Niobium-titanium capillaries tubes are used in low-temperature cables, usually referring to the core part of the conductor in superconducting cables. The typical chemical composition is niobium 53 titanium 47%. Nb53-Ti47 pipe is a common low-temperature superconducting material, while capillaries are hollow tubular materials with thin walls and fine inner pores. Making capillaries from niobium and titanium and using them to construct low-temperature cables is a key technology for manufacturing practical superconducting cables.
The excellent superconducting properties make niobium-titanium alloy an outstanding Class II superconductor at liquid helium temperatures. Its critical temperature is approximately 10K, and its critical magnetic field is very high, making it highly suitable for operation under strong magnetic fields and large currents. Niobium-titanium is one of the most commercially successful and widely used superconducting materials, and its preparation and processing techniques are very mature and reliable. In superconducting cables, niobium-titanium usually does not exist in the form of a single capillary but in the form of "multi-core composite superconducting wires", and the concept of capillaries is reflected in two aspects:
1. As a stable base or channel for superconducting wires
A practical niobium-titanium superconducting wire is composed of tens of thousands of extremely fine niobium-titanium superconducting fibers embedded in a high-purity copper or aluminum matrix. In this structure, the copper or aluminum matrix can be regarded as a stable body containing countless superconducting "capillaries". Mechanical support protects the brittle superconducting filament. Thermal stability and electrical stability: When the superconducting state partially loses its superconductivity due to certain disturbances (such as flux jumps), the copper/aluminum matrix can provide a low-resistance bypass channel for huge currents, while quickly conducting heat to the coolant, preventing the expansion of hot spots, and helping the superconducting state recover. In more sophisticated designs, the porous structure of the substrate itself or the gaps between the wire harnesses allow liquid helium to penetrate into the interior of the wire, achieving efficient cooling.
2. As an independent cooling channel inside the cable
In some cable designs, some NbTi capillary tubes made of pure copper or stainless steel are specially arranged as channels for forced cooling circulation. But the main character here is the cooling tube, not the superconductor itself. However, this concept illustrates the significance of "capillaries" in low-temperature systems - for the efficient and compact transportation of cooling media.
It is made by twisting or weaving multiple of the aforementioned niobium-titanium/copper multi-core composite superconducting wires and is used to transmit huge currents (up to several kA). Wrapped around the conductor, it usually adopts special low-temperature insulating materials. A vacuum sandwich pipe insulates the core part of the cable from the external environment. The liquid helium circulating in the thermostat keeps the core temperature of the cable below the superconducting critical temperature of niobium-titanium (about 4.2K or lower).
