bmaraldo@watdcsu.UUCP (05/27/87)
**** In Article 662 of sci.electronics: >From: mwtilden@orchid.UUCP (M.W. Tilden, Hardware) >I know that the current >hit-and-miss work going on with ceramic superconductors has yet to >get even a proper defining theory to explain the effect, and >that current densities have yet to exceed the microamp range. The research is not "hit and miss", as stated below, the reproducability of superconductivity is very very high. High Tc sc's are being fabricated by many a reserch team with very good results. And, the current densities available in the old type II sc's is on the order of 10^3 amps and 10^5 amps for the new granular sc. Read on..... I have just resubscribed to sci.elec and notice alot of hoopla about superconductors being tossed about. A couple of weeks ago, Dr. Chen, a noted researcher in the field, gave a talk at the U of Waterloo. Below are the notes I took at the talk ( previously posted to sci.physics ). I hope they clear up some misconceptions. Oh, one more thing: On May 21, 1987, a week after Dr. Chen's talk, the U of W physics department fabricated a 90K superconductor using the Y Ba C O properties Dr.Chen discussed. 1 2 3 (8-y) It just goes to show that *almost* anyone can produce high Tc superconductors. The talk begins.... Prof. Chen gave a very enlightening talk today on superconductors. I took four pages of notes which appear below. There are three major types of sc material. Type I material which has certain critical properties making it less appealing than Type II. Type II sc material has high flux densities due to vortex pinning. Type II material can exhibit magnetic fields around 10e5 Gauss; two orders of magnitude more the Type I. The third, and most interesting, type of sc material is called Granular Superconducor Material (GSM). The structure of GSM is much like ceramic; grains of material packed together until homogeneity is achieved. The sc properties of GSM are brought about by a bizzare interaction between the grains. Two grains of sc material, when brought close enough together, will super conduct; this system is known as a Josephson Junction. It is important that a vast majority of the grains be Josephson coupled or the material will loose it's sc properties. The Josephson effect exists in what would normally be considered to be an insulator between the grains. When the grains are brought within a critical distance, the Josephson effect causes system to exhibit sc properties. The Josephson effect is a function of sine and, hence, is an oscillating effect that can under-go polarity reversal. The obvious application here being switches similar to the semiconductors switches used today but with the ability to handle MORE current with faster propagations. High Tc superconductors are based on the Ba-La-Cu-O system. A sc based on x(Ba) = 0.75 will have Tc=11K. This infact was the material that started the sc revolution in September of 1986 by IBM with a paper published by Muller and Bendnel. Shortly there-after, a sc with the same structure was reported to have a Tc=30 to 40 K. The (Cu)x(O)y plane in these materials is responsible for the super conductivity of the end material. CuO is a sc material itself. Towards the end of 1986, AT&T reported a sc material based on Sr-Ba-Cu-O that has a Tc>40K, the revolution was on. The first sample that Prof. Chen and his collegues prepared was (La)1.8 (Ba)0.2 Cu(O)4-y, baked at 1000C for 10-12 hours. It had a Tc=28K. This sample had two phase transitions from 4.2 GHz to 4170GHz. At 4.2GHz the sample was a sc at it's best; but at 4170 GHz, the sample lost it's sc properties. The average current load on the La-Ba-Cu-O sample was 10e5 A/cm2!!!! It was shown that this material was P type through thermal induction. What if we replace La with Y? On Febuary 16, 1987, Alabama-Houston reported a Y-Ba-Cu-O based sc material with a Tc=82K, 5K above the bp of liquid Nitrogen. And on March 5, 1987 AT&T reported at sc material based on Y(1) Ba(2) Cu(3) O(9-x), known as the 123 sc material. And on March 18, 1987 it was found that Y could be replaced by La, Nb, Sn, Eu, Gd, Dy, Ho, Er, Yb, or Lu. This lead to the conclusion that sc materials are not weight dependent. Later that month, Prof. Chen produced a sc material based on Y with a Tc=90K to 100K. It was time for some tea. Through-out all this research certain things became apparent; the actual production of the sc material was extremely critical. The baking temperature, the pressure, and the over-all homogeneity of the material effect the super conductivity of the end product; making it sc at room temperature or not exhibiting sc properties at all. A different isotope of oxygen ws also tried (18 instead of 16). No differance was found, the sample was still sc. This showed there was no isotope effect. It was also shown that all Y-Ba-Cu-O based sc compounds are multiphased (123 sc material showed two phases at 4.2 GHz, one at 60K another at 90K). The next step was to experiment with a different compound, namely Y(1.8) Ba(0.2) Cu O(4-y). This compound showed a transition at 240K, where R dropped 40%. Next the sample was purified. It was verified that there was a transition at 240K. As well the oscilation of V was verified showing that the sample did exhibit the Josephson effect. The sample was clearly a sc at Tc=77K; at Tc=150K the V swing was +-5V, still sc; at 240K the V swing was +- 1V, not quite sc but damn close. It was found that the Vdc swing was a Bessel function of (z)(e)(v)(sin q)/((h)(f)) where v=amplitude of the ac current, Io. The greater f, the greater v. And this system will work in opposite polarity, this happens in fact in the new sc material at 8.25 GHz. This raises the question: 'Can there be a sc material at 240K?' "The ultimate proof", says Prof. Chen, "- Identify the phase!". We were also told that two days ago (May 8), the Japanese reported a reproducable sc material at Tc=150K. We are well on our way to pocket Cray's and desktop brians. brett maraldo bmaraldo@watdcsu