New concept in Humidity sensing: Role of Molecular Brownian energy and probabilistic Mean free path of Vapour molecule.

A break in traditional pore morphology approach in anodic alumina, is presented here to see its niche merit over the conventional sensors for water vapour detection. Cylindrical pore structure was replaced with normal cone for trace-, and inverse cone for RH- level detection. The normal conical pore was fabricated by sheer manipulation of reaction rates of electrolytes, anodic polarization rate and time; and the procedure made reversed in case of inverse cone structure. Sensor with normal cone geometry, exhibits excellent response in ppm and slightly extended to low RH level with detection range 120 ppm - 30% RH; having response- and recovery time of 6s and 255s, measured at 120 ppm. Lowering of minimum detection limit, further requires alteration of conical geometric parameters, in tandem with molecular dynamics of water vapour molecules within pore. In contrast, sensor developed from inverse conical structure shows response only in RH level, from 20% RH - 90% RH; with response and recovery time less than 60s over the entire range. The limitations such as nonlinear response, large response recovery time, and high hysteresis as observed in conventional anodic alumina based humidity sensors have been removed. The sensor response in conical and inverse conical pore morphology is compared with standard sensors having cylindrical pore morphology; with top pore diameter identical to the reported sensors. The standard sensors found to detect in the RH range only; with response- and recovery time below 20s. Sensing mechanism in both the structures, have been suitably demonstrated and ratified with experimental data. Trace level detection is interpreted with the statistical probabilistic approach in the light of kinetic theory of gases and Brownian energy. A correlation between top surface pore diameter (through which water molecules enters), and the optimized mean free path of vapour molecule is established, and demonstrated its effectiveness for humidity detection in trace level. The results are encouraging, and same concept may be tried for detection of other gaseous stimuli including organic vapours.