Lecture
Refractory metals. Refractory metals include metals with a melting point above 1700°C. As a rule, they are chemically stable at low temperatures but become active at elevated temperatures. Their use at high temperatures can be ensured in an atmosphere of inert gases or in a vacuum.
All refractory metals, with the exception of platinum, oxidize intensively on heating in air to high temperatures, forming volatile compounds. Therefore, they can be used only as heating elements operating in a vacuum or a protective environment.
The main refractory metals used are tungsten, molybdenum, tantalum, niobium, chromium, and rhenium.
Noble metals. Noble metals include the most chemically resistant metals: gold, silver, platinum, and palladium.
In electronic engineering, gold is used as a contact material and as a material for corrosion-resistant coatings of microwave resonators and the internal surfaces of waveguides. In gold-aluminum contacts, intermetallic compounds gradually form that have increased resistivity and brittleness. For this reason, contacts between thin aluminum and gold films are unreliable.
Silver differs from other metals in having the lowest resistivity. It is widely used to make contacts in equipment of various power ratings. The high values of specific heat capacity, thermal conductivity, and electrical conductivity of silver ensure, compared with other metals, the lowest heating of contacts and rapid removal of heat from the contact points.
A drawback of silver is its tendency to form non-conducting dark Ag2S sulfide films as a result of reaction with hydrogen sulfide present in the atmosphere.
Platinum, unlike silver, does not form sulfide films, which ensures stable transition resistance for platinum contacts.
Platinum is used to make thermocouples paired with a platinum-rhodium alloy, rated for operating temperatures of up to 1600°C.
Because of its low hardness, platinum is rarely used for contacts in its pure form. The most common are alloys of platinum with iridium, but these are very expensive and are used when it is necessary to ensure high contact reliability.
Palladium is close to platinum in properties and often serves as a substitute for platinum, since it is 4–5 times cheaper.
Metals with medium melting points. Among the metals with medium melting points, let us consider those most commonly used in electronic engineering – iron, nickel, and cobalt. In any application of these metals in equipment and devices, it should be kept in mind that they are ferromagnetic. In addition, they have an increased temperature coefficient of resistivity. Alloys of these metals are of very important practical significance.
Solders. Solders are special alloys used in soldering.
Solders are conventionally divided into two groups – soft and hard. Soft solders are those with a melting point up to 300°C, hard solders – above 300°C. In addition, solders differ significantly in mechanical strength. Soft solders have a tensile strength of 16…100 MPa, hard solders – 100…500 MPa.
Solders are selected taking into account the physicochemical properties of the metals being joined, the required mechanical strength of the joint, and the resistivity.
Soft solders are tin-lead alloys (POS) with a tin content ranging from 10 (POS–10) to 90% (POS–90), the balance being lead.
The most common hard solders are copper-zinc alloys (PMTs) and silver alloys (PSr) with various additives.
Auxiliary materials for obtaining a reliable solder joint are called fluxes. They must:
1) dissolve and remove oxides and contamination from the surfaces of the metals being soldered; 2) protect the surfaces from oxidation during soldering (including the solder itself);
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