The pressure fluctuations in fluidized beds contain useful information for indexing the quality of fluidization. However, the highly random and nonlinear signal is hard to investigate and far from being fully understood, which eventually hampers its industrial application. Many scholars have studied the causes of the pressure fluctuations (Fan et al., 1981, and references therein). Aside from the influence of the operating condition, Fan et al. (1981) demonstrated that the coalescence and motion of bubbles appear to be the major causes of the pressure fluctuations, while gas jetting, the formation of small bubbles above the distributor, and the raining of fluidized particles in the upper half of the beds also contribute to the pressure fluctuation. Roy et al.3 (1990) results showed that local pressure fluctuations appear to be characteristic of large fluctuations in other parts of the bed. The fractal feature was also explored in the gas-solids fluidized bed. Taking the long-term correlation property exhibited in the signal into account, Neogi et al. (1993) considered the signal as the product of a sine wave and fractional Gaussian noise. The multifractal character of the signal was also discussed by Zhong et al. (1996). Applying chaos theory to the analysis of pressure fluctuations in gas-solids fluidized beds has recently become popular, as it provides an interesting insight into the system's chaotic characteristics (Hay et al., 1995;Schouten et al., 1996).In this article, we study the signal composition from its correlation structure. A great deal of evidence of the relatively slow decay of the autocorrelation function via time lag for the signal (for example, see Figure 8 in Fan et al., 1981; Figures 5-10 in Neogi et al., 1988; Figures 9-12 in Neogi et al., 1993) suggests that the inherent long-term correlation component exists in the time series. We think that this featureCorrespondence concerning this article should he addressed to Z . He.Address For Z . He: 12-1-402, Youyi Xincun. Songmuchang, Hangzhou. 310007. P.R.China.can be attributed to the intrinsic causality of bubble growth and motion, the endurance of the pressure fluctuations they cause, and the statistical similarity of the bubble phase in the bed at different times. Also, the nonstationarity of the signal was recently demonstrated by He et al. (1997). In order to model this part of the pressure fluctuations, fractional Brownian motion (FBM) introduced by Mandelbrot and Ness (1968) appears to be an appropriate candidate. It is a zeromean, nonstationary Gaussian random process with statistical self-similarity. If one denotes FBM as B&), its covariance function where H is the self-similarity parameter. It is clear that B&) bears the infinite correlation property. The parameter H controls the "roughness" of FBM, which corresponds to the fractal dimension:The greater the parameter H , the more regular the FBM appears. In fact, one tends to search for periodicity in the signal for large H value. H also controls the shape of the average spectral d...

Owing to the simplicity, scalability, and costefficiency, solution-processable two-dimensional (2D) semiconductors have attracted great interest in electronic applications, especially as the channel material for field-effect transistors (FETs). Inkjet printing is a lithography-free technique to achieve drop-on-demand patterning of solution-processable 2D ink. However, thus far, inkjet-printed 2D FETs exhibit limited performance due to the coffee-ring effect and consequent discontinuity of the printed 2D material films. Here, we report high-performance and flexible inkjet-printed MoS 2 FETs with high mobilities and high on/off ratios and their gas sensing applications. By preparing high-quality MoS 2 ink comprised of MoS 2 nanoplates using electrochemical exfoliation and then applying a binary solvent comprised of 2-butanol and isopropanol, the obtained ink was printed to form a continuous and relatively uniform MoS 2 film, and high-performance printed MoS 2 FETs were demonstrated, with mobilities of 11 cm 2 V −1 s −1 and on/off ratios of 10 6 . Furthermore, low-voltage gate modulation was achieved by applying an ion gel gate, and robust electrical performance under tensile strain was observed for the ion gel-gated MoS 2 FETs printed on flexible substrates. As the printed MoS 2 film is abundant in edge sites and sulfur vacancies, we further demonstrated our MoS 2 FETs as high-performance gas sensors with a limit of detection of 10 ppb for NO 2 and 0.5 ppm for NH 3 , together with a fast recovery rate.

thickness, health and environment issues, fire resistance, moisture resistance, and durability issues. [1][2][3][4] In the past two decades, varieties of artificial structures, collectively called "acoustic metamaterials," have been designed to manipulate sound waves beyond natural limits. [5][6][7][8][9][10][11][12][13] For example, low-frequency (<500 Hz) sound waves have strong penetration ability, long propagation distance, and low attenuation coefficient, hence it is still a great challenge to deal with low-frequency noises. [1,3,4,14] In the last decade, diverse sound-absorbing metamaterials in subwavelength scales have been designed, which generally increase the density of states at targeted low-frequencies in the metamaterials in order to enhance the dissipation of sound energy. [15][16][17][18][19][20][21][22][23][24][25] However, for most acoustic metamaterials, their targeted frequencies and operating functions can hardly be adjusted after being fabricated. It is hence an intrinsic barrier for their applications in scenarios where the frequencies of noise can change over time.To overcome this stringent barrier, tunable acoustic metamaterial absorbers have been proposed and investigated in recent years. [26,27] Some tunable mechanisms, mainly employing the mechanical deformations, piezoelectric, and/or magnetic effects, have been presented. [28][29][30][31][32][33] However, they generally require rigid backings to impede sound transmissions, which forbid the transmission of fluids, such as air and water. The absorption of a ventilated acoustic metamaterial composed of subwavelength scatters is usually much smaller compared with its reflection. It is a challenge to break the limit and finally achieve a tunable absorber with simultaneously good absorption and ventilation at low frequencies. [33] Besides, their intricate mechanical, electronic, and magnetic structures are inconvenient and may be unstable for daily applications. [34,35] Moreover, previously designed adjustable metamaterial absorbers could not automatically perceive incident sound to realize intelligent adjustment and absorption. [21,[36][37][38][39][40][41][42][43] A simpler, more adjustable, and more automatic scheme is eagerly needed.Here, we demonstrate an automatically adaptive ventilated metamaterial absorber (AAVMA) for low-frequency sound that can achieve high-efficiency (>90%) absorption for onesided incidence with frequency varying in the working range. The ventilated absorber can automatically monitor the environmental noise and intelligently adjust the sound absorption An automatically adaptive metamaterial sound absorber is designed, which can absorb tunable low-frequency (<500 Hz) sounds under ventilated conditions by designing a feedback circuit to actively detect the noise signals and adjust the sliders on the reconfigurable absorbers. The automatically adaptive ventilated absorber provides an intelligent route to adapt for different low frequencies, through adjusting the sound absorption units directly in accordance with the...