Nature sub-journal of the City University of Hong Kong academician team: Composite phase diamond reaches its limit!

CPD essentially originates from the graphitization transformation of diamond. The synthesized CPD has a unique structure and is completely different from traditional graphitized diamond. As shown in Figure 1a, the HRTEM image clearly shows island-like amorphous carbon and graphite fragments evenly embedded in the diamond matrix. These amorphous carbon and graphite fragments together constitute the sp² hybridized carbon phase, while the diamond matrix exhibits an atomic spacing of 0.206 nanometers. This composite phase structure was verified by a variety of characterization methods. In particular, the average size of the amorphous carbon region is 3.55 nanometers (volume fraction 26%), and its atomic spacing is approximately 0.338–0.357 nanometers; while the size of the graphite fragment region is 3.01 nanometers (volume fraction 17%), and its atomic spacing is 0.338 nanometers.

The transformation process of diamond to graphite is quite controversial. It first directly transforms into a rectangular {0002} graphite face along the [_112] crystal axis direction, and then transforms into a fingerprint-like structure toward the<1010>crystal axis (Figure 1b). The entire transformation can be simplified It is d{111}→g{0002}. This transformation process is similar to the transformation of graphite into diamond. Low-magnification HRTEM images show that the nanocomposite phase structure exhibits a repeatable discontinuous periodic distribution in an area of at least 2 microns. The phase transformation process also produces uniform alternating tension and compressive stress fields. These structures and stress fields may be the key to enhancing diamond's high-temperature oxidation resistance.

Conductivity and temperature coefficient of CPD

As the temperature decreases from 400 K to 10 K, the resistance of the synthesized CPD increases almost linearly and rises significantly below 10 K. CPD has stable performance close to absolute zero, and its theoretical calibration accuracy is up to 8 mK in the range of 3 to 400 K, with residual errors mainly within ±0.008 arb. In particular, at low temperatures of<10 K, CPD achieves a temperature measurement resolution of 1 mK, which far exceeds the level of existing cryogenic sensors. For example, the resistance of CPD at 2 mK and 1 mK is 16.36614 Ω and 16.36652 Ω respectively, a difference that significantly exceeds the resistance change in adjacent temperature intervals. In addition, researchers have successfully synthesized CPDs at multiple scales, including micron-scale probes (Ø=1 µm), millimeter-scale bulk CPDs, and 3D printed complex structures. These different scales of CPD expand its application scope and significantly enhance its potential value.

CPD magnetoresistance

Cryogenic measurements are greatly limited by the presence of magnetic fields. At temperatures >14 K, the CPD is almost insensitive to magnetic field changes (Fig. 3a). Even at<14 K, the change in magnetoresistance of the CPD remains relatively small. At a temperature of 2 K and a strong magnetic field of 9 T, the resistance change rate of CPD is only ~3%. The insensitivity of CPD to magnetic fields is of great significance in applications such as nuclear magnetic resonance.

Summarize

The researchers successfully synthesized CPDs with excellent electrical properties and thermal stability through a simple method. CPD demonstrated its performance and stability as a temperature sensing material in extreme environments. The lowest temperature measurement limit of CPD reaches 1 mK, which can contribute to the advancement of low-temperature physics and quantum physics. In addition, the composite phase structure also has a great impact on diamond properties. This nanoscale crystal reconstruction transforms diamond from a superhard material into a functional material. The successful synthesis of CPD will open up new avenues for the application of diamond in precision measurement, quantum computing, medical equipment and space technology.

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