Quartz Crystals in War and Peace FRANK CONNOR Department of Commerce, Washington,D. C.
MANY millions of pounds of quartz crystals, vital element used to maintain the wave Length or frequency of radio and radar equipment, were examined and graded during the war period in a special testing laboratory a t the National Bureau of Standards, United States Department of Commerce. All crystalline quartz purchased by the Metals Reserve Company for the Government is graded in this laboratory. More than 6,000,000 pounds of crystals have been examined and graded initially and upwards of 10,000,000 pounds examined and re-examined in an effort t o make certain that no usable part of a crystal is discarded. The great bulk of quartz crystala used by the United States manufacturers of radio and radar equipment is imported from Brazil. When German U-boats were active in the Atlantic, the Caribbean, and the Gulf of Mexico shortly after Pearl Harbor, sinking American and Allied vessels right and left, this principal world source of crystalline quartz was temporarily cut offby sea. Resort was had to air transport and for months all quartz crystals from Brazil were flown in by commercial planes and militam transoorts. T o increase the flow of this essential material by air, large bombers were temporarily diverted from the task of bombing the Nazi and the Japanese and added to the air fleet Bying over jungle and sea. The transportation of Brazilian quartz by air came high. Plane shipments were in specially constructed boxes weighing 50 pounds. The freight rate was 81.50 per pound, or $75 per box. Meanwhile, the cost of crystals in Brazil had multiplied many times. Despite the high freight rate and material costs, however. maintenance of volume shioments was imoerative. The manufacturers of radio and radar equipment could not operate without crystals for oscillators and in their absence the h y and Navy could not maintain adequate communicationr. Every effort was made by the Government to find domestic
deposits of quartz crystals of radio quality. Scientific scouts scoured the United States. Deposits were found in several states, the most promising in Arkansas. However, the Arkansas deposits have yielded only about 5000 pounds of crystals in any one year, while the armed forces were calling for millions of pounds to make certain no shortage would develop. Other foreign sources were sought and modest quantities found in Madagascar and elsewhere. But Brazil retains a virtual monopoly on readily available quartz crystals of high grade in the quantities required. The bulk of it comes from the Brazilian States of Bahia and Minas Geraes. I t is of interest t o note that in the immediate prewar years Japan was the largest purchaser of Brazilian quartz clystals. I n 1936 Japan purchased 153,000 pounds. Germany 23,000 pounds, and the United States and Great Britain each 12,000 pounds. In the following year Japan's purchases aggregated 204,000 pounds and there were smaller increases in shipments to the other countries. Orders from the United States increased substantially in 1940 and before Pearl Harbor had multiplied many times. How the National Bureau of Standards responded to the demand for more and more graded crystal3 is shown by the fact that in June, 1942, a total of 65,000 pounds was examined, while in June, 1943, the volume handled was 500,000 pounds, with the testing laboratory working on a %hour basis. The current output of the Bureau's quartz laboratories is going into a stock pile for the anned forces, which ultimately will aeereeate -- - millions of ~ o u n d s . The 113 manufacturine olants making radio and radar equipment are being supplied by private importers. This arrangement is in conformity with a policy under which the Government refrains from buying quartz which the private importers can handle. The importers are protected by an agreement under which the Government undertakes t o
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purchase for stock-piling purposes any excess inventories they may have on hand when the war ends. At present the government stock pile contains many hundreds of thousands of pounds of graded quartz crystalsready for instant shipment. The current output of the testing laboratory goes into a chain of warehouses which, for security reasons, are located in different sections of the country. Operations a t the Bureau's testing laboratory have been reduced to one &hour shift, with the volume of graded crystals going into government stock pile averaging about 60,000 pounds per month. A much larger volume could be handled and the stock pile built up more rapidly, hut as long as private importers keep the industry adequately supplied, the Government avoids competition for available supplies. The reserves now held in one of the several government warehouses are sufficient t o keep the industry going for many months if an emergency should arise. Quartz is a crystalline form of silica showing certain welldefined constants. The crystals are next to topaz in hardness, but will fracture readily if dropped on a hard surface. Consequently, the deftfingered women, who constitute the larger part of the inspection staff, handle them with great care. Crystals are examined for various defects which would disqualify them for use in a finished oscillator-the heart of radio and radar equipment which fixes the frequency of operation. An oscillator plate or bar when correctly cut from the mother crystal can be fabricated so as t o give any one desired frequency out of a very considerable range. When inserted in a transmitter or a receiver, it maintains indefinitely the frequency for which it was designed. Each military radio, far example, is equipped with a set of quartz oscillator units of various frequencies in order that the user may keep in contact with his own force and yet not reveal his presence, position, or plans t o the enemy. Where communications are interrupted on one frequency, another oscillator unit having a different frequency is always handy. Interference may be artificial-created by the enemy-or from natural forces. But always there is another oscillator to plug in on a frequency where interference is not present. A great bomber winging over the Pacific probably carries a hundred oscillator units, while a lumbering tank carries them by the dozen. To inspect a piece of quartz properly, i t must be immersed in liquid. The liquid, having approximately the same index of refraction as the crystal, prevents reflections of the inspection lights from the quartz surfaces. This permits the inspector t o look into the crystal, spot defects if any exist, and determine its quality and grade. A water-white mineral oil has proved most satisfactory as a liquid bath. The oil is refiltered frequently to remove any particles of dirt or sediment which might interfere with the testing. T o detect and estimate the extent of inclusians-foreign chemical material in a crystal-a beam of white light from a carbon arc lamp is introduced into the oil bath through a glass window in the side of the small tank which holds the ail. The inspectors are expert and the presence of any disqualifying defects in a crystal is quickly discovered. On the other hand, the inspectors never cease being delighted when a perfect crystal is examined. A defect not infrequently found is known as optical twinning, which describes the occurrence of both left- and right-hand quartz in a single crystal. To detect and estimate the extent of optical twinning, the presence of which renders the material unusable for oscillator purposes, plane polarized light is used. I n this instance the light source is a mercury vapor lamp located under the oil bath. Ordinary or unpolarized light bas vibrations in all directions perpendicular to the ray, while in polarized light the paths of the vibrations are in a single plane. The mercury light is polarized as it enters the tank by passage through a sheet of "Polaroid." The crystal is then examined under an analyzing sheet of "Polaroid" attached to the top of the tank. . In addition to optical twinning, quartz may also be defective owing t o the prrseuce of clectricsl twinning, tests far which are rnadc by the manufacturers during the cutting stages of fabriction. The Bureau's testing laboratory confines itself t o the direct visual examination of raw quartz.
While twinned portions might be considered inclusions in the mother crystal, the term usually refers to foreign material occurring in the solid, liquid, or gaseous states. The presence of these inclusions in a finished oscillator affects its performance by altering its electromechanical properties to the extent that the crystal vibrates a t an undesired frequency, fractures during acceptance tests or later, or fails to show any response to the impressed alternating current. One example of these defects is the phenomenon of phantom crystals. These appear as if several crystals of various sizes but of the same orientation had grown within the mother crystal. Actually, they mark stages in the interrupted growth of the crystal and the phantom faces were once external faces that became weathered and stained with foreign matter during a period when conditions were not favorable to crystal growth. Similar signs of intermittent growth are seen in some colored crystals, in which the inner crystals are outlined b y vsriations in the depth of color. Another example is that in which "blue needles" occur in pairs apparently originating from a tiny bubble. Many inclusions have a needlelike appearance and are referred t o under the general termof needles. These range in size from gross inclusions having a diameter and length comparable to a lead pencil to tiny pale-blue needles which can he seen only under certain conditions of illumination, and then only with difficulty. The larger needles may consist of either tourmaline or mtile, and the inclusion of either of these minerals in a piece of quartz completely blocks its piezo (pressure) electric effect, making it unusable as an oscillator. On the other hand, the nature of the much smaller blue needles has never been definitely determined. This defect does not disqualify quartz far use in the manufacture of oscillator platesunless the ddect ispresent in excessiveamount. Bubbles are among the defects occurring in quartz. They appear in a wide range of sizes and number. Quartz containing a large number of bubbles per unit volume appears milky white. Small scattered bubbles have about the same effect on the usability of quartz as have blue needles. The carbon lamp also reveals a defect known as incipient fractures. Material coritaining this defect appears to have a grainy structure somewhat comparable to the appearance of weathered ice. Crystals containing incipient fraCtures are rejected because of mechanical weakness and also because such crystals may show extensive electrical twinning. Other visual defects which have irregular forms, some appearing as scales or mosslike aggregates, have been analyzed and found t o he oxides and sulfides of iron or magnesium-aluminum silicates. None of these foreign materials can be included in a finished oscillator. Raw crystals containing them are held out for salvage by trimming off the impure portion where possible, or they are consigned to the below-grade stack. These salvage operations recover a substantial amount of material of radio quality. The Bureau laboratory has on hand one crystal that weighs 270 pounds, too large to he handled through its present testing equipment. Most of the crystals inspected are comparatively small. Crystals weighing up into the tons have been mined. Radio-quality quartz is defined as National Bureau of Standards grades 1 and 2, 30 per cent usable or better in sizes 200 g. and up. Below 200 g., material 80 per cent eye clean or better is considered radio quality. One development in the industry which has resulted from the war is that manufacturers are using crystals smaller than were previously used. For example, the use of quartz crystals down t o 40 g. in weight was not considered practical even two years ago. It is now common practice. Finished crystal-oscillator specifications have become more exacting, requiring much closer inspection. The efficiency of the industry in terms of plates ~ epound r has greatly increased. T h e number of plants manufacturing radio and radar equipment in which oscillator plates are used has multiplied several times during the war and the output runs into tens of millions of quartz crystal plates each year.-Reprinted from Domestic Commerce (September, 1945)
