Dielectric behaviour of propylene glycol-water mixtures studied by time domain reflectometry

Dielectric relaxation measurements on water mixtures of propylene glycol across the entire concentration range were carried out using time domain reflectometry at 25°C over the frequency range from 10 MHz to 4 GHz. For all the mixtures, only one dielectric loss peak was observed in this frequency range. The relaxation in these mixtures can be described by a single relaxation time using the Debye model. A plot of the calculated relaxation time of the mixtures gives a straight line against the mole fraction of water, X_w. It is reasoned that the diameter of the water cluster is nearly the same as the length of propylene glycol. Further, a plot of the dielectric relaxation strength δ? against X_w suggests that there is a changing pattern of dielectric behaviour from below X_w = 0.5 to higher values of X_w. The excess permittivity, the excess inverse relaxation time and the activation free energy have been determined, to confirm the formation of hydrogen bonded homogeneous and heterogeneous cooperative domains, the dynamics of solute-solvent interaction and the hindrance to molecular rotation in the hydrogen bonded glass forming propylene glycol-water system.     

Low-limit detection of NO2 by longitudinal mode selection in a photoacoustic resonant system

The paper reports the pulsed laser-based photoacoustic (PA) spectroscopy of NO₂ in a resonant PA cavity with special filters made of stainless steel. The PA cell along with special types of sound filters are designed and fabricated to excite only the second-order longitudinal mode inside the cavity. The second harmonic, i.e. λ?= 532 nm pulse width, of 7 ns obtained from Q-switched Nd:YAG laser at 10 Hz repetition rate has been used to study the saturation behaviour of the PA signal and absorption coefficient with respect to the input laser energy. Generally, the Q-factor of longitudinal modes in the acoustic cavities is quite low. However, by modifying the design of the cell and the filter, we can achieve high value of Q = 30. The combination of special filter along with the experimental data acquisition technique helped us to achieve the minimum detection concentration of NO₂ of the order of 9 ppbV which is much better than the previous value of the same PA cell without filter [Yehya and Chaudhary, Appl. Phys. B 106, 953 (2012)].