STUDY OF REGULARITIES OF FORMATION SEMICONDUCTOR GAS-SENSITIVE FILMS BASED ON OXIDES OF METALLS Ti, Zn AND W

ABSTRACT

Semiconductor gas sensors based on nanomaterials obtained using sol-gel processes, sol-gel technology for the manufacture of ammonia semiconductor sensors, and studies of hydrolytic polycondensation of TEOS during the synthesis of gas-sensitive nanocomposite films have been investigated.

All over the world, the need for environmental monitoring of atmospheric air is growing, especially in economic sectors, especially in transport, energy and industry. In particular, the improvement of existing analytical methods and instruments, the creation of new semiconductor highly sensitive sensors, the scientific substantiation of the processes of selective gas-sensitive materials. Special attention is paid to the search and development of highly efficient semiconductor sensors that determine their metrological, analytical and operational characteristics.

АННОТАЦИЯ

Исследованы полупроводниковые газовые сенсоры на основе наноматериалов, полученных с использованием золь-гель процессов, золь-гель технологии изготовления полупроводниковых сенсоров аммиака, исследования гидролитической поликонденсации ТЭОС при синтезе газочувствительных нанокомпозитных пленок.

Во всем мире возрастает потребность в экологическом мониторинге атмосферного воздуха, особенно в отраслях экономики, особенно на транспорте, в энергетике и промышленности. В частности, совершенствование существующих аналитических методов и приборов, создание новых полупроводниковых высокочувствительных сенсоров, научное обоснование процессов селективной газочувствительности материалов. Особое внимание уделяется поиску и разработке высокоэффективных полупроводниковых датчиков, определяющих их метрологические, аналитические и эксплуатационные характеристики.

Keywords: semiconductor sensor, catalyst, selectivity, sensing elements, titanium oxide, tungsten oxide, titanium oxide, tetraethoxyxane.

Ключевые слова: полупроводниковый сенсор, катализатор, селективность, чувствительные элементы, оксид титана, оксид вольфрама, оксид титана, тетраэтоксиксан.

Purpose of the study: Study of the formation of thin-film sensor elements containing SiO₂ / TiO₂, and the creation of selective semiconductor gas sensors that determine the concentration of ammonia using nanomaterials obtained on the basis of sol-gel processes.

Research goals: determination of the regularities of the synthesis process of a gas-sensitive film based on tetraethoxysilane, the composition of the initial components of the process, the ratio and temperature-time regime;

development of a technological sequence of the process of forming a gas-sensitive material, obtaining a gas-sensitive material containing SiO₂: TiO₂, and creating sensor elements on its basis;

The object of the study was metal oxides (Ti, Zn, W, etc.), mineral fertilizers and exhaust gases of industrial enterprises, atmospheric air and standard gas mixtures.

The scientific significance of the results of the study is the study of the laws of synthesis of gas-sensitive materials sol-gel and the selection of optimal conditions for the production of selective gas-sensitive nanocomposite materials based on titanium oxide, as well as the creation of selective semiconductor sensors. The practical significance of the research results is the development of selective methods for the determination of gases, multi-component process gases and atmospheric air using developed semiconductors. The developed sensors are used to control social facilities and solve social, environmental and economic problems in ensuring the safety of a number of processes where there is a risk of explosion.

Depending on a solved problem, to obtain the semiconductor thin films with reproducible and stable characteristics, different methods are used. Getting gas sensing films by the sol-gel method is the most promising direction of the receiving material of the sensitive elements o f semiconductor gas sensors [1]. The sol-gel method, in contrast to the traditionally used methods allows to diversify the structural-phase state of oxide systems due to the change of synthesis conditions, varying the ratios of components and processing conditions. This method is simple to implement, does not require expensive and sophisticated equipment makes it possible to control the thickness, composition and microstructure of the material [2]. In the sol-gel technology for preparation of selective gas-sensitive layer must be strict compliance with the sequence and conditions processes. In [3], the optimum ratio of the initial components of the solution (TEOS:H₂O:alcohol:HCl =1:20:30:0.05) of the process sol-gel synthesis of gas-sensitive films gas sensors. The aim of this work was to study and develop on their basis, recommendations for the sol-gel synthesis of thin films containing metal oxides (Ti, Zn, and W) and obtaining materials sensitive elements of semiconductor gas sensors.    The oxides of metals Ti, Zn, and W are interesting because of their extremely high sensitivity to the composition of the gas phase and simple design of the sensor. These oxides have high chemical stability and do not form chemical compounds with adsorptive particles. The advantage o f TiO₂, WO₃ and ZnO as wide bandage semiconductor oxides is their stability in air and ease of production in ultrafme condition. Introduction to the silica matrix of the oxides of  Ti, Zn and W allows obtaining selective gas sensing nanocomposites. Thin film layers based on TiO₂, ZnO and WO₃ on the parameters is very promising, but little studied. In this regard, we studied the possibility of application of these oxides as gas sensitive layer. The results of these works allow extending the application of these materials as a gas sensitive layer of semiconductor gas sensors. Obtaining sols for the synthesis of gas sensitive thin films was carried out in three stages. In the first stage at room temperature for 30 minutes was carried out the reaction of the exchange interaction of tetraethoxysilane and ethanol:  

(C₂H₅O )₄Si + 4C₂H₅OH = (C₂H₅O )₄Si + 4C₂H₅OH (solution 1). In this case, TEOS and ethanol were mixed in a certain ratio. At this stage, for the preparation o f Sol solution in a molar ratio ethanol/TEOS=30 to 10 ml of freshly distilled TEOS was poured at 88.5 ml of 96% ethanol (which correspond to 85,0 ml absolute alcohol) and at temperature 200C intensively mixed for 30 minutes. The second stage was obtained the catalyst solution and dopant in water (solution 2). The third stage is the preparation of the sol (solution 3) containing all components. To the first solution in the small portions pour the aqueous solution of dopant (solution 2) under vigorous stirring at room temperature for 1 hour:  (C₂H₅O)₄Si + 4H₂O + HC1= Si(OH)₄+ 4C₂H₅OH + HCl. The resulting product of hydrolysis of orthosilicic acid in the second stage is polycondensation and forms a main chain of the polymer molecule: Si(OH)₄ + Si(OH)₄ = Si(OH)₃-O-Si(OH)₃ + H₂O the resulting solution (solution 3) - sol orthosilicic acid has film-forming properties. The results of the calculations of a number of dopant to obtain at least 500 sensitive elements are shown in table 1.

Table 1.

The number of dopant to obtain the gas sensitive elements when the ratio Of SiO₂:

metal oxide of 1:0.5 (TEOS:water:alcohol:HCl=1:20:30:0,05)

The number of components needed to prepare 30 ml of solution (required to receive not less than 500 sensitive elements) with optimal ratio of components equal to: TEOS - 2,80 ml, H₂O-4,60 ml of ethanol and 22.6 ml, HCl-0,07 ml. To 30 ml of the initial solution contains 0,7534 g SiO₂. For obtaining of the nano-composites the main components added alloying additives in the form of salts: TiCl₄, ZnCl₂, K₂WO₄. The required amount of metal salts to prepare the solution containing SiO₂:Me_xO_y =1:0,5 is shown in table 1. In the experiments the molar ratio of SiO₂: Me_xO_y in the ash ranged from 1:0.1 to 1:2,0. The experiments studied the influence of the composition of dopant on the viscosity and time of stability of film-forming solution. With this aim, for 30 days at room temperature, studied the gelation of solutions with daphantom at a ratio of SiO₂:Me_xO_y =1:0.5 mol. The results of these experiments for SiO₂:Me_xO_y =1: 0.5 mol given in table 2.

Table 2.

The dependence of the viscosity and stability of the solution composition of dopant

(TEOS:H₂O :C₂H₅OH:HC1 = 1:20:30:0,05; SiO₂:Me_xO_y = 1,0:0,5; temperature 20°C experience, n=5)

The table shows that the dynamic viscosity of solutions with daphantom (2,6-3,8 sPa) is greater than the viscosity of the fluid (2,1 sPa) without dopant. Term stability dopant containing solutions less than of solution of this composition without dopant. Thus, supplements dopant to the solution of TEOS:H₂O :C₂H₅OH:HC1 = 1:20:30:0,05 (at a ratio of SiO₂\ Me_xO_y=2) lead to an increase in viscosity and decrease in the term stability. He study of the solutions for obtaining nano-composites of SiO₂-TiO₂, SiO₂ - ZnO, SiO₂-WO₃ has allowed to establish that the active period of film formation corresponds to low-viscosity sol in the range of viscosity from 2 to spa 5, which corresponds to the initial additive value of viscosity of initial components and a gradual increase in the viscosity of the sol, associated with the formation of primary polymeric products of the reaction. A further increase in the viscosity of the solution, corresponding to a deeper passage of the hydrolytic polycondensation of TEOS, develops, the aging process, associated with the growing polymer network in the whole volume of the solution and rolling transition sol in the gel. Depending on the conditions of implementation of hydrolytic polycondensation of TEOS solutions at the stage of low viscosity rolling sol can exist up to several tens of hours.

Conclusion. Study of kinetics of process of maturation of film forming solutions based on tetraethoxysilane with the addition of dopant and without it showed that the joint hydrolysis of tetraethoxysilane and daphantom is slower than the hydrolysis of pure tetraethoxysilane. Selected and recommended temperature and time regimes (T=450 °C, t=30 min) of synthesis of film structures of nano-composites of S i02-Me_xO_y, sol-gel method. Methods differential thermal analysis (DTA) model systems established that the processes of solvent removal and major products of chemical reactions in gels of the system SiO₂-Me_xO_y mostly completed at 450 °C.

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