Nanostructured SnO2- and ZnO-Based Gas Sensors for Early Warning of Electrical Fires
Authors: Chen Wenjia , Han Jia , Yi Jianxin 
Published: May 2020

Introduction

A key to prevent and control fires lies in the early and accurate detection of fire signatures. Over-heated or burned electrical cable with PVC (polyvinyl chloride) insulation or sheath is a frequent cause of fire. PVC cable insulation contains a large amount of various additives like plasticizers, which can be substantially emitted under overheating conditions. Our recent study showed that two widely-used plasticizers, Dioctyl phthalate (DOP) and 2-Ethylhexanol (2-EH), are ubiquitously present in the vapors of different overheated PVC cables, and can be used as a reliable signature of cable overheating [1]. These compounds, especially DOP, have low vapor pressure under normal conditions, and are usually measured through analytical methods like gas-chromatograph mass spectroscopy. These techniques are generally expensive and time-consuming, and thus not suitable for wide applications in fire detection.
Metal oxide semiconductor (MOS) gas sensors offer a promising alternative for low-cost and real-time detection of plasticizers. Nevertheless, MOS sensors sensitive and selective to DOP and 2-EH are not yet available. MOS sensors with high gas sensing performance are often made of nanostructured materials based on SnO2, ZnO, etc. Modification of the materials surface by noble metals or metal oxides may efficiently further enhance the sensing performance. In this contribution, we report on the development of gas sensors targeting detection of the fire signature gases and cable overheating, with an emphasis on modified SnO2 hollow nanofibers [2] and ZnO flowers.

Results and Conclusions

SnO2 nanofibers obtained via electrospinning were porous and hollow, with a diameter of around 200 nm and length of 1-2 mm. They were loaded with various amounts of NiO, In2O3, and ZnO oxides via a facile impregnation-calcination route (Fig. 1). Samples loaded with 2.8 at.% In2O3 (SIn-0.1) and 5.0 at.% ZnO (SZn-0.5) exhibited excellent response to the fire signature gases, DOP and 2-EH, whilst NiO modification deteriorated the sensing performance. A maximum response of 44 and 27 to 100 ppm DOP and 2-EH, respectively, was achieved, ~5 or 9 times larger than that of a commercial TGS822 VOC sensor (Fig. 2). The nanofiber gas sensors exhibited very fast response speed with typical response time of 3-5 s at 260 °C. In contrast, the recovery was much slower, taking ~10 min for DOP. Moreover, low cross-sensitivity towards common interfering gases such as ethanol and H2 were observed. Large-scale simulation tests performed in a 182 L chamber showed that the modified sensors started to respond to the fire hazard earlier than a conventional smoke detector, particularly at lower cable temperatures (below 200 °C), whilst the TGS822 sensor exhibited almost no response (Fig. 3).
ZnO flowers fabricated via a hydrothermal route were in a size of a few microns, assembled with nano-columns of ~200 nm diameter. Samples were decorated with various amounts of CuO, Cr2O3, and Au. The modification led to decrease of the operation temperature and enhanced response to the signature gases. The modified ZnO samples presented a maximum response of 48 and 28 to 100 ppm DOP and 2-EH, respectively (Fig. 4). Sensitive response to the cable overheating was also achieved.
These results showed that MOS sensors based on modified SnO2 nanofibers and ZnO flowers can effectively detect the overheating of PVC cables, promising for early warning of electrical fires.

References

[1] J. Han,W. Chen, A. Yu, J. Yi, Detection of semi-volatile plasticizers as a signature of early electrical fire, Frontiers in Materials (2019) 6:250. doi: 10.3389/fmats.2019.00250.
[2] J. Yi, W. Chen, J. Han, D. Chen, Sensitive and selective detection of plasticizer vapors with modified-SnO2 hollow nanofibers for electrical fire warning, Sensors & Actuators B. Chemical 287 (2019) 364–370. doi: 10.1016/j.snb.2019.02.025.

Figure 1