Ferrite Composite Oxide (the Mfe <sub> 2 </ Sub> O <sub> 4 </ Sub>) Series Of Gas-sensing Material Preparation And New Type Of Gas Sensor Research

Gas sensitive materials of spinel type ferrite (MFe₂O₄) composite oxides were studied systematically in the thesis. NiFe₂O₄, MgFe₂O₄, CdFe₂O₄ and Zn₀.5Ni₀.5Fe₂O₄ powders were prepared by inverse titration chemical co-precipitation. The method and the technology of preparation, the structures, gas sensitive properties and gas sensitive mechanism of the materials were researched, and reaction kinetics of NiFe₂O₄ powders and MgFe₂O₄ powders were discussed. Principles of new combined structure gas sensors and choosing materials were also studied. The combined structure gas sensors based on the powders were prepared. The results are as follows:1. The preparation techniques, structures and gas sensing properties of NiFe₂O₄, MgFe₂O₄, CdFe₂O₄ and Zn₀.5Ni₀.5Fe₂O₄ nano-powders.(1) By applying inverse titration chemical co-precipitation to obtain NiFe₂O₄, MgFe₂O₄, CdFe₂O₄ and Zn₀.5Ni₀.5Fe₂O₄ nano-powders, the best techniques of above materials were acquired.(2) The sizes of NiFe₂O₄, MgFe₂O₄, CdFe₂O₄ and Zn₀.5Ni₀.5Fe₂O₄ nano-powders were analyzed by means of powder XRD. The diameter of NiFe₂O₄ powders sintered at 350⁷00℃ is about 9-39 nm, MgFe₂O₄ powders at 500⁷00℃ about 14-99 nm, CdFe₂O₄ powders at 700 ℃ about 80 nm, Zn₀.5Ni₀.5Fe₂O₄ powders at 600℃ about 50 nm. The size of the powders through TEM is about 100 nm.(3) CdFe₂O₄ powders were synthesized by solid reaction at room temperature, comparing with inverse titration chemical co-precipitation. The results exhibited that pure CdFe₂O₄ powders sintered at 700℃ were obtained by inverse titration chemical co-precipitation, but CdFe₂O₄ powders were complex oxides with CdO and Fe₂O₄, and the reaction was so tense that the reaction energy was not controlled.(4) Reaction kinetics of NiFe₂O₄ powders and MgFe₂O₄ powders were studied. The crystallite sizes increase with the increase of calcining temperature, whichfollows Arrhenius equation. The changes of 2LnD with 1/T for the NiFe2O4 materials and MgFe2O4 materials are approximately linear , and the activation energy of the NiFe2O4 materials sintered at 350- 700°C is 42.00 kJ/mol. The activation energy of the MgFe2O4 materials sintered at 500⁷00°C is 17.58 kJ/mol.(5) The gas sensing properties of NiFe2C>4, MgFe2O4, CdFe2C>4 and Zno.5Nio.5Fe204 powders were also studied. It was discovered that sensors based on NiFe2C>4 powders exhibited higher sensitivity and better selectivity to toluene (C6H5CH3) at 300°C; that the sensors based on MgFe2C>4 powders higher sensitivity and better selectivity to CH3COCH3; that the sensors based on CdFe2O4 powders higher sensitivity and better selectivity to ethanol (C2H5OH); that the sensors based on Zn0.5Nio.5Fe204 powders higher sensitivity and better selectivity to ethanol (C2H5OH) and acetone (CH3COCH3). The work was reported for the first time. 2. Gas sensitive mechanism of spinel type ferrite (MFe2O4) materials.(1) The reversible adsorption and the irreversible adsorption of gases for NiFe2O4powders were examined, comparing with the sensitive properties of sensors based on NiFe2O4 powders for gases at- different operating temperature. It is found that the total adsorption bears a liner relation to the sensitivity for gases and vise versa.(2) According to the MFe2O4 gas-sensing materials which deviated normal chemical components, to adsorption properties and surface principle, the gas-sensing mechanism of n-type and p-type MFe2O4 materials were put forward. The work was not reported.3 N+p and n+n structure semiconductor gas sensors based on the compensation-multiplication and compensation-feedback principle were studied in advance. The sensors are composed of two sensitive body A and B. The sensing properties of the structure semiconductor gas sensors have been improved when the sensitive body A and B satisfied certain conditions.4 The combined structure semiconductor gas sensors based on thecompensation-feedback and compensation-multiplication principle by choosing sensitive body A and B were designed, and its sensing properties were studied.(1) The n+p combined structure gas sensor based on the compensation-multiplication principle for ozone (O3) was obtained. hr^C^ and Pt doped p-type NiFe2O4 material used as sensitive body B, Nb2O5 doped n-type ZnO material used as sensitive body A which has better sensitivity for ozone compared with doping WO3. The gas sensor showed higher sensitivity and better selectivity to ozone gas.(2) The n+p combined structure gas sensor for ethanol (C2H5OH) was researched in this thesis. In2O3 and Pt doped n-type CdFe2O4 material used as sensitive body A, and sensitive body B is p-type C02O3 complex oxides. The experimental results exhibited that the sensor displayed higher sensitivity and selectivity, better thermal stability for ethanol.(3) The n+n combined structure gas sensor based on the compensation-feedback principle for acetone (CH3COCH3) was reported in this thesis. ZnO and ZrC>2 doped n-type MgFe2C>4 material used as sensitive body A, Nb2Os, Sb2C>3 and Pt doped n-type SnO2 material used as sensitive body B. The experimental results revealed that the sensor had higher selectivity for acetone.