响应三甲胺气体低阻TiO2基气体传感器研制

[09-12 18:41:33]   来源:http://www.88dzw.com  传感技术   阅读:8378

文章摘要:孙海波,王强,裴素华,孙振翠(山东师范大学半导体研究所,山东 济南 250014)摘要:二氧化钛是一种良好的响应三甲胺气体的敏感材料,然而用它做成传感器后存在一个明显的缺点:阻值太高。文中采用不同气氛热退火和掺杂方法来提高TiO2基材料的电导率,从而制成一种新型低阻敏感元件,对三甲胺灵敏度高、选择性好。 关键词:二氧化钛;热退火;掺杂;三甲胺1 IntroductionThe detection of food freshness is important in various fields of food industry. As for fish freshness,it is gener

响应三甲胺气体低阻TiO2基气体传感器研制,标签:传感技术知识,传感器与检测技术,http://www.88dzw.com
孙海波,王强,裴素华,孙振翠
(山东师范大学半导体研究所,山东 济南 250014)
摘要:二氧化钛是一种良好的响应三甲胺气体的敏感材料,然而用它做成传感器后存在一个明显的缺点:阻值太高。文中采用不同气氛热退火和掺杂方法来提高TiO2基材料的电导率,从而制成一种新型低阻敏感元件,对三甲胺灵敏度高、选择性好。

关键词:二氧化钛;热退火;掺杂;三甲胺

1 Introduction

The detection of food freshness is important in various fields of food industry. As for fish freshness,it is generally detected by calculating the percentage of inosine and hypoxanthine among the adenosine triphosphate(ATP)related compounds in fish muscle at home[1]. But the method requires a lot of time and efforts.

It is reported[2]that freshness of fish can be determined by detecting the concentration changes of trimethylamine(TMA)evolved from dead fish. In 1988,Egashira et al. reported[3,4]that a sensor element,made from sintered TiO2 powder mixed with Ru,was very sensitive to TMA,and could be used to detect the fish freshness. However,there is an important problem that electrical resistance of the TiO2based elements is extremely high:109~1010Ω at their operating temperature. It needs complicated and expensive electrical circuits or instrument to measure such high resistance,which greatly impeded the development of it towards miniaturization and lowpowder consumption.

In this paper,a new type of gas sensor with high sensitivity to TMA was reported,whose conductivity is relatively high by annealing and adding method.

2 Experimental

2.1 Materials

The analytical pure TiO2 sensor materials without any additives were prepared by annealing at 1000 ℃ for 6 h in the ambience of O2(600 mL/min)and N2(600 mL/min)respectively in high temperature furnace,to make comparison to the influence of different ambience to resistance of elements.

The above mentioned materials are chosen as optimal base and added with In,Nb2O5 and V2O5 with different proportion of quantity to form the sensitive materials,in order to analyze the effects of it to resistance.

2.2 Sensitive elements fabrication

The different sensitive materials were grinded in an agate mortar for 4~5 hours to ensure the additives and TiO2 are fully mixed. It was mixed with a small amount of distilled water,and the resulting paste was applied on the surface of an alumina tube(1.2 mm in diameter and 3.5 mm in length)on which two Au electrodes were printed. Then it was sintered at 800 ℃ for 6 h after drying in air. At last,it was soldered on the tube socket along with a heater,and the sensitive element was fabricated after packagnse.

2.3 Properties measurement

A gassensor characterization rig(GSCR)with the function of controlling the sensor elements temperature was used to measure all the properties of elements,such as electrical resistance in air(Ra)and in samples gas(Rg),gas sensitivity(K=Ra/Rg),selectivity,stability and so on.

3 Result and discussion

3.1The crystal structure of TiO2

Titanium consists of three different crystal structures,i.e. anatase,brookite and rutile,among which the rutile structure is the most stable at high temperature[5]. The device is not stable and susceptible to fracture if it is mainly of anatase or brookile. In order to avoid these adverse effects,titanium were preprocessed in 1000 ℃ for 1 h in air to obtain rutile structure. The XRD analysis of asprepared material are shown in Fig.1.

3.2The effects on resistance in different ambience

The sensitive elements are made from sensitive materials annealed in N2 and O2 respectively and added with adhesion agent,and the resistance in air(Ra)of which are measured by GSCR after ageing. The results are shown in Table 1.

From above results,we can see the resistance of elements annealed in N2 is apparently low in comparison to that of elements annealed in O2.

Titanium is a kind of nonstoichiometric oxide which preferable to show oxygen vacancy. The resistance of it can be modified through controlling the stoichiometry as reported[6]. In contrast with O2,N2 is liable to introduce oxygen deficiency in the annealing,which can be shown in two feasible cases,i.e. oxygen vacancy and metal interstitial atoms. But at any case,a donor level can be introduced in the forbidden band. At certain condition,electrons at donor level can be excited into the conduct band,and decline the resistance.

3.3 The effects of dopants on resistance and sensitivity of elements

The resistance in air and sensitivity of elements made from titanium with different additives are shown in Table 2.

From above data,we can see when only doped with In(2%),the elements show comparatively high resistance and relatively low sensitivity to TMA. While when doped with Nb2O5 or V2O5,both the resistance and sensitivity were greatly improved. In terms of resistance,V2O5 is better. Yet in terms of sensitivity Nb2O5 is much better. In comprehensive comparison,Nb2O5 is superior to V2O5 in the improvement of the TiO2based sensitive elements properties.

The resistance of the V2O5 is much lower than that of TiO2,while the diffusion coefficient of oxygen vacancy in it is relatively large. The atom radius of V5+ is of the similar size to that of Ti4+,so when doping with V2O5,Ti4+ can be readily substituted by V5+. The defect reaction is

V2O5=2Vn′+2e′+4Ox0+1/2(g)

where Vn′ is the ionizing donor located in Ti lattice. The concentration of electron is equal to that of Vn′. With the increase of V2O5,the concentration of electron increased,therefore,the resistance of ntype titanium is decreased.

The cause of the improvement in resistance by Nb2O5 is similar to that by V2O5. The exact mechanism of the improvement in sensitivity by both is not clearly at present.

3.4 Comprehensive performance of sensor

The main sensitive material consists of TiO2,In(2%)and Nb2O5(10%),in which adhesion agent was added to make elements. Its performance is shown as follows:

(1)Resistance in air(Ra)

The resistance of elements in air is below 1MΩ,measured by the GSCR with heating power PH=5.4 V×165 mA=891 mW,and much lower than that of In/TiO2(Ra≈78 MΩ,PH=1400~1500 mW),which provide premise for the sensitive elements with lowpower consumption at room temperature.

(2)Sensitivity versus concentration of TMA

The sensitivity of elements to various concentration of TMA gas370 ℃ is shown in Fig.2.

As is shown in Fig.2,the elements have excellent sensitivity in wide range of concentration with the minimum of ×10-5(10 ppm)(K=18). With increase of concentration the sensitivity increased and when concentration of TMA is ×10-3(1000 ppm),sensitivity is more than 200,which increased by factor of seven to that of In/TiO2.

(3)Selectivity of TMA gas

The interference to TMA gas mainly is due to ammonia in background,and is shown in Fig.3.

We can see that the elements show very low sensitivity to ammonia,which shows its excellent selectivity.

(4)Time of response and recovery

The time of response and recovery is measured in experiment,where their response time is about 10~15 seconds with TMA of 100×10-6(100 ppm)and the recovery time is about 2 min. It can be concluded that elements have quick response and recovery properties with high concentration of TMA,and can be used as an excellent element sensitive to TMA.

4 Conclusion

Annealing in N2 can introduce oxygen deficiency into titanium and produce oxygen vacancy or metal interstitial atoms,and as a result the electrical resistance declines apparently.

Doping with Nb205 and V205 into TiO2 both can improve conductivity of sensitive elements apparently. In consideration of sensitivity,Nb205 is an excellent dopant which can improve both the conductivity and sensitivity to TMA.

Rutile titanium after annealing in N2 added with Nb2O5(10%)was used as sensitive material,and the sensor made from which has a low resistance,high sensitivity to TMA,excellent selectivity and short response & recovery time. This method has a potential for realizing a rapid nondestructive analysis of fish freshness.

本文摘自《微纳电子技术》


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