Increasing the specific superficial area of gas sensing layer is a direction to improve the performance of sensors. Considering the special features, high specific superficial area of fractal geometry and the fractal properties of thin film in the deposition and growth progress, many studies have explored the relationship between fractal geometry and gas sensing characteristics of the sensors. In this paper, the application and development of fractal geometry in gas sensors are reviewed, the superiority of fractal geometry in the application of gas sensors, as well as the formation mechanism and influencing factors of fractal geometry are indicated. The advantages of fractal analysis in the characterization of film morphology and properties are also discussed. This review demonstrates that fractal geometry can be successfully used in the design of macro and micro structure of gas sensors to improve the performance of sensors. Films with appropriate fractal structure and best gas sensing performance can be obtained by controlling the deposition and parameters. 3D printing and MEMS technologies provide an implementation scheme to prepare sensors with fractal structure, such as electrodes and heater based on Hilbert curve and substrates based on Menger sponge.

Gas sensor plays a key role in many applications with sensitivity being a critical performance characteristic. Increasing the surface area of gas sensing material is one approach that can increase sensitivity. Fractal geometries, which have the large specific surface area and special fractal dimension, have previously been successfully used in the design of macrostructure and microstructure of gas sensors to improve their performance. In this article, the influence of geometrical structure of the substrate on the gas sensor performance has been investigated. Two fractal structures (Koch snowflake and Menger sponge) and one traditional structure (Cylinder) were fabricated by 3-D printing and coated in Ag-doped multiwalled carbon nanotube (Ag:MWCNT)-based sensing materials. The fabricated sensors were tested with nitrogen dioxide at different temperatures and humidity. Experimental results show that the sensitivity of gas sensors with fractal structures is increased more than twice that of those with traditional geometrical structures.

An important component of any chemiresistive gas sensor is the way in which the resistance of the sensing film is interrogated. The geometrical structure of an electrode can enhance the performance of a gas-sensing device and in particular the performance of sensing films with large surface areas, such as carbon nanotubes. In this study, we investigated the influence of geometrical structure on the performance of gas sensors, combining the characteristics of carbon nanotubes with a novel gas sensor electrode structure based on fractal geometry. The fabricated sensors were tested with exposure to nitric oxide, measuring both the sensor resistance and capacitance (RC) of the sensor responses. Experimental results showed that the sensors with fractal electrode structures had a superior performance over sensors with traditional geometrical structures. Moreover, the RC characteristics of these fractal sensors could be further improved by using different test frequencies that could aid in the identification and quantification of a target gas.

In this brief, a joint design for cooperative spectrum sharing framework of multiple-input multiple-output (MIMO) radar and MIMO communication system in radar multi-target optimization is proposed. The interesting design Degrees of Freedom (DOFs) include radar transceiver beamforming vector and communication system codebook, whereby the designed problem is a constrained minimization of total radar effective interference power under power limited conditions and communication rate constraint guaranteeing both the two coexistence systems performance. As to the non-convex multi-variables problem, an alternating iteration approach is developed to solve the three suitably transformed subproblems, the closed form solutions are derived, and the convergence of the proposed design is guaranteed. Finally, numerical results show the spectrum sharing performance of the proposed approaches.