A novel solid-state thermal rectifier based on reduced graphene oxide.

Recently, manipulating heat transport by phononic devices has received significant attention, in which phonon--a heat pulse through lattice, is used to carry energy. In addition to heat control, the thermal devices might also have broad applications in the renewable energy engineering, such as thermoelectric energy harvesting. Elementary phononic devices such as diode, transistor and logic devices have been theoretically proposed. In this work, we experimentally create a macroscopic scale thermal rectifier based on reduced graphene oxide. Obvious thermal rectification ratio up to 1.21 under 12 K temperature bias has been observed. Moreover, this ratio can be enhanced further by increasing the asymmetric ratio. Collectively, our results raise the exciting prospect that the realization of macroscopic phononic device with large-area graphene based materials is technologically feasible, which may open up important applications in thermal circuits and thermal management.

Static behavior of a graphene-based sound-emitting device.

Due to the extremely high thermal conductivity and low heat capacity per unit area of graphene, it is possible to fabricate an efficient sound-emitting device based on the thermoacoustic effect with no mechanical vibration. In this paper, the fundamental performance of this new graphene sound-emitting device (G-SED) is investigated in terms of its static behavior. The sound amplitude mapping shows that the G-SED has good sound performance under 0.01 W. The sound frequency spectra measured at different distances and angles show that the G-SED has good sound directivity. It is possible to realize sound wave manipulation by using an array of G-SEDs. The relationship between the temperature of graphene and the sound frequency was investigated by a thermal imaging instrument. The fast transient sound response in real time was recorded by applying 60 µs short time multi-pulses and single-pulse. The stable sound emission at a constant sound pressure amplitude with low noise was observed for continuous operation under a fixed frequency over several hours. Such significant performances in this G-SED indicate broad applications, and shed light on the use of graphene in the field of acoustics.

Single-layer graphene sound-emitting devices: experiments and modeling.

Single-layer graphene (SLG) was demonstrated to emit sound. The sound emission from SLG had a significant flat frequency response in the wide ultrasound range from 20 kHz to 50 kHz. SLG can produce a sound pressure level (SPL) as high as 95 dB at a distance of 5 cm with a sound frequency of 20 kHz. The SPL value is among the highest reported to date for sound-emitting devices (SEDs) based on the thermoacoustic effect. A theoretical model was established to analyze the sound emission from SLG. The theoretical results are in good agreement with the experimental results. Conventional acoustic devices with a large size can be reduced to the nano-scale by using this novel SLG-SED material. It has the potential to be widely used in speakers, buzzers, earphones, ultrasonic transducer, etc.

Graphene-on-paper sound source devices.

We demonstrate an interesting phenomenon that graphene can emit sound. The application of graphene can be expanded in the acoustic field. Graphene-on-paper sound source devices are made by patterning graphene on paper substrates. Three graphene sheet samples with the thickness of 100, 60, and 20 nm were fabricated. Sound emission from graphene is measured as a function of power, distance, angle, and frequency in the far-field. The theoretical model of air/graphene/paper/PCB board multilayer structure is established to analyze the sound directivity, frequency response, and efficiency. Measured sound pressure level (SPL) and efficiency are in good agreement with theoretical results. It is found that graphene has a significant flat frequency response in the wide ultrasound range 20-50 kHz. In addition, the thinner graphene sheets can produce higher SPL due to its lower heat capacity per unit area (HCPUA). The infrared thermal images reveal that a thermoacoustic effect is the working principle. We find that the sound performance mainly depends on the HCPUA of the conductor and the thermal properties of the substrate. The paper-based graphene sound source devices have highly reliable, flexible, no mechanical vibration, simple structure and high performance characteristics. It could open wide applications in multimedia, consumer electronics, biological, medical, and many other areas.