Humidity and Gas Sensitivity of NI and Co Ferrites pellets with nanoscale grain size at Room Temperature

Ni and Co ferrite pellets were prepared by co-precipitation method at 100 o C as a reaction temperature and then sintered at 400 o C for the phase formation. The identified phase was confirmed by xray diffraction analysis in which all intensity peaks of Ni and Co ferrites were observed which clearly indicates that the single phase of inverse spinel ferrite. Surface morphology of pellet was studied by Scanning Electron Microscopy (SEM), which shows high reacting surface area due to small grain size ≈ 32 to 40 nm. The pellets were prepared as gas sensor elements. We have studied humidity sensing with Ni and Co ferrite in pellet form. Also with Ni and Co ferrite, sensing to NH4 and acetone gas has been studied. It has been found that the gas sensing for test gases is found to be in the order of humidity, NH4, acetone and Cl2. Here it is shown that CoFe2O4 acts as a good sensor as compared to NiFe2O4 especially for acetone and ammonia gases. key word: coprecipitation method; x-ray diffraction; gas sensor. طرلا ةبو تانيعل زاغلا ةيساسحو تمبوكلاو لكينلا تيا رف عم يبيبحلا مجحلا ةفرغلا ةرا رح ةجرد دنع سيسعناذبع يزوف عيًسناذبع ميهخ ىيهاربا فهخ ىيهاربا هدايع ذيجًنا ذبع ثيركج ةعياج – هيبرحنا ةيهك – ءايسيفنا ىسق ولاحسا خيراج :ثحبنا 81 / 9 / 1188 :ثحبنا لوبق خيراج 81 / 81 / 1188

respiratory diseases like asthma, allergic bronchial asthma, and rhinitis [1][2][3][4]. Gas sensors based on metal oxides are commonly used in the monitoring of toxic pollutants and can provide the necessary sensitivity, selectivity and stability required by such systems [5]. Spinels of the typeM 2+ M 2 3+ O 4 attract research interest because of their versatile practical applications [6][7][8].
In the case of M 3+ = Fe, the resulting spinel ferrites having a general chemical composition of MFe 2 O 4 (M = Mn, Mg, Zn, Ni, Co, Cd, etc.) are widely used as magnetic materials. Currently it is a topic of increasing interest to study the gas sensing properties of ferrites [9][10][11][12][13]. Gas sensing at room temperature is of great interest; most of the currently available sensors, expect a few types of polymer-based gas sensor operate at elevated temperature [14][15].

Experimental
The

Gas sensing setup
A specially designed gas sensing system was used for sensing of various toxic gases. The sensing setup consisted of a dome vessel of a 20L volume made of thick glass having 12 in.
diameter. The dome vessel was kept on a circular metallic base. There was a gas-handling unit attached to the system, which measured the exact amount of gas inserted in the vessel. Also one can insert gas with the help of a syringe through a valve provided on a vertical wall of the metallic base of the system. The connections from the sensor were drawn as discussed in the fabrication of sensors. The sensor was placed in the vessel and outputs were connected to a Keithley (2000 Multimeter) meter. Commercially available gases of 99% purity were used to study the sensing response, while Cl 2 gas was prepared by the reaction of HCl and bleaching powder with some water vapour. The gas HCl mixture with water vapours was passed through a cold trap for the water vapour to get condensed. Thus, we obtained pure Cl 2 gas of 99.9% purity [16]. A known amount of the gases namely Cl 2 , ammonia and acetone, was inserted with the help of a syringe and the change in resistance was noted for a fixed concentration of a gas as a function of time. The sensor response to an analyte gas is defined as the ratio of the change in resistance of a sensor in the analyte to the resistance in air [17]: Where R a and R g are the resistance in air and in gas, respectively, and ΔR is the change in resistance.

Results and discussion
Ni and Co ferrite powders were characterized at several stages of synthesis using X-ray

Fig. 2: Scanning Electron Micrographs of Ni and Co Ferrites.
To get elemental analyis, The Ni ferrite powder annealed at 400 o C was characterized by EDAX.  Table 1.   Fig 3 (a-b). The resistance of the pellet sensor was measured in air In these cases, electrons are drawn from the oxide, resulting in an increase in resistance. At room temperature there would be no oxygen adsorption as observed in the case of other oxide semiconductor gas sensors [21]. Therefore, the oxygen adsorption-desorption mechanism is not employed to sense the Cl 2 gas, and hence in the present case chlorine adsorption on the surface of the sensor is favorable mechanism. The sensing mechanism can be understood in a similar line to that of Cl 2 sensing.