Comparative study of rare- earth based ferrites for flammable and volatile organic compound gas sensors
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Date
2023
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University of Zululand
Abstract
The monitoring of flammables and VOCs is still a challenge. In addition, acetone (CH3CH3CO) is a one of the toxic and harmful volatile organic compounds (VOCs) commonly used as a solvent in various laboratories and industries. Its high level of inhalation and ingestion can cause low, acute and chronic poisoning. Another commonly used highly flammable gas is liquefied petroleum gas (LPG), commonly known as a cooking gas, which is widely available in almost all communities and working environments to create fires for various uses. This gas causes many fatalities due to its nature of complexities in detection and can easily ignite. This, therefore, suggests that it is one of the main air pollutants that require constant detection and monitoring. Most studies in the chemical gas sensing community focus on just the normal sensing of these gases. In an attempt to solve the long existing problem, we have undertaken this study, where MgCexFe2-xO4 (0 ≤ x ≤ 0.4) nanoparticles have been produced by the glyco-thermal technique and characterised by X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, Mössbauer spectroscopy and gas sensing analyses. The X-ray diffraction results indicated that a pure cubic spinel phase was formed for samples having a low concentration of Ce, but the high Ce doping (x ≥ 0.2) of magnesium ferrite resulted in the formation of secondary phases. The crystallite size of the compounds ranged from 2.2 nm to 15.3 nm. The 57Fe Mössbauer spectra showed transformation from an ordered to a paramagnetic spin state with an increase in Ce concentration. Gas sensors fabricated from the spinel ferrites were tested towards various organic compound vapours (acetone, methanol, p-xylene, ethylbenzene, toluene, and benzene) and flammable gases (LPG, Methane, Propane, Butane and Ammonia) at an operating temperature of 225 °C. The MgCe0.2Fe1.8O4 nano-ferrites proved to possess quality sensor characteristics of high sensitivity and selectivity to acetone vapour, with a response of over 500@100 ppm concentration as well as reproducibility, reversibility, and stability of over 120 days. This sensor not only displayed high responses, but could also maintain them over 1, 3, 5, 10, 20, and 30 min of acetone exposure time. On flammable gases, the addition of cerium to magnesium ferrites proved to kill the sensing. The best performing sensor was found to be the undoped Magnesium ferrite on LPG gas. It was resilient and sensitive to an oxygen reduced, inert ambient environment. Under relative humidity, the response was reduced, but stable, due to physisorbed water molecules. In addition, this SMO sensor was tested at 200 ˚C on vegetables and fruit to evaluate their off-shelf freshness during the ripening process. These plants show a dramatic increase in the respiration rate over their maturation or ripening process over time.
OKUHUNYUSHWE NGOLIMI LWESIZULU
IMFINGQO
Ukuqapha izinto ezivuthayo kanye nama-VOC kuseyinselelo. Ngaphezu kwalokho, i-acetone (CH3CH3CO) ingenye yezinhlanganisela eziphilayo ezinobuthi neziyingozi (VOCs) ezivame ukusetshenziswa njengesinyibilikisi kumalabhorethri nezimboni ezihlukahlukene. Izinga layo eliphezulu lokuhogela kanye nokumunca lingabangela ubuthi obuphansi, obunamandla futhi obungapheli. Enye igesi evame ukuvutha esetshenziswa kakhulu i-liquefied petroleum gas (LPG), eyaziwa ngokuthi igesi yokupheka, etholakala cishe kuyo yonke imiphakathi nasezindaweni zokusebenza ukuze kubase imililo esetshenziselwa ukusetshenziswa okuhlukahlukene. Le gesi idala ukufa kwabantu abaningi ngenxa yemvelo yayo yobunzima ekubonweni futhi ingavutha kalula. Ngakho-ke, lokhu kuphakamisa ukuthi ingenye yezinto ezingcolisa umoya ezidinga ukubonwa nokuqapha njalo. Ucwaningo oluningi emphakathini ozwa igesi yamakhemikhali lugxile ekuzwaneni nje okujwayelekile kwala magesi. Emzamweni wokuxazulula inkinga ende ekhona, senze lolu cwaningo, lapho i-MgCexFe2-xO4 (0 ≤ x ≤ 0.4) nanoparticles ikhiqizwe indlela ye-glyco-thermal futhi ibonakala nge-X-ray diffraction, i-electron microscopy, i-X-ray photoelectron spectroscopy, i-Mössbauer spectroscopy kanye nokuhlaziywa kokuzwa kwegesi. Imiphumela ye-X-ray diffraction ibonise ukuthi isigaba se-cubic spinel esihlanzekile sakhiwe amasampula ane-concentration ephansi ye-Ce, kodwa i-Ce doping ephezulu (x ≥ 0.2) ye-magnesium ferrite ibangele ukwakheka kwezigaba zesibili. Ubukhulu be-crystallite bezinhlanganisela busukela ku-2.2 nm kuya ku-15.3 nm. I-spectra ye-57Fe Mössbauer ibonise ukuguqulwa ukusuka ku-oda ukuya esimweni sokujikeleza kwe-paramagnetic kanye nokwenyuka kokugxila kwe-Ce. Izinzwa zegesi ezakhiwe ngama-spinel ferrites zahlolelwa ukushunqa inhlabathi ehlukahlukene (i-acetone, i-methanol, i-p-xylene, i-ethylbenzene, i-toluene, ne-benzene) namagesi avuthayo (LPG, Methane, Propane, Butane kanye ne-ammonia) ezingeni lokushisa elingu-225°C. I-MgCe0.2Fe1.8O4 nanoferrites ibonakale inezici zezinzwa zekhwalithi zokuzwela okuphezulu nokukhetha kumhwamuko we-acetone, nempendulo yokugxila okungaphezu kuka-500@100 ppm kanye nokuphindaphindeka, ukuhlehla, nokuzinza kwezinsuku ezingaphezu kweziyi-120. Le nzwa ayizange ibonise ukusabela okuphezulu kuphela, kodwa futhi ibikwazi ukuzigcina ngaphezu kwe-1, 3, 5, 10, 20, kanye nemizuzu engama-30 yesikhathi sokuchayeka kwe-acetone. Kumagesi avuthayo, ukungezwa kwe-cerium kuma-magnesium ferrites kufakazele ukubulala inzwa. Inzwa esebenza kahle kakhulu itholwe iyi-Magnesium ferrite engafakwanga i cerium kugesi ye-LPG. Yayikwazi ukumelana nezimo futhi izwela umoya-mpilo oncishisiwe, indawo ezungezile engenzi lutho. Ngaphansi komswakama ohlobene, impendulo yancishiswa, kodwa yazinza, ngenxa yama-molecule amanzi afakwe i-physisorbed. Ukwengeza, le nzwa ye-SMO ihlolwe ku-200 ˚C ezitshalweni nasezithelweni ukuze kuhlolwe ubusha bazo obungekho eshalofini ngesikhathi sokuvuthwa. Lezi zitshalo zibonisa ukwanda okumangalisayo kwezinga lokuphefumula phezu kokuvuthwa kwazo noma inqubo yokuvuthwa ngokuhamba kwesikhathi.
Description
A Thesis submitted to the Faculty of Science, Agriculture and Engineering in fulfilment of the requirements for the Degree of Doctor of Philosophy in Physics in the Department of Physics and Engineering at the University of Zululand, South Africa [2023].