US1708571A - Rectifying element - Google Patents

Description

A1339, 1929' M. HARTMANN ET AL 1,708,571

RECTIFYING ELEMENT Filed Feb'. 21, 1925 INVENTORS MAX- a m,

y/zw 122M come.

Patented Apr. 9, 1929.

PATENT OFFICE.

MINER L. HAR'IMANN AND MORROW C. MILLER, OF ITIAGARA FALLS, NEW YORK, AS- SIGNORS TO THE CARBOBUNDUM COMIANY, OF NIAGARA FALLS, NEW YORK, A

CORPORATION OF PENNSYLVANIA.

RECTIF YING ELEMENT.

Application filed February 21, 1925. Serial No. 10,794.

The present invention relates to detectors of the crystal type for high frequency electrical oscillations, such as those used in wireless or radio telephony and telegraphy, and more particularly to an improved rectifying elementfor use in such detectors.

It is well known that rectification of high frequency oscillating currents can be effected at the contact between two different specific materials, one of'which is usually a natural mineral and the other a metal or another mineral pressed in light contact upon the first material. One of the most widely used rectifiersof this crystal type consists of a galena or lead sulphide-fragment upon which rests with the lightest touch a small metallic wire and through this device high frequency alternating current, such as radio signals, are changed'from alternating current to direct pulsating current. Other minerals and other contacting members have been used, but in all ofthese combinations the rectifying action takes place at the light contact between the two members. Inherent in this combination of two materials is a high electrical resistance, which, when incorporated in a receiving device fcr'radio signals, introduces disadvantages which have heretofore not been over- The amount of current which will flow" through the rectifying device is, of course, proportional to the resistance or imped'ance, of the circuit and any decrease in this resistance or impedance will allow a V greater 1 current to flow and, consequently, produce. a stronger signal .in the receiving apparatus. In the several forms of rectifying device disclosed in our co-pending application, Serial No. 10,7 95,'filed of even dateheiewith, weuse a fragment of pure silicon carbide, as one of the rectifying elements, against which is'pressed with a relatively heavy pressure a vcryhard steel plate. This combination of members with the heavy pressure acting between them gives of itself a low resistance rectifier. Our present invention, however, has to do not with the rectification contact, but with the terminals or other devices for'carryi'ng the current to and away from the rectifying contact, and more particularly with the treatment ofthe silicon carbidefragment to remove surface impurities and with the mounting of the fragment in a metal base." It is well known thatordinary silicon particularly to the decreasing of this joint resistance at the other terminal of the fragment of silicon carbide not involved in the rectifying action.

In the drawings:

Figures '1, 2 and 3 are perspective views illustrating a silicon carbide fragment at successive stages in the coating and mounting thereof, as hereinafter specifically described, the parts being greatly magnified for clearness of illustration;

e have discovered that if a fragment of silicon carbide, such asthe fragment 2 shown in the drawings, is carefully cleaned of all surface in'ipuritiesby treatment, first, with a strong caustic solution, and, subsequently by acids, preferably anoxidizing acid which is a solvent for silica as for eXample,-nitric acid and hydrofluoric acid, that a closely adhering coating 3 of some highly conductive metal, such as copper or silver, preferably by electroplating will provide intimate electrical contact between the surface of the silicon carbide and the metallic coating, thereby greatly reducing the joint resistance. F urthermore, by removing the surface impurities from'the silicon carbide fragment, the resistance of the rectifying contact is also reduced. Also, by the removal of the normal surface impurities, the leakage path in the non-conduction direction of the crystal is removed. e do not restrict ourselves to the above cleaning agents for it is well known that other equivalent materials may be used to remove the normal surface impurities, which consist of carbides of iron, aluminum, calcium, and silicides and oxides of these metals again. or alloyed with a non-oxidizing metal 4, such as an alloyof tin and lead, in order to preserve the thin metallic film, which has been deposited electrolytically or by spraying or otherwise, and also to facilitate ready con- .tact with 'inounting metal "as described" below.

After the fragment of silicon carbide has been treated and prepared as above described,

it is set in an expanding alloy as, for QX- ample, solder, in a steel or metal mold so 7 arranged and so rigid that upon coolingthe metal is'forced to contract around the fragment, thus providing a shaped metal base. 5. In, thus effecting the solidification of the molten metal in a rigid container, the: metal I close contact with the metal coated portion ofthe silicon carbide f fragment, thereby producing practically. perfeet electrical contact. With suitable base metals and coating metals. this intimate contact causes the alloying of'the metal coated silicon carbide and the base metal. 7

The advantages of low "electrical resistance. exclusive of the rectifyingcontact,-in arectifying device will be apparent to those skilled in the art. Low electrical resistance allows the passing of a greater amount of current, whereby signals are. more easily made apparent either by the direct use of. a telephone or by the use of onev of the several known methods of audio frequency amplification, supplementedby some device re producing sound from electrical impulses. Morejeificient rectification results due to the low leakage path for high frequency currents,farising from the removal of the surface impurities. Ordinary crystals as here tofore employed will rectifycurrents efli ciently only to values up to 100 microamperes,

,whereas a crystal treated according to'the presentinvention will rectify and stand up under current of a value up to at least 75 milliamperes. I I While we have disclosed one preferred embodiment of our invention, it will be understood'tha't' the invention is not limited to its illustrated: embodiment but may be otherwise embodied within the scope of the appended claims. e r 1 i i .We claim: 1

A rectifying element comprising-a fragment of silicon carbide whose surfaceis free of those mineralimpurities that are formed on thefcrystal in the manufacture thereof. '2. .Arectifying element comprising a frag,- ment. of 'siliconrcarbide which has: been move such compounds on the surface of the crystal which form'aleakage path forhigh frequency currents. v 3. A rectifying element comprising 'a cleaned fragment of silicon carbide free of the surface impurities which form on the carbide during its manufacture and having a,

coating over a portion of the surface thereof of metallic copper, and having a second coating of non-oxidizing metal over the first coating. V p

. 4. A rectifying element comprising afragment of silicon carbidefwho'se surface has been freed of the impurities which form thereon in the-inani'ifacture of the SlliCOlICiti'blClQ and which either-increase the contact resistance of the silicon carbide or form a leakagepath for. high-frequency currents. in the non-conducting direction of the silicon carbide, said crystal having, al'cofatin'g of copper electroplated thereon Over a portion only thereof and having a second coating of rion-oxidizing alloy over the copper coating.

5. A I rectifying element comprising a cleaned fragment of silicon carbide free of.

surface impurities which form :thereon during the manufacture of the silicon carbide and whichincrease the contact resistance thereof or form a leakage path for high-frequency currents, said crystal having a coating over a portion of its surface of a metal which forms a good electrical contact with the silicon carbide, a second protective coat- '7 ing of metal overthe first coating.

6. A rectifying element comprising a fragmentofsilicon carbide whose'surface is sufficiently free of those. impurities which form on'the carbide in the process of its manufacture having a coating of metal over a portion only of the surface thereof which metal is in intimate contact with the: silicon carbide and is a good conductor of electricity, and a. metal base in which'the coatedportion of the fr inent is set. I w 1 r 7 i A rectifying element comprising a frag ment of-si'licon carbide; coated over the surface its lower portion with a metal, and

having a" second nonoxidizing protective coating over the firstcoatingyand havmg a 'metal base cast" aboutthe coated portion thereof, leaving theuncoated portion of the fragment exposed. g

8. "The-method of makingfa low-resistance rectifying element comprising treating a fragment of silicon carbide with chemicals which remove normal surface impurities which are formed thereon in the manufacture of silicon carbide and which increase the contact resistance of silicon carbide cr' form a -leakage path for high-frequency currents int-he non-conducting direction-of the. siliconcarbide, then coating a substantial portion of the surfaceof said 9. lhe method of making a low-resistance rectifying element which comprises treating a fragment of silicon carbide'withchemicals which remove normal surface impurities v p I I "cleaned fragment with a conducting metal. cleanedwith caustic solution and acid to .re-I:

formed thereon in the manufacture, of the silicon carbide, 1 coating a substantialportion of the surface of said fragments with a metal which is a goodconductor, then forming a second coating over the first coating of a thus creating a pressure forcing the still metal which is more resistant to oxidation molten interior metal into intimate contact 15 than the first coating. with the metal coated silicon carbide frag- 10. The method of making a low resist ment.

ance rectifying element, comprising coating 7 11. The method of making a low-resistant a fragment of silicon carbide with a conductrectifying element which comprises first ing metal over a substantial portion of its treatingafragment of silicon carbide with an i0 surface, providing a second coating over the alkaline solution and then with acid.

first coating of non-oxidizing metal, and In testimony whereof we have hereunto molding the coated portion of the silicon carset our hands.

bide fragment into a shaped metal base by V pouring around the fragment molten metal,, MINER L. HARTMANN.

causing this to solidify in the outer portions, MORROW C. MILLER.

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