| Protein in fish foodstuffs such as fish, shellfish, shrimp and the foodstuffs made from them can readily be hydrolyzed into volatile salt-based nitride, NHa, histamine and trimethylamine (TMA) by the action from themselves and the outer microbes with increasing of time during the storage and transportation, which decreases freshness and quality of the meat. Nowadays, the global market capacitance of fish foodstuffs is increasing by the ratio of 30% per year. In this condition, it is vital to strictly check and evaluate the fish freshness, for it is not only an important symbol to measure and judge the quality of food, but also related to the health and lives of general consumers. Consequently, the maintenance of the freshness offish foodstuffs and sensors to test the freshness of them attach much attention.The deterioration of fish foodstuffs is a gradual and complicated process. At present, the main method at home and abroad to test the freshness of fish is to test the concentration of amine or ammonia in fish meat using such physical-chemical analytic methods as spectrophotometry and thin-layer chromatography, etc. All the above-mentioned methods have such shortcomings as complicated manipulation, high cost, much effort and time, low consistency and repetition of measurement, which urgently calls for a simple and efficient gas sensor to test the fish freshness. All kinds of TMA sensors for detection of fish freshness have been made recently at home and abroad using sintering method, metal organic chemical vapor deposition (MOCVD) method, plasma enhanced chemical vapor deposition (PECVD) method, etc, which have such shortcomings as too high resistance and low sensitivity. In view of too high resistance, complicated circuits and apparatus are needed, which hasgreatly hampered the realization of micromation, low power and integration of the sensor. So it is urgent to find low-resistance materials sensitive to TMA.Among all the semiconductor metal oxides, titanium dioxide (TiOa) is most sensitive to TMA, and it has been testified abroad. Yet it has such chemical and energy band properties as weak n-type, wide band gap, and low activity of outer shell electrons of Ti, etc, which seriously affect the decrease of its resistance.Based on the above-mentioned structural properties of TiOa and the analysis in this paper about the current investigation states of testing fish freshness and TiOa-based gas sensor, the methods for decreasing resistance of the TiC^-based sensitive materials and improving their sensitive properties are introduced, which consists of pre-treatment, heavy doping, long-time heating, and structural modification of the electrodes. The effect of above-mentioned methods to the resistance (Ra) and sensitivity (K) of TiO2-based materials is also discussed therein. At last the TMA-sensing mechanism of TiCVbased sensitive materials and the catalytic mechanism of additives are discussed extensively. The innovations of this paper are shown as following:1. TiOj ultra-fine power is annealed in N2 ambience at 1050癈 for 6 hours. Due to the serious insufficiency of oxygen, the deflection of stoichiometric proportion of TiOj and the self-semiconductivity are improved. TiO2 material is changed from weak n-type to strong n-type, and its resistance decreased to 2 ~ 3 X 107 Q . Controlling of the stoichiometric proportion deflection provides a new way for resistance change, meanwhile pre-treatment at high temperature makes TiOa utterly changed to rutile structure.2. The main part in this paper is the resistance decreasing of TiOi by doping. Nb2O5(10%), Sb2O3(0.5%) and In(2%), as additives, can decrease the resistance(Ra) to below 10M Q and partly improves the sensitivity of element, owing to the strong catalysis of additives. The minimum concentration of TMA the sensor sensitive to inThe result is the decrease of adsorbed oxygen density, release of electrons and decrease of electron voltage barrier on crystal boundary. Consequently it increased the conductivity of sensitive materials and r...
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