Research Notes (Oct. 1987)

Optical fibers: bandwidth breakthrough

Researchers at West Germany's Standard Elektrik Lorenz AG have constructed an optic-fiber system that will transmit data at a rate of 5Cbit/s for 90km with out any intermediate amplification. Until now, such distances could only be achieved at much lower data rates because of dispersion within the fiber. The problem is the difficulty of constructing a fiber in which the wavelength of minimum dispersion coincides with the wavelengths for minimum attenuation. It is therefore usually something of a trade-off between distance and bandwidth.

What the West Germans have done is to adopt a special doping profile to increase the wavelength of minimum dispersion to a point at which it is almost coincides with the wavelength of minimum attenuation. These figures are now respectively 1525mm and 1550mm.

The result of this research is a system with a single optimum wavelength and a distance/ bandwidth product about an order of magnitude better than anything in commercial operation.

Atomic bonding for reliability

Mitsubishi's quarterly publication Advance Vol. 38, 1987, contains details of a new technique for welding together dissimilar materials without the need for heat or high pressures. This development is significant in view of the number of solid-state de vice failures that can be attributed to defective bonds.

Conventional welding, whether arc, resistance or beam, depends on extremely high temperatures to induce localized melting. This in turn can create thermal stresses or even destroy semiconductor materials. High processing temperatures can also cause chemical effects and molecular migration, leading to chip diseases such as the once-dreaded 'purple plague'.

Diffusion welding is a more recent technique in which heat is partially traded for pressure. Nevertheless it still requires many materials to be heated to temperatures at which thermal degradation occurs.

Mitsubishi's atomic joining process gets round the problem by reference to the fundamental physics of bonding. If one imagines a typical solid material such as a metal, each atom is closely and tightly joined to the next by means of chemical bonds made up of clouds of electrons.

In any solid these bonds are by definition strong, otherwise the material would liquefy or vaporize. In theory, therefore, all that is necessary to create a strong bond between two lumps of solid material is to create two adjoining surfaces that closely replicate the inter-atomic structure.

The problems in practice are firstly the need to polish the surface to a mirror-like finish and then the need to avoid any chemical contamination. Even the oxygen in the atmosphere can bind with the free chemical bonds at the surfaces and deactivate them. If it were not so, any fracture could be repaired merely by pressing the broken pieces together.

To achieve atomic bonding, Mitsubishi first polish the surfaces under atmospheric conditions. Then in an ultra-high vacuum, better than 10^9Pa, the surfaces are bombarded with argon ions that perform a sort of atomic-scale sandblasting function. These ions are themselves too big to embed themselves in the atomic lattice, but carry away any contaminants from the free chemical bonds. If two surfaces that have been treated in this way are pressed together, they then unite instantly with a bond strength as great as that of the bulk material.

The secret of the whose process is partly the initial polishing process, which can remove surface atoms down to an accuracy of about 10^9m. Then. if the bond is to be as strong as the parent material, the vacuum must he of an extremely high quality. Mitsubishi use a combination of sputter-ion and titanium-evaporation pumps which act as sacrificial surfaces to absorb gases left behind by conventional pumps.

Although the process may seem more suited to the laboratory than to chip production it does open up possibilities denied to conventional welding. Amorphous materials such as ceramics can easily be atomically joined to metals. Heat-sensitive semiconductors can also be joined reliably to each other and to various substrates. Moreover, as Mitsubishi point out. the whole process would be very much simpler and more economic if it were carried out on board a space station where the necessary vacuum comes free.

Plastics hold the record

ERA Technology has developed a novel technique to ascertain the thermal history of samples of most common plastics. Given a few milligrams of polyethylene, polypropylene, nylon, p.t.f.e. for example, they can reveal the maximum temperature to which it has been subjected during ser vice.

The technique depends on the fact that all these plastics consist of long-chain polymer molecules that take up different configurations when held for a certain time at a particular temperature.

When cooled down, the plastic holds a memory of the maximum temperature reached.

ERA has now solved the problem of how to decode the in formation locked away in the molecular structure using a differential scanning calorimeter.

This is simply a device which plots temperature rise against heat input. When a sample of plastic reaches again the maxi mum temperature it attained in service it exhibits a phenomenon analogous to latent heat of fusion; there is a non-linear relationship between heat input and temperature rise.

This technique of retrospective temperature measurement is likely to have numerous applications in practice. It is possible, for example, to dispense with sensors during a product's testing and development; it is merely necessary to cut up a moulding or a hearing or an insulator afterwards and test samples taken from different areas. In the case of a component that has failed, the technique can establish the maximum temperature of the material at the very point of fracture.

Although ERA's thermal analysis procedure requires a sample to be taken from a component. this can be obviated in some cases by building a sacrificial element into the original component design. This element could be made of a plastic specially chosen for its maximum sensitivity over the temperature range of interest.

The test has already been applied to ascertain the maximum temperature reached by high-voltage power cables. It also has possible interesting applications in forensic science or industrial espionage. In the case of a fire, it should be. possible to determine from a sample of cable whether it was the cause or the result of the fire. In the first case a sample of the plastic insulation will show a temperature gradient from the inside out; in the latter case it will be the other way round.

As for discovering other people's secrets. it should be possible to find out the temperature at which a sample of a special plastic has been treated during manufacture. This can often be the clue to its particular properties.

ERA are, needless to say, not too keen on this last possibility, but they are interested in hearing from anyone with a legitimate application for this technique.

New superconductivity puzzle

An unexpected and unexplained property of the new ceramic superconductors is reported by a group of physicists from the University of Wellington, New Zealand and from the Department of Scientific and Industrial Research in the same country, but with the assistance of AB Kaiser from the \Vest German Max-Planck Institute of solid state physics at Stuttgart (Nature Vol. 328 No 6127). What A. Mawdsley and his colleagues have done is to measure the thermoelectric power of several samples of superconducting ceramic oxides of the yttrium barium-copper-oxide type. Thermoelectric power is a measure of the voltage generated when there are differences of tempera ture between one part of an electrical conductor and another, except that the thermo electric power of superconductors is expected to be zero (as confirmed by the New Zealand experiments).

The surprise that now emerges from the measurement of several samples of yttrium-barium-copper-oxide, all of which became superconducting between 91 and 93 degrees above absolute zero, is that the thermoelectric power seems to in crease dramatically as samples are cooled to the temperature of the transition to superconductivity, before plunging to zero at the onset of the superconducting stage.

The thermoelectric power of a material arises from differences in the way electrons are distributed between the various energy states accessible to them at different temperatures. The New Zealand authors say that measurements like theirs, apparently the first of their kind to have been carried out with the new super-conducting materials. should throw light on several still puzzling features of electrical conduction in the ceramic superconductors. But they confess that they had expected the thermoelectric power of their materials to decline steadily towards zero as their materials were cooled to the superconducting point.

Superconductivity: the temperature keeps rising

One of the best attested hints of superconductivity near room temperature (at minus 43 degrees Celsius) is reported (Na ture, Vol. 328 No 6129) by Professor C. W. Chu from the University of Houston and a group of his colleagues there and the Lockheed Palo Alto Laboratory and the National Magnet Laboratory at MIT. In an accompanying editorial note, Dr David Caplin from Imperial College, London, says that the report from Chu and his colleagues may be best explained if the high conductivity of the specimens resides only in the boundaries between grains.

The material used in the experiments at Houston contains the rare earth element europium, and is chemically similar to the more familiar ceramic superconductors of the yttrium-barium-copper-oxide type, but with the replacement of yttrium by europium. Chu and his colleagues describe how the electrical resistance of one of the samples decreased at least 10.000-fold as it was cooled over an interval of 10°C beginning at minus 35°C. The current used in these measurements was kept small to "avoid damaging the sample". Even so, these hints of some kind of superconductivity disappeared after the sample was heated and cooled a number of times.

Caplin says in his comment on Chu's research that reports of "possible, if transient, superconductivity" at these higher temperatures "should not be pushed aside", hut that the successful search for room-temperature superconductors may require that people should replicate on a "macroscopic scale the molecular structure of grain boundaries"

Groundplanes and interference

ERA Technology Ltd has under taken a detailed study of the effect of PCB ground-planes on the susceptibility of micro processor circuitry to electro magnetic fields. The purpose of this research was to try and discover what could be achieved without the cost penalty associated with filters, screened cables and metal enclosures. As ERA point out in their report (87-0096R), microprocessor-based control systems are increasingly finding their way into industrial and domestic environments where hostile fields exist and where cost constraints are severe.

The work on which this latest paper is based is an extension of an earlier study at the Research Laboratories of the British Gas Corporation in which different PCB designs were compared for their susceptibility to transient over-voltages on the supply leads. ERA Technology have now looked at how different boards compare in the presence of high r.f. fields.

Two circuit boards were constructed. each carrying an 8040 microprocessor, a crystal oscillator, a circuit to monitor oscillation and a fiber-optic output device. Component layouts were nominally identical, except that one board had a copper ground-plane on the upper side to provide all the zero-voltage returns. Power in each case came from the a.c. mains via a filter.

Each board was subjected on its own to a range of frequencies from 10MHz to 500MHz with and without a square-wave modulation at 2.5kHz. Tests were then repeated with a 2m length of twin cable connected to the output of the circuit and resistively terminated to simulate a practical working load.

As might have been predicted, the presence of an output lead reduced the level of external r.f. field at which problems began to occur and also created a few 'problem' frequencies due to resonances. Taking the results as a whole, however, it was immediately apparent that, with or without external connections, the circuit employing a ground-plane was consistently 6dB more immune to electromagnetic fields than the conventional single-sided circuit board. ERA Technology add that with ground-plane construction it is much easier to retro-fit addition al suppression components if they should prove necessary.

Success for deep-frozen squid

A team from the department of physics at Birmingham University has developed what is probably the first practical application for the recently discovered yttrium-barium-copper-oxide high-temperature superconductors. It is a sensitive magneto meter called an r.f. SQUID (Superconducting Quantum Interference Device).

Conventional r.f. SQUIDS consist of a metal ring made superconducting at liquid helium temperatures. This ring which has a gap analogous to the gap in a tape head, is coupled to a tuned LC circuit and fed from a constant current r.f. source of around 20MHz. At particular r.f. current levels the voltage across the LC circuit becomes critically dependent on changes in the magnetic flux entering the SQU1D's gap. This is due to changes in the hysteretic losses in the ring.

As a sensitive detector of magnetic fields there is nothing to beat a SQUID; it can pick up the fields due to the tiny electric currents that flow in the human brain. It also offers immense possibilities for navigation, especially in the tiny magnetic fields of space and for military applications such as submarine detection. The only snag is the need to cool the metal of the SQUID ring to within a few degrees of absolute zero. At least that was the case until the Birmingham group made a ceramic SQUID.

In their recent paper (Nature Vol.328. No. 6125) they describe a SQUID ring made of a material with the composition Y1,2 Ba0,8 CuO4 that works, albeit with reduced efficiency at temperatures up to 45° K. At the lower temperature of 4.2°K, the ceramic SQUID exhibits a novel and un expected property. Unlike the metal version it requires no gap to intercept the external field.

The implications of this finding is that, unlike a bulk metallic superconductor which is impervious to magnetic fields. the ceramic material can be penetrated to a significant extent, even at high frequencies.

As yet the Birmingham team has no fully developed theory to explain the behavior of the novel ceramic r.f. SQUID. They think. however, that in some way the ceramic has built-in magnetic 'weak links' that disappear as the temperature is raised. At the moment they are testing out other SQUID geometries in order both to optimize performance and also to elucidate precisely what is happening at the molecular level.

Research notes is compiled by John Wilson.

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(adapted from: Wireless World , Oct. 1987)