A Chat on Crystal Chemistry

Radio Record, Volume I, Issue 49, 22 June 1928, Page 15

A Chat on Crystal Chemistry

HOW CHEMICAL COMPOSITIONS INFLUENCE RECTIFICATION —

WHen one begins to consider the . number of different forms of. crystalline minerals which exist in various quarters of the world, one is often struck by the relatively small propor-. tion of these naturally occurring minerals which are of any practical use for the purpose of radio rectification. ; Apart from its crystalline nature and physical form, therefore, it follows from the above observation that 2 mineral, to be an efficient rectifier, must possess another quality, that of partial electrical conduction. A crystal of rock salt or of quartz (silica) is a tolerably good insulator, and it is also a non-rectifier. A diamond is also a non-conductor and a nonrectifier, but, at the same time, it forms an extremely well-defined crystal. At the other end of the scale we find substanees which are very good conductors of electricity, but which, at the same time, do not possess any rectifying powers. Such materials may be looked for among all the true metals, their alloys, and one or two of their pseudo-metallic compounds. WELL-KNOWN GROUP OF CRYSTALS. Tz will thus be evident that a erystalline substance must be intermediate in character between a = conductor and a non-conductor if it is to be used as a rectifier of R.F. currents, and, in fact, practically all of the mineral rectifiers which haye been found to be of any use at all conform to the above provision, There are 2 few rectifying materials which are elementary in nature, That is to say, they are composed entirely of a material which cannot be split up into any simpler forms of matter |

by the ordinary means ef chemical analysis, Such rectifiers are Silicon, Tellurium, Arsenic, and Graphite. The first three of this list are in nature and general characteristics half-way between a metal and 2 non-metal, and they are generally referred to as "metalloids." With the exception of graphite, which is a good conductor, these elementary rectifying substances are poor conductors. They offer considerable resistance to the passage of direct current, but yet they are capable of producing good rectification. The other rectifying minerals which are in use for wireless purposes are almost entirely confined to the sulphide and oxide class of chemical compounds, although there are a few other rectitiers besides these which can be used satisfactorily, and which we will deul with later on in this article. The sulphides of metals fourm the most widely-used class of mineral rectifiers. If you have a piece of crystal whose composition is unknown to you, you may wager quite a ¢considerable amount that it is composed of a metallic sulphide. Of course, it may not have such a composition, but the chances are that it will contain sulphur in some form or other. A sulphide, of course, is a compound which is formed by the direct union of sulphur and a metal. Thus, for instance, if we heat a few pieces of scrap copper to redness and then throw a quantity of flowers of sulphur over the metal, combination’ between the two elements will take place. The metal will glow almost to white-heat, and 2 blackish mass of copper snilphide will remain. LIST of the sulphide rectifiers which are used for wireless purposes will be. seen in Table I, Ié

will be seen that Galena, the basic { material of all the proprietary "ites," is included in the category of sulphide rectifiers. Now, just as sulphur can combine with metals to form sulphides, tellurium and arsenic. are able to enter into a similar combination with metals with the formation of tellurides and arsenides. Many of these latter compounds behave as efficient rectifiers, Hessite, a telluride of silver, is about the best-known mineral of this class, and Niccolite, or nickel arsenide, is an‘other example of this category of minerals. | From the table it will also be seen that the "pyrites" minerals are all sulphides. Some of them are complex sulphides containing more than one kind of metal in their composition. Thus it will be seen that whilst galena, molybdenite, and iron pyrites contain only one metal, minerals such as copper pyrites, Bornite, and Bournonite contain two or three different metals in each case. It is interesting to note that Argentite, a sulphide of silver very similar in general type of composition to galena, is, to all intents and purposes, a complete non-rectifier, but, nevertheless, When a small proportion of this mineral is fused with galena it is able to increase the sensitive properties of the latter mineral. Most samples of ‘argentite have a very much lower electrical resistance than galena, and probably this fact may account in ‘some way for their non-rectifying properties, THE OXIDE GROTP. PASSING on to the oxide group of minerals which are able to act as radio rectifiers, we notice from Table II that the most important member of the group is the well-known Zincite, which ¢an be used in combination with so many other rectifying minerals, Zincite is a naturally occurring oxide of zine, and its ruby-red colour is imparted to it by the existence of slight traces of manganese compounds in the mineral. This trace of manganese in the mineral seems to have a lot to do with the efficiency of its recitfying powers, for zincite which has been deyoid of such impurities is found to be a poor rectifier.

Mineral, Chemical Composition, Galena ....... Lead sulphide Galena forms Molybdenite .. Molybdenum sulphide Single the _- basis Coyelitte ..... Copper sulphide of most Stibnite ...... Antimony sulphide sulphides. proprietary Tren pyrites .. Iron sulphide erystals, Copper pyrites Sulphide of iron and capper Bornite -...... Sulphide of iron and copper Bournonite .. Mnlphide cf copper, antimony, and Complex or mixed sulphides ear Mispickel +e. Sulphide of iron and arsenie . Tin pyrites ... Sulphide of vopper, iren, and Table I.-The above table indieates the composition of most of the sulphide group of mincral rectifiers, Note that these minerals can be further divided inty "single? and "mixed" sulphides,

METALLIC RECTIFIERS, PRON and capper oxides have Leen used experimentally as rectifiers o£ RE. currents under the names of Magnetite and Cuprite respectively, but owing to the varying sensitivities of different samples of these minerals they are not used with any frequency in general amateur work. A number of oxide rectifiers which do not give very good rectification under ordinary conditions can have their rectifying powers very much increased by the application of a small local potential across the rectifying contact. Such mineral rectifiers include the two oxides of manganese, Manganite and Pyrolusite; Cassiterite, an oxide of tin; Anatase, or titanium oxide; and one or two sther similar compounds, It is the rectifying nature of many metallic oxides which is often responsible for rectification at the point Mineral, Chemical Composition, Zincite .... Zine oxide (containing traces of manganese). Magnetite . Iron oxide (magnetic), Cuprite .... Copper oxide. Cassiterite . Tin oxide. Anatase ... Titanium oxide. Brookite ... . Pyrolusite . Manganese Tellurite ,. Tellurium oxide. Iimenite .., Oxide of iron and titanium, Table II.-Indieating the composition of a number of materials which may be included in the oxide category of rectifiers. A large number of other metullie oxides will produce rectification, but only when they are present in very thin films on the surface of their constituent metals. The above, however, are able to rectify in their mass condition, of eontact of two metals, Fer instance, if a strip of clean metallic copper is placed for a minute or two in the flame of a spirit lamp and then Withdrawn and allowed to cool, its surface will be covered with a film of tarnish consisting, for the most part, of oxide of capper. Such a strip of copper will give good rectilfieation when an extremely light contact is made with it either with an ordinary fine cat’s-whisker or with a fragment of zincite, A few experiments of this nature, using different varieties of metals and alloys, will be of interest to the amateur should he be keen on the fascinating subject of crystal rectifieation, An explanation similar to the one given above accounts for the often surprising phenomenon of "rectification by means of the crystal cup alone.’ In these cases, the crystal cup has become slightly tarnished, and its film of oxide has such 2 physical form that it is able to display strong rectifying properties, The last type of rectifier which we have to deal with in our brief survey of the chemistry of erystals is the compound carborundum. Carborundum has the honour of being the first rectifier to be employed for any practical purposes in radio reception, and its use in this direction dates back to the year 1906, when it was brought into service by General Dunwoody, of the United States Army, | sarborundum is really a compound of two elements, carbon and cilicon, , hoth of which are rectifiers, The

‘substance is thus silicon carbide, or, as some prefer to call it, a carbon silicide, Both names, however, mean the same thing. | Carborundum and also silicon are. the only commonly used rectifying materials which are not found in Nature, and which have to be produced artificially. Carborundum, as is well known, requires a local potential for its proper functioning, but, alt the same, it can be used without the application of such a potential if the material is of good rectifying quality to begin with, ORGANIC CRYSTALS. fee whole range of mineral rectifying substances may thus’ be divided up into a few classes: the elementary class, the sulphide class, and the oxide class. Apart from a few exceptions to this classification, such as carborundum (silicon carbide) and one or two other little-known materials, all the crystal rectifying substances are contained in the aboye categories, Experiments have been made with a view to producing well-defined crystals of an organic nature which contain metallic atoms in their composition and which would be suitable for rectifying purposes. Such experiments appear to have proved fruitless up to the present time, but they represent an interesting line of research, and doubtless, at some future date, they may provide the erystallographer and scientist generally with much interesting data of a theoretical and a practicably applicable nature,