Introduction

Currently, the number of materials used in electronic technology for various purposes is several thousand. According to the most general classification, they are divided into four classes: conductors, semiconductors, dielectrics and magnetic materials. Among the most important and relatively new materials are semiconductor chemical compounds, among which compounds of the type A II B VI are of the greatest scientific and practical interest. One of the most significant materials of this group is CdS.

CdS is the basis of modern IR technology, since its photosensitivity spectrum overlaps the atmospheric transparency window (8-14 microns), in which all environmental objects radiate. This allows it to be used in military affairs, ecology, medicine and other branches of human activity. To date, CdS is obtained in film form by a hydrochemical method.

The purpose of this course project is to implement a project for the production of sensitive elements of photoresistors based on CdS by the hydrochemical method with a capacity of 100 thousand pieces / year, as well as familiarization with the calculation method designed to preliminary determine the conditions for the formation of CdS, cadmium hydroxide and cyanamide.

1. Characteristics of cadmium sulfide

The diagram of the Cd - S system has not been built, there is one CdS compound in the system that exists in two modifications: α (hexagonal) and β (cubic). CdS occurs naturally as the minerals greenockite and howleyite.

1.1 Crystal structure

Compounds of type A II B VI usually crystallize in the structure of sphalerite or wurtzite. The structure of sphalerite is cubic, type B-3, space group F4 3m (T d 2). The structure of wurtzite is hexagonal, type B-4, space group P 6 3 mc (C 6 v 4). These structures are very similar to each other; they have the same number of atoms in both the first and second coordination spheres - 4 and 12, respectively. The interatomic bonds in the tetrahedra of both modifications are very close.

Cadmium sulfide has been obtained with both sphalerite and wurtzite structures.

1.2 Thermodynamic and electrophysical properties

Cadmium sulfide is a one-sided phase of variable composition, always having an excess of cadmium. Cadmium sulfide, when heated to 1350 ᵒС, sublimates at atmospheric pressure without melting, in a vacuum at 180 ᵒС it distills without melting and without decomposition, under a pressure of 100 atm it melts at a temperature of about 1750 ᵒС. The degree of dissociation of cadmium at temperatures above 1000 ᵒС reaches 85-98%. The heat of formation of CdS Δ H 298 0 \u003d -34.71 kcal / mol.

Depending on the conditions of production and heat treatment, the properties of CdS can be different. Thus, crystals grown in an excess of cadmium vapor have a significantly higher thermal conductivity than crystals grown under conditions of a stoichiometric composition. The specific resistance of CdS, depending on various factors, can vary over a wide range (from 10 12 to 10 -3 ohm * m).

Deviations from stoichiometry have a decisive influence on the electrophysical properties of CdS. The introduction of oxygen into the samples leads to a strong decrease in electrical conductivity. The band gap of CdS, determined from optical data, is 2.4 V. Cadmium sulfide typically has an n-type conductivity due to the lack of sulfur relative to the stoichiometric composition.

The solubility of cadmium in water is negligible: 1.5 * 10 -10 mol / l.

2. Methods for obtaining metal chalcogenides

At present, metal chalcogenides are obtained both by physical (vacuum evaporation and cathode sputtering) and chemical methods (aerosol spraying of the reaction mixture onto a substrate heated to 400–600 K or precipitation from an aqueous solution). Let's consider each method in more detail.

Vacuum condensation method

The essence of the method consists in heating the substance in vacuum (P ≥ 10 -3 mm Hg) to a temperature when the pressure exceeds the residual vapor pressure by several orders of magnitude, followed by condensation on the substrate.

Process steps:

Evaporation of a substance;

The flight of atoms of a substance to the substrate;

Deposition (condensation) of vapor on a substrate, followed by the formation of a film structure.

Method of cathodic vacuum sputtering.

The method is based on the destruction of the cathode by bombarding it with working gas molecules. The cathode is a material that is to be deposited in the form of a film. First, air is pumped out of the working area, then the working gas (argon or nitrogen) is let into the chamber. A voltage (3-5 kV) is applied between the cathode and the anode, which causes a breakdown of the gas gap. The operation of the installation is based near the plasma discharge.

Types of cathode sputtering:

Physical: no chemical reaction occurs in the system;

Reactive: involves a chemical reaction, a reactive gas (oxygen, nitrogen, carbon monoxide) is added to the working gas, with the molecules of which the sprayed substance forms a chemical compound. By changing the partial pressure of the working gas, it is possible to change the composition of the film.

It should be noted that the vacuum production of thin-film structures, having wide possibilities and versatility. It has a number of significant drawbacks - it requires complex expensive equipment, and also does not ensure the uniformity of properties.

The most attractive of the methods for obtaining sulfide films in terms of its simplicity and efficiency is the technology of hydrochemical deposition. At present, there are three main varieties of this method: chemical deposition from solutions, electrochemical deposition, and spraying of solutions onto a heated substrate followed by pyrolysis.

During electrochemical deposition, anodic dissolution of the metal in an aqueous solution of thiourea is carried out. The process of sulfide formation proceeds in two stages:

the formation of metal ions at the anode;

interaction of metal ions with a chalcogenizer.

Despite the advantages of the method: controllability and a clear dependence of the film growth rate on the current strength, the method is not economical enough; thin, uneven and amorphous films are formed, which prevents the wide application of this method in practice.

The method of spraying a solution onto a heated substrate (pyrolysis)

A solution containing a metal salt and thiourea is sprayed onto a substrate heated to 180..250 ᵒС. The main advantage of the pyrolysis method is the possibility of obtaining films of mixed composition. The hardware design includes a spray device for solutions and a heater for the substrate. To obtain films with metal sulfide, the stoichiometric metal-sulfur ratio is optimal.

Chemical precipitation from aqueous solutions is of particular attractiveness and wide prospects in terms of final results. The hydrochemical deposition method is distinguished by high productivity and economy, simplicity of technological design, the possibility of depositing films on a surface of complex shape and different nature, as well as doping the layer with organic ions or molecules that do not allow high-temperature heating, and the possibility of “soft chemical” synthesis. The latter allows us to consider this method as the most promising for obtaining compounds of metal chalcogenides of complex structure that are metastable in nature.

Hydrochemical precipitation is carried out in a reaction bath containing a metal salt, alkaline and complexing agents, and a chalcogenizer. The process of sulfide formation is realized through a colloid-chemical stage and represents a set of topochemical and autocatalytic reactions, the mechanism of which is not fully understood.

3. Application of films basedCDS

Thin-film cadmium sulfides are widely used as photodetectors, photoluminescent materials, thermoelements, solar cells, sensor materials, decorative coatings, and promising nanostructured catalysts.

4. Description of production technologyCDS

The technological scheme for manufacturing sensitive elements of photoresistors includes the following operations:

1. substrate preparation (cleaning, etching, washing);

Chemical deposition of a semiconductor film;

Film washing and drying;

Heat treatment of the semiconductor layer under the charge layer at 400 ᵒС for 2 hours;

Vacuum deposition of AI-contacts;

Scribing;

Output control of FR chips parameters.

.1 Preparation of substrates for film deposition

Film deposition is carried out on previously degreased substrates. The substrates are thoroughly degreased with soda, rinsed with tap water, and after installation in a fluoroplastic fixture, they are placed for 20 seconds in a dilute Dash solution to etch the surface in order to increase film adhesion. After treatment in the Dash etchant, the substrates are rinsed with a large amount of heated distilled water and stored in a beaker under a layer of distilled water until the start of the process.

The quality of the substrate surface preparation is controlled by the degree of its wettability: on a carefully prepared substrate, distilled water spreads in an even layer. It is strictly forbidden to take the fat-free substrate with your hands.

4.2 Chemical deposition of a semiconductor film

Sitall is used as the substrate material for deposition of CdS films.

The following chemical reagents are used for the synthesis of CdS semiconductor films:

cadmium chloride, CdCl 2 ∙H 2 O;

thiourea, CSN 2 H 4, high purity;

aqueous ammonia solution, NH 3 aq, 25%, chemically pure.

The order of draining the reagents for the preparation of the working solution is strictly fixed. The need for this is due to the fact that the process of precipitation of chalcogenides is heterogeneous, and its rate depends on the initial conditions for the formation of a new phase.

The working solution is prepared by mixing the calculated volumes of the starting materials. The films are synthesized in a 100 ml molybdenum glass reactor. First, the calculated volume of cadmium salt is introduced into the reactor, then aqueous ammonia is introduced and distilled water is added. Then thiourea is added. The solution is stirred and the prepared substrate is immediately immersed in it, fixed in a fluoroplastic device. The substrate is installed in the reactor with the working surface downwards at an angle of 15 - 20°. From this moment, with the help of a stopwatch, the countdown of the time of the synthesis process begins. The reactor is tightly closed and placed in a U-10 thermostat. The accuracy of maintaining the synthesis temperature is ±0.01°C. For some time, no changes occur with the solution. Then the solution begins to become cloudy, and a yellow mirror film forms on the surface of the substrate and the walls of the reactor. Its settling time is 60 min. Precipitation is carried out at a temperature of 70 °C.

4.3 Processing of the deposited film

After the end of the specified synthesis time, the reactor is removed from the thermostat, the substrate with the holder is removed and washed with a large amount (0.5-1.0 l) of heated distilled water. After that, the substrate is removed from the holder, the working surface of the substrate (the one on which the film was deposited) is gently wiped with cotton wool soaked in distilled water, and the sediment is removed from the back side. Then the substrate with the film is washed again with distilled water and dried on filter paper until visible traces of moisture are removed.

4.4 Heat treatment

Thoroughly washed and dried - the substrates go to the next operation: heat treatment. It is carried out in muffle furnaces PM-1.0-7 or PM-1.0-20 to eliminate stress and improve the electrical properties of the films. The process lasts 2 hours at a temperature of 400 °C, followed by cooling to room temperature.

4.5 Vacuum deposition of AI contacts

Metal films are used in the production of semiconductor devices and microcircuits as non-rectifying (ohmic) contacts, as well as passive components (conductive tracks, resistors, capacitors, inductors). The main method for obtaining metal films is vacuum deposition (thermal evaporation in vacuum) of various metals (aluminum, gold, etc.), as it has several advantages: purity and reproducibility of deposition processes, high productivity, the possibility of deposition of one or more metals on semiconductor wafers in one operation and fusing the deposited metal film and vacuum to protect it from oxidation, the ease of controlling the deposition process and the possibility of obtaining metal films of various thicknesses and configurations when deposition of metals using masks.

Spraying is also carried out in a vacuum installation with a residual pressure under the cap of the order of 6.5∙10 Pa (5∙10 -6 mm Hg). Such a pressure is chosen so that there are no collisions between the evaporated metal atoms and the molecules of the residual gas under the hood of the installation, which lead to the formation of films of a disturbed structure.

In the production of semiconductor devices for the deposition of various films on semiconductor wafers and other substrates, several models of vacuum deposition installations are used, which differ from each other in various design solutions, primarily a cap device, as well as a vacuum system, a power supply system for monitoring process parameters and controlling operating modes. , conveying and auxiliary devices for evaporation or spraying.

For thermal film deposition and sputtering in these installations, respectively, resistive and electron-beam devices are used, and for sputtering by ion bombardment, discharge devices. Despite some disadvantages (difficulty in evaporation of refractory materials, high inertia, change in the ratio of components during the evaporation of alloys), installations with electron-beam and especially with resistive evaporators are widely used in semiconductor production due to their ease of operation. Therefore, we will focus on units with resistive evaporators, the basic model of which is the UVN-2M unit.

4.6 Scribing

From a substrate with a film deposited on it, chips of a given size are cut out by scribing (the standard time is 25 min per one substrate). The semi-automatic machine for scribing ZhK 10.11 is designed for applying a grid of notches on semiconductor wafers. They break the plates with the applied risks by rolling them with a rubber roller manually or on special installations. The semiautomatic device is installed in a spacesuit fixed on the table, which serves to create a microclimate. They work on a semiautomatic device in rubber gloves built into the front wall of the suit. The workplace is illuminated by daylight lamps installed in the upper part of the suit. Drawing marks is made by the diamond cutter fixed in the swinging support.

cadmium sulfide electrophysical vacuum

4.7 Output control of "chip" parameters

Initially, the chips are subjected to visual control for the quality of the coating. Layer heterogeneities, spots, irregularities, areas with poor adhesion are noted.

The output control is carried out on the K.50.410 units (the standard time is 2 minutes per “chip”).

5. Settlement part

.1 Calculation of formation boundary conditionsCDS, CD(Oh) 2 andCdCN 2

It is necessary to find the boundary conditions for the precipitation of lead sulfide, hydroxide, and cyanamide at the following initial concentrations, mol/l:

0,4

The basis of hydrochemical synthesis is the reaction:

CdL x 2+ + N 2 H 4 CS(Se) + 4OH - \u003d CdS + CN 2 2- + 4H 2 O

In the reaction mixture, the formation of the following complex compounds is possible (Table 1):

Table 1 Initial data for calculating the conditions for hydrochemical precipitation of CdS, Cd(OH) 2 , CdCN 2

Compound (complex ion)

Let's calculate α Me z + , for this we use the expression:

where α Me z + - fractional concentration of uncomplexed metal ions; L is the ligand concentration; k 1 , k 1.2 ,…k 1.2… n - instability constants of various complex forms of metal.

For the ammonia system, the expression has the form:
8,099∙10 -9

Let's build a graphical dependence pC n =f (pH) (Fig. 2).

Rice. 2. Boundary conditions for the formation of cadmium sulfide, hydroxide, and cyanamide.

Based on the graph, we can conclude that in this system it is possible to form a CdS film at pH = 9.5-14, Cd(OH) 2 at pH = 10.5-14, and CdCN 2 is not formed at all.

Introduction

Currently, the number of materials used in electronic technology for various purposes is several thousand. According to the most general classification, they are divided into four classes: conductors, semiconductors, dielectrics and magnetic materials. Among the most important and relatively new materials are semiconductor chemical compounds, among which compounds of the type A II B VI are of the greatest scientific and practical interest. One of the most significant materials of this group is CdS.

CdS is the basis of modern IR technology, since its photosensitivity spectrum overlaps the atmospheric transparency window (8-14 microns), in which all environmental objects radiate. This allows it to be used in military affairs, ecology, medicine and other branches of human activity. To date, CdS is obtained in film form by the hydrochemical method.

The purpose of this course project is to implement a project for the production of sensitive elements of photoresistors based on CdS by the hydrochemical method with a capacity of 100 thousand pieces / year, as well as familiarization with the calculation method designed to preliminary determine the conditions for the formation of CdS, cadmium hydroxide and cyanamide.

Characterization of cadmium sulfide

The diagram of the Cd - S system has not been built, there is one CdS compound in the system that exists in two modifications: b (hexagonal) and c (cubic). CdS occurs naturally as the minerals greenockite and howleyite.

Crystal structure

Compounds of type A II B VI usually crystallize in the structure of sphalerite or wurtzite. The structure of sphalerite is cubic, type B-3, space group F4 3m (T d 2). The structure of wurtzite is hexagonal, type B-4, space group P 6 3 mc (C 6v 4). These structures are very similar to each other; they have the same number of atoms in both the first and second coordination spheres - 4 and 12, respectively. The interatomic bonds in the tetrahedra of both modifications are very close.

Cadmium sulfide has been obtained with both sphalerite and wurtzite structures.

Thermodynamic and electrophysical properties

Cadmium sulfide is a one-sided phase of variable composition, always having an excess of cadmium. Cadmium sulfide, when heated to 1350 ° C, sublimates at atmospheric pressure without melting, in a vacuum at 180 ° C it distills without melting and without decomposition, under a pressure of 100 atm it melts at a temperature of about 1750 ° C. The degree of dissociation of cadmium at temperatures above 1000 °C reaches 85-98%. The heat of formation of CdS D H 298 0 \u003d -34.71 kcal / mol.

Depending on the conditions of production and heat treatment, the properties of CdS can be different. Thus, crystals grown in an excess of cadmium vapor have a significantly higher thermal conductivity than crystals grown under conditions of a stoichiometric composition. The specific resistance of CdS, depending on various factors, can vary over a wide range (from 10 12 to 10 -3 ohm * m).

Deviations from stoichiometry have a decisive influence on the electrophysical properties of CdS. The introduction of oxygen into the samples leads to a strong decrease in electrical conductivity. The band gap of CdS, determined from optical data, is 2.4 V. Cadmium sulfide typically has an n-type conductivity due to the lack of sulfur relative to the stoichiometric composition.

The solubility of cadmium in water is negligible: 1.5 * 10 -10 mol / l.

Cadmium(II) oxide

When heated in air, cadmium ignites, forming cadmium oxide CdO (molecular weight 128.41). The oxide can also be obtained by calcining nitrate or carbonate salts of cadmium. In this way, the oxide is obtained in the form of a brown powder, which has two modifications: amorphous and crystalline. When heated, an amorphous oxide becomes crystalline, crystallizing in the cubic system: it adsorbs carbon dioxide and behaves like a strong base. The heat of transformation of CdO AMORPH CdO CRIST is 540 cal.

The density of artificially prepared oxide ranges from 7.28 to 8.27 g/cm 3 . In nature, CdO forms a black coating on the galmy, with a density of 6.15 g/cm 3 . Melting point 1385°.

Cadmium oxide is reduced by hydrogen, carbon and carbon monoxide. Hydrogen begins to reduce CdO at 250-260° according to the reversible reaction:

CdO + H 2 Cd + H 2 O,

Which ends quickly at 300°.

Cadmium oxide is highly soluble in acids and in a solution of zinc sulfate according to a reversible reaction:

CdO + H 2 O + ZnSO 4 CdSO 4 + Zn (OH) 2.

Cadmium sulfide

Sulfide (CdS, molecular weight 144.7) is one of the important compounds of cadmium. It dissolves in concentrated solutions of hydrochloric and nitric acids, in boiling dilute sulfuric acid and in solutions of ferric iron; in the cold, it dissolves poorly in acids, and is insoluble in dilute sulfuric acid. Solubility product of sulfide 1.4·10 -28 . Crystalline sulfide occurs in nature in the form of grenakite as an admixture to ores of heavy and non-ferrous metals. It can be obtained artificially by fusing sulfur with cadmium or cadmium oxide. When metallic cadmium is fused with sulfur, the development of the reaction of sulfide formation is inhibited by CdS protective films. Reaction

2CdO+3S=2CdS+SO2

begins at 283° and passes at 424° at high speed.

Three modifications of CdS are known: amorphous (yellow) and two crystalline (red and yellow). The red variety of crystalline sulfide is heavier (sp. weight 4.5) yellow (sp. weight 3). Amorphous CdS becomes crystalline when heated to 450°C.

Cadmium sulfide, when heated in an oxidizing atmosphere, oxidizes to sulfate or oxide, depending on the firing temperature.

cadmium sulfate

Cadmium sulfate (CdSO 4 , molecular weight 208.47) is a white crystalline powder that crystallizes in the orthorhombic system. It is easily soluble in water, but insoluble in alcohol. The sulfate crystallizes from an aqueous solution in a monoclinic system with 8/3 water molecules (CdSO 4 8 / 3H 2 O), is stable up to 74 °, but at a higher temperature it turns into one-water sulfate (CdSO 4 H 2 O). With an increase temperature, the solubility of sulfate increases slightly, but with a further increase in temperature, it decreases as shown in table 3:

Table 3

The existence of three modifications of sulfate was established: b, c, and d. After the isolation of the last water molecule at 200° from the 3CdSO 4 ·8H 2 O crystalline hydrate, a b-modification is formed, which is stable up to 500°; with a further increase in temperature, the s-modification arises, which, at temperatures above 735 °, passes into the z-modification. The high-temperature modifications (c and d) transform into the b-modification upon cooling.

The invention can be used in inorganic chemistry. The method for obtaining crystalline cadmium sulfide includes placing sulfate-reducing bacteria in a synthetic medium containing metals and adding nutrients, including solutions of vitamins, salts, cofactors. When cultivating, sulfate-reducing bacteria Desulfovibrio sp. A2, and a synthetic medium containing a source of cadmium ions - a solution of cadmium chloride. The concentration of cadmium ions in the synthetic medium is 150 mg/l. An aluminum foil was placed in the culture vessel, culture was carried out at 28°C for 18 days. The precipitate collected from the foil and from the bottom of the vial, containing crystals of cadmium sulfide, is dried. EFFECT: invention makes it possible to obtain cadmium sulfide from wastewater and liquid waste from metallurgical enterprises. 2 ill., 3 tables, 1 pr.

Drawings to the RF patent 2526456

The invention relates to a method for obtaining pure cadmium sulfide (CdS) from solutions containing metals using sulfate-reducing bacteria (SRP).

The proposed method can be used to obtain pure cadmium sulfide from wastewater containing metal ions, including cadmium, and liquid waste from mining and processing metallurgical enterprises. When using the proposed method, it is possible to selectively precipitate cadmium in the form of sulfides. This feature makes it possible to use liquid waste from metallurgical enterprises and wastewater as a secondary source of raw materials for the production of cadmium sulfides. Cadmium sulfide is used in semiconductor lasers, is a material for the manufacture of photocells, solar cells, photodiodes, light-emitting diodes, phosphor, pigments for art paints, glass and ceramics. Cadmium sulfide pigments are valued for their good temperature stability in many polymers, such as engineering plastics. By replacing some of the sulfur atoms with selenium in CdS crystals, a wide variety of dye colors can be obtained from green-yellow to red-violet. Cadmium sulfide is a wide-gap semiconductor. This property of CdS is used in optoelectronics, both in photodetectors and in solar cells. From single crystals of cadmium sulfide, scintillators are made for detecting elementary particles and gamma radiation.

In nature, cadmium sulfide exists as the minerals greenockite and howleyite, which occur as yellow deposits on sphalerite (ZnS) and smithsonite. Since these minerals are not widely distributed in nature, cadmium sulfide is obtained by synthesis for industrial use and scientific and technical work.

Cadmium sulfides are obtained by chemical methods - by heating sulfur with cadmium or by passing hydrogen sulfide over cadmium, cadmium oxide or chloride when heated. A known method for producing powdered sulfides of cadmium and lead (RF patent, No. 2203855, C01G 11/02, C01G 21/21, 2003). The invention relates to methods for producing powder materials in molten salts. The synthesis is carried out in a molten medium. The molten medium is formed by crystalline thiourea, and as a metal-containing component it includes anhydrous cadmium or lead acetates. Synthesis is carried out by mixing powders of one of the indicated salts and thiourea at a 2-4-fold molar excess of thiourea and further holding at 160-180°C for 20-30 minutes. The practical yield of products obtained by the proposed method is over 95%. In addition, they contain an admixture of elemental sulfur (3-4 wt.%), which, depending on the further use of the product, can be removed by washing with an organic solvent (toluene, carbon tetrachloride, etc.). The disadvantages of this method is the energy consumption of production, the need to use special, expensive equipment. In addition, chemical production has a negative impact on the environment.

The formation of cadmium sulfide crystallites on the cell surface by the bacteria Klebsiella pneumonia and Clostridium thermoaceticum is known (Aiking H. et al. Detoxification of mercury, cadmium, and lead in Klebsiella aerogenes NCTC 418 growing in continuous culture // Appi Environ Microbiol. 1985 Nov;50(5 - P.1262-1267; P. R. Smith et al. PHOTOPHYSICAL AND PHOTOCHEMICAL CHARACTERIZATION OF BACTERIAL SEMICONDUCTOR CADMIUM-SULFIDE PARTICLES // Journal of the Chemical Society. Faraday transactions. - 1998, 94(9). - pp.1235-1241 ).

CdS crystallites synthesized on the surface of the K. pneumonia bacteria effectively absorb UV light, which protects the bacterium from its harmful effects. The deep sea fluorescent bacterium Pseudomonas aeruginosa removes cadmium from the medium by forming CdS crystallites on the cell wall (Wang CL et al. Cadmium removal by a new strain of Pseudomonas aeruginosa in aerobic culture // Appl. Environ. Microbiol. - 1997, 63. - pp .4075-4078). The sizes of cadmium sulfide crystallites vary from tens of microns outside cells to tens of angstroms inside cells or on their surface. Cadmium sulfide crystallites are formed only under certain conditions for organisms to endure unfavorable environmental conditions.

The closest in essence and the achieved result to the claimed invention is a method for removing low concentrations of cadmium ions using a bioreactor with sulfate-reducing bacteria (Hiroshi H. et al. Removal of Low Concentrated Cadmium Ions Using Fixed-bed Sulfate-Reducing Bioreactor with FS Carrier // Journal of the Mining and Materials Processing Institute of Japan, 2003, V.119, No. 9, pp.559-563). The recovery of heavy metal ions from water took place in a bioreactor using sulfate-reducing bacteria immobilized on fibrous slag, which was used as a biocarrier. In this process, sulfate ions in the liquid are biologically converted to hydrogen sulfide (H 2 S), which reacts with metal ions to form ultrafine metal sulfide particles. Then the resulting particles are collected on the surface of the carrier in the upper part of the reactor, resulting in the accumulation of heavy metal ions and their sulfides. With continuous treatment of water contaminated with 6 mg/l cadmium, almost complete removal was carried out over a period of about 30 days.

The disadvantage of this method is that its use is possible only at low concentrations of cadmium ions in the environment and does not form crystalline cadmium sulfide.

The objective of the present invention is to develop a method for obtaining crystalline cadmium sulfide from solutions with a high content of cadmium ions (up to 150 mg/l), not containing impurities of other metal sulfides, using sulfate-reducing bacteria resistant to high concentrations of cadmium ions.

The problem is solved by placing SRP, highly resistant to cadmium ions, into a synthetic medium simulating wastewater containing metals, with the addition of nutrients, including solutions of vitamins, salts, cofactors, lactate, sodium sulfide, with further cultivation in a thermostat and drying, but , unlike the prototype, SRBs are used that are resistant to cadmium ions, aluminum foil is added to the medium, cultivation is carried out at a temperature of 28°C for 18 days.

Cultivation is carried out in a synthetic medium (table 1 - the composition of the synthetic medium) with the introduction of nutrients that stimulate the growth of bacteria. Nutrients and divalent cadmium are added to the synthetic medium before the inoculation of the bacterial culture. The composition of nutrients and the sequence of their introduction are shown in Table 2. All nutrients, except for vitamins, are autoclaved at 1 atm for 30 minutes. The vitamins are sterilized by filtration with a bacterial filter (0.20 µm).

Sowing is carried out in sterile containers with embedded foil, the volume of the inoculum (CRP culture) in the amount of 10% of the volume of the container. Tanks with inoculum are filled with synthetic medium (with all nutrients added) to the top. The pH of the medium is adjusted to 7.0-7.8 with a solution of NaHCO 3 . The vials are closed with aluminum caps, sealed and placed in a thermostat at 28°C. The formation of cadmium sulfide crystals occurs on the foil and partly on the bottom of the vial. After cultivation, the precipitate is collected from the foil and centrifuged from the bottom of the vial and dried in air. Examples of the invention in the laboratory are given below.

A pure culture of SRB Desulfovibrio sp. A2 was cultured on a synthetic medium containing divalent cadmium at a concentration of 150 mgCd/l and aluminum foil. Cadmium sulfide crystals were obtained on foil and partly on the bottom of a 120 ml vial. The aluminum foil vials were sterilized by dry heat in a sterilizer at 160° C. for 2.2 hours.

Sowing was carried out in a sterile laminar flow cabinet, which was previously disinfected with ultraviolet light for 30 minutes. Before inoculation, the synthetic medium (Table 1) was brought to a boil and then rapidly cooled under running cold water to remove dissolved oxygen. Nutrients were added to the medium cooled to room temperature (table 2) (per 1 l) in the following sequence: vitamins (2 ml), salt solution (10 ml), cofactor solution (1 ml), organic substrate - lactate (1 .6 ml), NaHCO 3 solution (pH was adjusted to 7.0-7.8), sodium sulfide solution (2 ml). A stock solution of cadmium (CdCl 2 ×2.5H 2 O 2 g per 100 ml of water) was added in an amount of 16.72 ml per 1 liter of synthetic medium (thus a cadmium concentration in the medium of 150 mg/l was achieved).

About 50 ml of synthetic medium with additives and 10 ml of inoculum (bacteria culture) were added to foil vials, after which the medium was topped up. Rubber stoppers were rubbed to the edges of the vials with a sterile needle, which reduced the possibility of air oxygen penetration. At the end of seeding, the flasks were closed with aluminum caps, the flask was sealed with a seamer, and the thermostat was placed at 28°C. Crystallization of cadmium sulfide begins after 10 days of cultivation; after 18 days of cultivation, cadmium sulfide crystallizes completely. The formed precipitate was collected from the foil and centrifuged from the bottom of the vial and dried in air. The mass of the precipitate formed is 0.38 g.

The study of the precipitates formed was carried out using scanning electron microscopy (Philips SEM515 with EDAX ECON IV analyzer). The crystalline phase was determined by X-ray phase analysis on a Shimadzu XRD 6000 diffractometer.

The size of the crystals, determined under a scanning electron microscope, was 50-300 μm, Figure 1 - Micrographs (SEM) of sediments obtained during the cultivation of Desulfovibrio sp. A2 in the presence of Cd ions (150 mg/l) for 18 days, and the corresponding emf. Sediments obtained during the cultivation of the strain Desulfovibrio sp. A2 contained cadmium, sulfur, iron, oxygen, carbon and sodium, with carbon and oxygen coming from the carbon substrate on which the sample lay. The ratio of elements is presented in table 3 - the elemental composition of sediments obtained during the cultivation of Desulfovibrio sp. A2 in the presence of Cd ions (150 mg/l) for 18 days (elements C and O originate from the substrate on which the sample lay).

When studying precipitation using X-ray phase analysis, the formation of crystalline cadmium sulfide was shown for 18 days (Figure 2 - diffraction pattern of precipitation obtained by cultivating Desulfovibrio sp. A2 in the presence of an initial concentration of Cd (150 mg/l) for 18 days. Symbols on the diffraction pattern : CdS - cadmium sulfide).

In the control sediments obtained by incubation without the addition of an inoculum, no crystalline phase was observed and the main elements were cadmium and oxygen. The proposed method includes the possibility of using sewage and liquid waste from mining and processing metallurgical enterprises as a synthetic medium for the production of cadmium sulfide.

Table 1
Reagent	Concentration, mg/l
Na2SO4	4000
MgCl 2 6H 2 O	400
NaCl (25%)	0,0125*
FeSO 4 * 7H 2 O	2,1
N 3 IN 3	0,03
MnCl 2 *4H 2 O	0,1
CoCl 2 *6H 2 O	0,19
NiCl 2 *6H 2 O	0,024
CuCl 2 *2H 2 O	0,002
ZnSO 4 *7H 2 O	0,144
Na 2 MoO 4 * 2H 2 O	0,036
CuSO 4 * 7H 2 O	750
H2O	1 l
* - ml/l

table 2
Solution (introduced amount per 1 liter of synthetic medium)
	Reagent	Concentration
	4-aminobenzoic acid	4 mg/l
	Biotin (vitamin H)	1 mg/l
	Nicotinic acid (vitamin B 5)	10 mg/l
1. Vitamins (2 ml/l)	Calcium pantothenate (vitamin B 3)	5 mg/l
	Pyridoxine dihydrochloride (vitamin B 6)	15 mg/l
	Cyanocobalamin (Vitamin B 12)	5 mg/l
	Thiamine (vitamin B 1)	10 mg/l
	Riboflavin (vitamin B 2)	0.5 mg/l
	Folic acid	0.2 mg/l
	KH2PO4	20 g/l
	NH4Cl	25 g/l
2. Salt solution (10 ml/l)	NaCl	100 g/l
	KCl	50 g/l
	CaCl2	11.3 g/l
	H2O	1 l
3. Solution of cofactors (1 ml/l)	NaOH	4 g/l
	Na 2 SeO 3 × 5H 2 O	6 mg/l
	Na 2 WO 4 × 2H 2 O	8 mg/l
4. Lactate solution (1.6 ml/l)
	lactate	40%
5. Na 2 S solution (2 ml/l)
	Na 2 S × 9H 2 O	4.8 g

Table 3
Element	Weight fraction (Wt%)	Atomic fraction (At %)
FROM	7,56	15,1
O	2,75	4,1
Na	0,41	0,4
S	23,3	44,5
CD	64,7	35,4
Fe	1,28	0,5

CLAIM

A method for obtaining crystalline cadmium sulfide by placing sulfate-reducing bacteria in a synthetic medium containing metals with the addition of nutrients, including solutions of vitamins, salts, cofactors, characterized in that the cultivation uses sulfate-reducing bacteria Desulfovibrio sp. A2, use a synthetic medium containing a source of cadmium ions - a solution of cadmium chloride, and the concentration of cadmium ions in the synthetic medium is 150 mg/l, while aluminum foil is placed in the culture vessel, cultivation is carried out at a temperature of 28°C for 18 days, and the precipitate collected from the foil and from the bottom of the bottle, containing crystals of cadmium sulfide, is dried.

Traditionally, cadmium sulfide has been used as a dye. It can be seen on the canvases of such great artists as Van Gogh, Claude Monet, Matisse. In recent years, interest in it is associated with the use of cadmium sulfide as a film coating for solar cells and in photosensitive devices. This compound is characterized by good ohmic contact with many materials. Its resistance does not depend on the magnitude and direction of the current. Due to this, the material is promising for use in optoelectronics, laser technology, and LEDs.

general description

Cadmium sulfide is an inorganic compound that occurs naturally as the rare minerals zincblende and howliite. They are of no interest to the industry. The main source of cadmium sulfide is artificial synthesis.

In appearance, this compound is a yellow powder. Shades can vary from lemon to orange-red. Due to its bright color and high resistance to external influences, cadmium sulfide has been used as a high quality dye. The substance has been widely available since the 18th century.

The chemical formula of the compound is CdS. It has 2 structural forms of crystals: hexagonal (wurtzite) and cubic (zinc blende). Under the influence of high pressure, a third form is also formed, like that of rock salt.

Cadmium sulfide: properties

A material with a hexagonal lattice structure has the following physical and mechanical properties:

• melting point - 1475 °C;
• density - 4824 kg / m 3;
• linear expansion coefficient - (4.1-6.5) μK -1;
• hardness on the Mohs scale - 3.8;
• sublimation temperature - 980 °C.

This compound is a direct semiconductor. When irradiated with light, its conductivity increases, which makes it possible to use the material as a photoresistor. When alloyed with copper and aluminum, the effect of luminescence is observed. CdS crystals can be used in solid state lasers.

The solubility of cadmium sulfide in water is absent, in dilute acids it is weak, in concentrated hydrochloric and sulfuric acid it is good. It also dissolves Cd well.

A substance has the following chemical properties:

• precipitates when exposed to a solution of hydrogen sulfide or alkali metals;
• when reacting with hydrochloric acid, CdCl 2 and hydrogen sulfide are formed;
• when heated in an atmosphere with an excess of oxygen, it oxidizes to sulfate or oxide (this depends on the temperature in the kiln).

Receipt

Cadmium sulfide is synthesized in several ways:

• during the interaction of cadmium and sulfur vapors;
• in the reaction of organosulfur and cadmium-containing compounds;
• precipitation from solution under the influence of H 2 S or Na 2 S.

Films based on this substance are made using special methods:

• chemical precipitation using thiocarbamide as a source of sulfide anions;
• pulverization followed by pyrolysis;
• the method of molecular beam epitaxy, in which crystals are grown under vacuum;
• as a result of the sol-gel process;
• ion sputtering method;
• anodizing and electrophoresis;
• screen printing method.

To make the pigment, the precipitated solid cadmium sulfide is washed, calcined to obtain a hexagonal crystal lattice, and then ground to a powder.

Application

Dyes based on this compound have high thermal and light resistance. Additives of selenide, cadmium telluride and mercury sulfide make it possible to change the color of the powder to green-yellow and red-violet. Pigments are used in the manufacture of polymer products.

There are other applications for cadmium sulfide:

• detectors (recorders) of elementary particles, including gamma radiation;
• thin film transistors;
• piezoelectric transducers capable of operating in the GHz range;
• production of nanowires and tubes, which are used as luminescent labels in medicine and biology.

Solar panels on cadmium sulfide

Thin film solar panels are one of the latest inventions in alternative energy. The development of this industry is becoming more and more urgent, as the reserves of minerals used to generate electricity are rapidly depleted. The advantages of solar panels based on cadmium sulfide are as follows:

• lower material costs in their manufacture;
• an increase in the efficiency of converting solar energy into electrical energy (from 8% for traditional types of batteries to 15% for CdS/CdTe);
• the possibility of generating energy in the absence of direct rays and the use of batteries in foggy areas, in places with high dust content of the air.

Films used for the manufacture of solar cells have a thickness of only 15-30 microns. They have a granular structure, the size of the elements of which is 1-5 microns. Scientists believe that thin-film batteries in the future will be able to become an alternative to polycrystalline due to unpretentious operating conditions and long service life.