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.

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

Currently, 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 fixture. 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 producing metal films is vacuum deposition (thermal evaporation in vacuum) of various metals (aluminum, gold, etc.), as it has a number of 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.

Sulfides of some other metals (insoluble in water), for example, iron (II), manganese, zinc, do not precipitate from an acidic solution, since they are soluble in dilute mineral acids, therefore, not hydrogen sulfide is used for their precipitation, but ammonium sulfide (or sodium).

FeSO 4 + (NH 4) 2 S \u003d FeS (precipitate) + (NH 4) 2 SO 4

Some insoluble sulfides are able to dissolve in an excess of ammonium sulfide or ammonium polysulfide solution (due to the formation of complex salts), while others cannot.

As 2 S 3 (precipitate) + 3 (NH 4) 2 S \u003d 2 (NH 4) 3 (solution)

Previously, the property of sulfides to precipitate out of solution under the action of hydrogen sulfide or ammonium sulfide (as well as to dissolve or not dissolve in an excess of solutions of sulfides or polysulfides of monovalent cations) was actively used in analytical chemistry for the qualitative analysis and separation of mixtures of metals (hydrogen sulfide methods of analysis). Moreover, metal cations in analytical chemistry were classified into groups depending on their behavior under the action of hydrogen sulfide, ammonium sulfide solution and polysulfides (of course, this was not the only feature by which cations were classified in analytical chemistry, but one of the main ones).

In our time, hydrogen sulfide analysis methods have almost lost their relevance, since hydrogen sulfide is poisonous. Moreover, hydrogen sulfide is not only poisonous, but also insidious. At first, the characteristic smell of hydrogen sulfide (rotten eggs) is clearly noticeable even in low concentrations, but with prolonged exposure to hydrogen sulfide on the experimenter, the smell of hydrogen sulfide ceases to be felt. As a result, you can be exposed to dangerous levels of hydrogen sulfide without even knowing it. In the past, when working with hydrogen sulfide was the order of the day in analytical chemistry labs, this happened a lot.

Over the years, analytical chemists have been able to come up with a replacement for hydrogen sulfide and sulfides (the so-called non-hydrogen sulfide methods of analysis). In addition, physicochemical and instrumental methods of analysis are increasingly being used in analytical chemistry.

I decided to get some insoluble sulfides from solutions of metal salts and hydrogen sulfide. The choice fell on copper and cadmium (there was another thought about mercury, but I refused it, since there was little mercury, and it was in the form of a metal). Experiments were carried out on the street. Working at home with hydrogen sulfide is a kamikaze occupation. This is only allowed if there is a fume hood.

I took copper sulfate and cadmium acetate (both qualifications "Ch"). Salt dissolved in warm water. First, copper sulfate was treated with hydrogen sulfide. The tube quickly filled with black flakes of copper sulfide CuS. I left the test tube for a while, moved away (do not forget - hydrogen sulfide is poisonous!). When he arrived, he found in the test tube instead of liquid a dark porridge from a solution and sediment.

I rinsed the gas outlet pipe after copper and proceeded to cadmium. A yellow film of cadmium sulfide quickly formed on the walls at the top of the liquid. Soon the solution was covered in flakes. Walked away again. Fifteen minutes later he came, found porridge with yellow-orange stains in a test tube. This is cadmium sulfide CdS.

Despite the toxicity of cadmium, cadmium sulfide is still used as a pigment due to its beautiful color, lightfastness, and chemical resistance. Sometimes a solid solution is used between cadmium sulfide and cadmium selenide Cd(S, Se): by changing the ratio of selenium and sulfur in the pigment, its color can be varied.

__________________________________________________

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.

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) CD , is the chemical element 12 ( IIb ) group of the Periodic system. Atomic number 48, relative atomic mass 112.41. Natural cadmium consists of eight stable isotopes: 106 Cd (1.22%), 108 Cd (0.88%), 110 Cd (12.39%), 111 Cd (12.75%), 112 Cd (24.07%), 113 Cd (12.26 %), 114 Cd (28.85%) and 116 Cd (7.58%). The oxidation state is +2, rarely +1.

Cadmium was discovered in 1817 by the German chemist Friedrich Stromeyer (

Stromeyer Friedrich ) (1776–1835).

When checking zinc oxide, produced by one of the Shenebek factories, it was suspected that it contains an admixture of arsenic. When the drug was dissolved in acid and passed through a solution of hydrogen sulfide, a yellow precipitate appeared, similar to arsenic sulfides, but a more thorough check showed that this element was not present. For the final conclusion, a sample of suspicious zinc oxide and other zinc preparations (including zinc carbonate) from the same factory were sent to Friedrich Stromeyer, who from 1802 held the chair of chemistry at the University of Göttingen and the post of general inspector of Hanoverian pharmacies.

After calcining zinc carbonate, Strohmeyer obtained oxide, but not white, as it should have been, but yellowish. He suggested that the coloration was caused by an admixture of iron, but it turned out that there was no iron. Stromeyer fully analyzed the zinc preparations and found that the yellow color was due to the new element. It was named after the zinc ore in which it was found: the Greek word

kadmeia , "cadmium earth" - the ancient name of smithsonite ZnCO 3 . This word, according to legend, comes from the name of the Phoenician Cadmus, who allegedly was the first to find a zinc stone and noticed its ability to give copper (when smelted from ore) a golden color. The same name was given to the hero of ancient Greek mythology: according to one of the legends, Cadmus defeated the Dragon in a difficult duel and built the fortress of Cadmeus on its lands, around which the seven-gate city of Thebes then grew.The prevalence of cadmium in nature and its industrial extraction. The content of cadmium in the earth's crust is 1.6·10–5%. It is close in prevalence to antimony (2·10–5%) and twice as common as mercury (8·10–6%). Cadmium is characterized by migration in hot groundwater along with zinc and other chemical elements prone to the formation of natural sulfides. It concentrates in hydrothermal deposits. Volcanic rocks contain up to 0.2 mg of cadmium per kg, among sedimentary rocks, clays are the richest in cadmium - up to 0.3 mg / kg, to a lesser extent - limestones and sandstones (about 0.03 mg / kg). The average content of cadmium in the soil is 0.06 mg/kg.

Cadmium has its own minerals - greenockite

CdS, otavite CdCO 3, monteponite CdO . However, they do not form their own deposits. The only industrially significant source of cadmium is zinc ores, where it is contained in a concentration of 0.01–5%. Cadmium also accumulates in galena (up to 0.02%), chalcopyrite (up to 0.12%), pyrite (up to 0.02%), stannite (up to 0.2%). The total world resources of cadmium are estimated at 20 million tons, industrial - at 600 thousand tons.Characterization of a simple substance and industrial production of cadmium metal. Cadmium is a silvery solid with a bluish luster on a fresh surface, soft, malleable, malleable metal, rolls well into sheets, and can be easily polished. Like tin, cadmium sticks crackle when bent. It melts at 321.1°C, boils at 766.5°C, density is 8.65 g/cm 3 , which makes it possible to refer it to heavy metals.

Cadmium is stable in dry air. In humid air, it quickly dims, and when heated, it easily interacts with oxygen, sulfur, phosphorus and halogens. Cadmium does not react with hydrogen, nitrogen, carbon, silicon and boron.

Vapors of cadmium interact with water vapor to release hydrogen. Acids dissolve cadmium to form salts of this metal. Cadmium reduces ammonium nitrate in concentrated solutions to ammonium nitrite. It is oxidized in aqueous solution by cations of certain metals, such as copper (

II ) and iron(III ). Unlike zinc, cadmium does not interact with alkali solutions.

The main sources of cadmium are intermediate products of zinc production. Metal precipitates obtained after purification of zinc sulfate solutions by the action of zinc dust contain 2–12% cadmium. The fractions formed during the distillation production of zinc contain 0.7–1.1% cadmium, and the fractions obtained during the rectification purification of zinc contain up to 40% cadmium. Cadmium is also extracted from the dust of lead and copper smelters (it can contain up to 5% and 0.5% cadmium, respectively). The dust is usually treated with concentrated sulfuric acid and then the cadmium sulfate is leached with water.

Cadmium sponge is precipitated from cadmium sulfate solutions by the action of zinc dust, then it is dissolved in sulfuric acid and the solution is purified from impurities by the action of zinc oxide or sodium carbonate, as well as by ion exchange methods. Cadmium metal is isolated by electrolysis on aluminum cathodes or zinc reduction.

To remove zinc and lead, cadmium metal is melted under a layer of alkali. The melt is treated with aluminum to remove nickel and ammonium chloride to remove thallium. Applying additional purification methods, it is possible to obtain cadmium with an impurity content of 10–5% by weight.

About 20 thousand tons of cadmium are produced per year. The volume of its production is largely related to the scale of zinc production.

The most important field of application of cadmium is the production of chemical current sources. Cadmium electrodes are used in batteries and accumulators. The negative plates of nickel-cadmium batteries are made of iron meshes with sponge cadmium as the active agent. Positive plates coated with nickel hydroxide. The electrolyte is a potassium hydroxide solution. On the basis of cadmium and nickel, compact batteries for guided missiles are also made, only in this case, not iron, but nickel grids are installed as the basis.

The processes occurring in a nickel-cadmium alkaline battery can be described by the overall equation:

Cd + 2NiO(OH) + 2H 2 O Cd(OH) 2 + 2Ni(OH) 2 Nickel-cadmium alkaline batteries are more reliable than lead (acid) batteries. These current sources are distinguished by high electrical characteristics, stable operation, and long service life. They can be charged in just one hour. However, nickel-cadmium batteries cannot be recharged without being fully discharged first (they are inferior to metal hydride batteries in this respect).

Cadmium is widely used for anti-corrosion coatings on metals, especially in cases of their contact with sea water. The most important parts of ships, aircraft, as well as various products designed for operation in tropical climates are cadmated. Previously, iron and other cadmium metals were immersed in molten cadmium, but now the cadmium coating is applied electrolytically.

Cadmium coatings have some advantages over zinc coatings: they are more resistant to corrosion, and they are easier to make even and smooth. The high plasticity of such coatings ensures the tightness of threaded connections. In addition, cadmium, unlike zinc, is stable in an alkaline environment.

However, cadmium has its own problems. When cadmium is applied electrolytically to a steel part, the hydrogen contained in the electrolyte can penetrate into the metal. It causes so-called hydrogen brittleness in high-strength steels, leading to unexpected failure of the metal under load. To prevent this phenomenon, titanium is added to cadmium coatings.

In addition, cadmium is toxic. Therefore, although cadmium tin is used quite widely, it is forbidden to use it for the manufacture of kitchen utensils and food containers.

Approximately one tenth of the world's production of cadmium is spent on the production of alloys. Cadmium alloys are mainly used as antifriction materials and solders. An alloy containing 99% cadmium and 1% nickel is used for the manufacture of bearings operating in automobile, aircraft and marine engines at high temperatures. Since cadmium is not sufficiently resistant to acids, including organic acids contained in lubricants, sometimes cadmium-based bearing alloys are coated with indium.

Alloying copper with small additions of cadmium makes it possible to make wires on electric transport lines more wear-resistant. Copper with the addition of cadmium almost does not differ in electrical conductivity from pure copper, but noticeably surpasses it in strength and hardness.

Cadmium is included in Wood's low-melting alloy (Wood's metal), containing 50% bismuth, 25% lead, 12.5% ​​tin, 12.5% ​​cadmium. Wood's alloy can be melted in boiling water. It is curious that the first letters of the components of Wood's alloy form the abbreviation WAX It was invented in 1860 by a not very famous English engineer B. Wood (

b. Wood ). Often this invention is mistakenly attributed to his namesake - the famous American physicist Robert Williams Wood who was born only eight years later. Low-melting alloys of cadmium are used as a material for producing thin and complex castings, in automatic fire-fighting systems, and for soldering glass to metal. Solders containing cadmium are quite resistant to temperature fluctuations.

A sharp jump in demand for cadmium began in the 1940s and was associated with the use of cadmium in the nuclear industry - it turned out that it absorbs neutrons and they began to make control and emergency rods of nuclear reactors from it. The ability of cadmium to absorb neutrons of strictly defined energies is used in the study of the energy spectra of neutron beams.

cadmium compounds. Cadmium forms binary compounds, salts, and numerous complex compounds, including organometallic compounds. In solutions, the molecules of many salts, in particular halides, are associated. Solutions have a slightly acidic environment due to hydrolysis. Under the action of alkali solutions, starting from pH 7–8, basic salts are precipitated.

cadmium oxide

CdO obtained by the interaction of simple substances or by calcination of cadmium hydroxide or carbonate. Depending on the "thermal history", it can be greenish yellow, brown, red, or almost black. This is partly due to the particle size, but to a greater extent is the result of defects in the crystal lattice. Above 900°C cadmium oxide is volatile, and at 1570°C it sublimates completely. It has semiconductor properties.

Cadmium oxide is easily soluble in acids and poorly in alkalis, it is easily reduced by hydrogen (at 900 ° C), carbon monoxide (above 350 ° C), carbon (above 500 ° C).

Cadmium oxide is used as an electrode material. It is part of lubricating oils and charge for the production of special glasses. Cadmium oxide catalyzes a number of hydrogenation and dehydrogenation reactions.

cadmium hydroxide

Cd(OH ) 2 precipitates as a white precipitate from aqueous solutions of cadmium salts ( II ) by adding alkali. Under the action of very concentrated alkali solutions, it is converted into hydroxocadmates, such as Na 2 [Cd (OH ) four ]. Cadmium hydroxide reacts with ammonia to form soluble complexes:Cd (OH) 2 + 6NH 3 H 2 O \u003d (OH) 2 + 6H 2 OIn addition, cadmium hydroxide goes into solution under the action of alkali cyanides. Above 170°C, it decomposes to cadmium oxide. The interaction of cadmium hydroxide with hydrogen peroxide in an aqueous solution leads to the formation of peroxides of various compositions.

Cadmium hydroxide is used to obtain other cadmium compounds, and also as an analytical reagent. It is part of the cadmium electrodes in current sources. In addition, cadmium hydroxide is used in decorative glass and enamels.

cadmium fluoride

CDF 2 is slightly soluble in water (4.06% by weight at 20°C), insoluble in ethanol. It can be obtained by the action of fluorine on a metal or hydrogen fluoride on cadmium carbonate.

Cadmium fluoride is used as an optical material. It is part of some glasses and phosphors, as well as solid electrolytes in chemical current sources.

Cadmium chloride

CdCl 2 is highly soluble in water (53.2% by weight at 20°C). Its covalent nature is responsible for its relatively low melting point (568.5°C) and ethanol solubility (1.5% at 25°C).

Cadmium chloride is obtained by reacting cadmium with concentrated hydrochloric acid or by chlorinating a metal at 500°C.

Cadmium chloride is a component of electrolytes in cadmium electrochemical cells and sorbents in gas chromatography. It is part of some solutions in photography, catalysts in organic synthesis, fluxes for growing semiconductor crystals. It is used as a mordant in dyeing and printing textiles. Cadmium compounds are obtained from cadmium chloride.

Cadmium bromide

CdBr 2 forms scaly crystals with a pearly sheen. It is very hygroscopic, highly soluble in water (52.9% by weight at 25°C), methanol (13.9% by weight at 20°C), ethanol (23.3% by weight at 20°C).

Cadmium bromide is obtained by bromination of the metal or by the action of hydrogen bromide on cadmium carbonate.

Cadmium bromide serves as a catalyst in organic synthesis, is a stabilizer for photographic emulsions, and is a component of vibrating compositions in photography.

cadmium iodide

CdI 2 forms shiny crystals in the form of leaflets, they have a layered (two-dimensional) crystal structure. Up to 200 polytypes of cadmium iodide are known, differing in the sequence of layers with hexagonal and cubic close packing.

Unlike other halogens, cadmium iodide is not hygroscopic. It is highly soluble in water (46.4% by weight at 25°C). Cadmium iodide is obtained by iodinating the metal when heated or in the presence of water, as well as by the action of hydrogen iodide on cadmium carbonate or oxide.

Cadmium iodide serves as a catalyst in organic synthesis. It is a component of pyrotechnic compositions and lubricants.

Cadmium sulfide CdS was probably the first compound of this element that the industry was interested in. It forms lemon yellow to orange red crystals. Cadmium sulfide has semiconductor properties.

This compound is practically insoluble in water. It is also resistant to the action of alkali solutions and most acids.

Cadmium sulfide is obtained by the interaction of cadmium and sulfur vapors, precipitation from solutions under the action of hydrogen sulfide or sodium sulfide, reactions between cadmium and sulfur organic compounds.

Cadmium sulfide is an important mineral dye, formerly called cadmium yellow.

In the painting business, cadmium yellow subsequently began to be used more widely. In particular, passenger cars were painted with it, because, among other advantages, this paint resisted locomotive smoke well. As a dye, cadmium sulfide was also used in the textile and soap industries. Appropriate colloidal dispersions were used to obtain colored transparent glasses.

In recent years, pure cadmium sulfide has been replaced by cheaper pigments - cadmopone and zinc-cadmium litopone. Kadmopon is a mixture of cadmium sulfide and barium sulfate. It is obtained by mixing two soluble salts - cadmium sulfate and barium sulfide. As a result, a precipitate is formed containing two insoluble salts:

CdSO 4 + BaS = CdS

Ї + BaSO 4 Ї

Cadmium zinc lithopone also contains zinc sulfide. In the manufacture of this dye, three salts precipitate simultaneously. Lithopone is cream or ivory.

With the addition of cadmium selenide, zinc sulfide, mercury sulfide and other compounds, cadmium sulfide gives thermally stable pigments with a bright color from pale yellow to dark red.

Cadmium sulfide gives the flame a blue color. This property is used in pyrotechnics.

In addition, cadmium sulfide is used as an active medium in semiconductor lasers. It will happen as a material for the manufacture of photocells, solar cells, photodiodes, light-emitting diodes, phosphors.

Cadmium Selenide CdSe forms dark red crystals. It is insoluble in water, decomposed by hydrochloric, nitric and sulfuric acids. Cadmium selenide is obtained by fusing simple substances or from gaseous cadmium and selenium, as well as by precipitation from a solution of cadmium sulfate under the action of hydrogen selenide, by the reaction of cadmium sulfide with selenous acid, by the interaction between cadmium and organoselenium compounds.

Cadmium selenide is a phosphor. It serves as an active medium in semiconductor lasers, is a material for the manufacture of photoresistors, photodiodes, and solar cells.

Cadmium selenide is a pigment for enamels, glazes and art paints. Ruby glass is stained with cadmium selenide. It was he, and not chromium oxide, as in the ruby ​​itself, that made the stars of the Moscow Kremlin ruby ​​red.

Cadmium telluride CdTe can be dark gray to dark brown in color. It is insoluble in water, but decomposed by concentrated acids. It is obtained by the interaction of liquid or gaseous cadmium and tellurium.

Cadmium telluride, which has semiconductor properties, is used as an X-ray and

g -radiation, and mercury-cadmium telluride has found wide application (especially for military purposes) in IR detectors for thermal imaging.

When the stoichiometry is violated or impurities (for example, copper and chlorine atoms) are introduced, cadmium telluride acquires light-sensitive properties. This is used in electrophotography.

Organocadmium compounds CdR 2 and CdRX (R = CH 3 , C 2 H 5 , C 6 H 5 and other hydrocarbon radicals, X are halogens, OR, SR, etc.) are usually obtained from the corresponding Grignard reagents. They are thermally less stable than their zinc counterparts, but generally less reactive (generally non-flammable in air). Their most important field of application is the preparation of ketones from acid chlorides.

The biological role of cadmium. Cadmium is found in the organisms of almost all animals (in terrestrial animals, about 0.5 mg per 1 kg of body weight, and in marine animals, from 0.15 to 3 mg/kg). However, it is considered one of the most toxic heavy metals.

Cadmium is concentrated in the body mainly in the kidneys and liver, while the content of cadmium in the body increases with age. It accumulates in the form of complexes with proteins that are involved in enzymatic processes. Getting into the body from the outside, cadmium has an inhibitory effect on a number of enzymes, destroying them. Its action is based on the binding of the –SH group of cysteine ​​residues in proteins and the inhibition of SH enzymes. It can also inhibit the action of zinc-containing enzymes by replacing zinc. Due to the proximity of the ionic radii of calcium and cadmium, it can replace calcium in bone tissue.

People are poisoned by cadmium by drinking water contaminated with cadmium-containing waste, as well as vegetables and grains growing on lands located near oil refineries and metallurgical enterprises. Mushrooms have a special ability to accumulate cadmium. According to some reports, the content of cadmium in mushrooms can reach units, tens and even 100 or more milligrams per kg of their own weight. Cadmium compounds are among the harmful substances found in tobacco smoke (one cigarette contains 1-2 micrograms of cadmium).

A classic example of chronic cadmium poisoning is a disease first described in Japan in the 1950s and called itai-itai. The disease was accompanied by severe pain in the lumbar region, pain in the muscles. There were also characteristic signs of irreversible kidney damage. Hundreds of itai-itai deaths have been recorded. The disease became widespread due to the high environmental pollution in Japan at that time and the specific diet of the Japanese - mainly rice and seafood (they are able to accumulate cadmium in high concentrations). Studies have shown that sick "itai-itai" consumed up to 600 micrograms of cadmium per day. Subsequently, as a result of environmental protection measures, the frequency and severity of syndromes such as "itai-itai" decreased markedly.

In the United States, a correlation has been found between atmospheric cadmium levels and the incidence of deaths from cardiovascular disease.

It is believed that about 1 μg of cadmium per 1 kg of body weight can enter the human body per day without harm to health. Drinking water should not contain more than 0.01 mg/l of cadmium. The antidote for cadmium poisoning is selenium, but eating foods rich in this element leads to a decrease in the sulfur content in the body, in which case cadmium again becomes dangerous.

Elena Savinkina

LITERATURE Popular library of chemical elements. M., Nauka, 1977
Karapetyants M.Kh., Drakin S.I. General and inorganic chemistry. M., Chemistry, 1992
Greenwood N.N., Earnshaw A. Chemistry of the Elements, Oxford: Butterworth, 1997