrigants provides improved canal disinfection, smear layer removal, and better subsequent sealing. Although evidence suggests that passive activating endodontic devices increase the effectiveness of irrigation, no study exists to quantitatively compare and validate vibrational characteristics and cavitation produced by different ultrasonic endodontic devices. The current study aims to compare the efficiency of various commercially available ultrasonic endodontic activating devices (i.e., EndoUltra™, EndoChuck, Irrisafe™, and PiezoFlow®). The passive endodontic activating devices were characterized in terms of tip displacement and cavitation performance using scanning laser vibrometry (SLV) and sonochemical analysis, respectively. The obtained results showed that activator tip displacements and speed correlate to established cavitation thresholds. The EndoUltra™ tip speed was measured to be 14.5 and 28.1 m/s at 45 and 91 kHz, respectively, which is greater than the threshold. The EndoUltra™ was found to be the only device that exceeds the cavitation thresholds (i.e. tip speed and displacement), as evident from laser vibrometry analysis, and subsequently yielded measurable cavitation quantified via sonochemical analysis. All other passive endodontic activation devices, despite ultrasonic oscillation, were unable to produce cavitation.

On-chip detection of gel transition temperature using a novel micro-thermomechanical method.

We present a new thermomechanical method and a platform to measure the phase transition temperature at microscale. A thin film metal sensor on a membrane simultaneously measures both temperature and mechanical strain of the sample during heating and cooling cycles. This thermomechanical principle of operation is described in detail. Physical hydrogel samples are prepared as a disc-shaped gels (200 µm thick and 1 mm diameter) and placed between an on-chip heater and sensor devices. The sol-gel transition temperature of gelatin solution at various concentrations, used as a model physical hydrogel, shows less than 3% deviation from in-depth rheological results. The developed thermomechanical methodology is promising for precise characterization of phase transition temperature of thermogels at microscale.

A patterned single layer graphene resistance temperature sensor.

Micro-fabricated single-layer graphenes (SLGs) on a silicon dioxide (SiO2)/Si substrate, a silicon nitride (SiN) membrane, and a suspended architecture are presented for their use as temperature sensors. These graphene temperature sensors act as resistance temperature detectors, showing a quadratic dependence of resistance on the temperature in a range between 283 K and 303 K. The observed resistance change of the graphene temperature sensors are explained by the temperature dependent electron mobility relationship (~T-4) and electron-phonon scattering. By analyzing the transient response of the SLG temperature sensors on different substrates, it is found that the graphene sensor on the SiN membrane shows the highest sensitivity due to low thermal mass, while the sensor on SiO2/Si reveals the lowest one. Also, the graphene on the SiN membrane reveals not only the fastest response, but also better mechanical stability compared to the suspended graphene sensor. Therefore, the presented results show that the temperature sensors based on SLG with an extremely low thermal mass can be used in various applications requiring high sensitivity and fast operation.