What Oxygen-Free Copper Is
Oxygen-free copper is copper that has been melted and cast under conditions that keep oxygen out of the metal, rather than copper that has been deoxidised by a reactive addition. The distinction matters: tough-pitch copper contains oxygen as dispersed oxide particles, and phosphorus-deoxidised copper contains dissolved phosphorus, whereas oxygen-free copper contains neither to any significant extent. The result is a metal of very high purity that retains high conductivity, welds without embrittlement and releases very little gas when heated in vacuum.
TU3 is the designation used in the Chinese wrought-copper system, and C10200 is the corresponding UNS grade, commonly described as OF or oxygen-free high-conductivity copper. In practice the two designations are used interchangeably in commercial enquiries, but the purity and oxygen limits stated in the two systems are not identical, so the requirements of the standard named on the order govern the delivered material.
Composition Requirements and Grade Comparisons
| Grade | Designation | Copper content | Oxygen | Character |
|---|---|---|---|---|
| TU1 | Chinese system | 99.97 % min | Very low | Highest-purity oxygen-free grade |
| TU2 | Chinese system | 99.95 % min | Very low | Standard oxygen-free grade |
| TU3 | Chinese system | 99.90 % min | Very low | Cost-effective oxygen-free grade |
| C10100 | UNS | 99.99 % min | 0.0005 % max | Oxygen-free electronic grade |
| C10200 | UNS | 99.95 % min | 0.001 % max | Oxygen-free grade |
| C11000 | UNS | 99.90 % min | Residual oxygen present | Tough pitch, not oxygen-free |
Two points should be read carefully from this table. The first is that the Chinese and American purity limits do not line up grade for grade: TU3 at 99.90 % minimum is a step below C10200 at 99.95 % minimum in purity terms, even though the two are frequently treated as equivalents. The second is that oxygen-free is a statement about oxygen content, not about electrical grade alone; C10100 and C10200 are both oxygen-free, but C10100 imposes a much tighter limit on oxygen and on the total of all other elements.
The governing documents are GB/T 5231 for wrought copper and copper alloys in the Chinese system, ASTM B152 for plate, sheet, strip and rolled bar and ASTM B170 for oxygen-free electrolytic copper in the American system, with EN 1412 and the corresponding EN product standards used for European orders.
Physical, Electrical and Mechanical Properties
| Property | Typical value | Comment |
|---|---|---|
| Density | 8.93 g/cm³ | At 20 °C |
| Electrical conductivity | 100 % IACS and above | Annealed condition; among the highest of all commercial metals |
| Thermal conductivity | about 390 W/(m·K) | At 20 °C |
| Melting range | about 1083 °C | Copper base |
| Modulus of elasticity | about 117 GPa | Tension |
| Tensile strength, soft temper | 200–250 MPa | Temper dependent |
| Elongation, soft temper | 40 % and above | Temper dependent |
| Outgassing rate | Very low | Suitable for vacuum and electron-device service |
| Hydrogen embrittlement | Not susceptible | No oxide particles present |
Because the metal is so pure, its mechanical properties are controlled almost entirely by temper rather than by alloying. Soft-annealed material is ductile and formable; cold-worked temper raises strength and hardness while reducing elongation. The product standard for the specific form and temper gives the minimum tensile strength, elongation and grain size, and certificate values should be checked against that table rather than against generic figures.
Why Conductivity and Low Oxygen Matter Together
Dissolved impurities scatter electrons, so every element present in solid solution reduces conductivity. Silver, which is the one common impurity that does not degrade conductivity, is counted with the copper. Oxygen is different: in tough-pitch copper it exists largely as oxide particles, and although those particles also reduce conductivity slightly, the more serious problem is their reaction with hydrogen at elevated temperature. Oxygen-free copper removes both problems at once, which is why the grade is specified wherever copper must combine maximum conductivity with welding, brazing or high-temperature vacuum service.
The low outgassing rate is the second consequence of the grade's purity and low oxygen content, and it is the reason oxygen-free copper is used for vacuum chamber components, accelerating structures and electron-device parts. In these duties the material must not release gas into an evacuated space, and copper with a low residual gas content is the practical choice.
Processing and Fabrication
Oxygen-free copper is readily cold worked by drawing, rolling, bending and deep drawing, with intermediate anneals scheduled as the work-hardening builds. It can be soldered, brazed and welded using the standard copper processes, and it brazes particularly cleanly because there are no oxide particles to react with the furnace atmosphere. Annealing is carried out in a controlled or reducing atmosphere, and because the grade contains no oxygen there is no risk of the embrittlement that would affect tough-pitch copper in the same furnace.
Surface cleanliness is critical in electrical and vacuum work. Copper surfaces oxidise slowly at room temperature and rapidly when heated in air, so parts that will be joined by welding, brazing or diffusion bonding are normally cleaned and, where the application demands it, protected or processed promptly. Handling marks, embedded iron from tooling and fingerprints are all sources of contamination that should be removed before a vacuum or high-conductivity joint is closed.
Applications
Vacuum and electron devices: vacuum chamber liners and components, accelerating structures, magnetron and klystron parts, sputtering targets.
Electrical conductors: busbar, bus tube, transformer windings, switchgear conductors and high-current connections.
Power transmission and distribution: substation components and heavy-current joints where joint resistance must stay low.
Photovoltaic and battery plant: array and inverter connections, and battery tabbing and busing material.
Waveguides and radio-frequency components where surface conductivity governs performance.
Glass-to-metal seals and brazed assemblies that require a clean, oxygen-free substrate.
Inspection and Ordering Checklist
An oxygen-free copper order should state the grade by both the Chinese and the UNS designation where both are relevant, the product form, the temper, the dimensions and tolerances, and the standard against which the material will be inspected. Inspection normally covers the chemical analysis, including the total of all other elements, an oxygen determination, the conductivity, the tensile and elongation properties, and the grain size; joints and vacuum parts may add helium leak or ultrasonic examination at the assembly stage.
The most frequent ordering error is to treat the various oxygen-free grades as interchangeable. If the application requires the purity of TU1 or the tighter oxygen and impurity limits of C10100, ordering TU3 or C10200 will deliver a material that is oxygen-free but not to the required purity. The correct grade and the correct standard must both appear on the enquiry, and the certificate must be read against the right table.
FAQ
Q: Is TU3 the same as C10200?
They are the closest equivalents in their respective systems, but the purity limits differ: TU3 requires 99.90 % copper minimum, while C10200 requires 99.95 %. The standard named on the order governs.
Q: What is the oxygen content of oxygen-free copper?
Very low - ASTM specifications limit oxygen to 0.001 % for C10200 and 0.0005 % for C10100. The low oxygen content is what prevents hydrogen embrittlement and limits outgassing.
Q: Why use oxygen-free copper instead of tough-pitch copper?
Tough-pitch copper contains oxide particles that react with hydrogen at high temperature and crack the metal. Oxygen-free copper welds, brazes and vacuum-anneals without that risk, while keeping conductivity at 100 % IACS or above.
Q: How conductive is C10200 copper?
Annealed oxygen-free copper reaches 100 % IACS or slightly above, together with a thermal conductivity of roughly 390 W/(m·K), placing it among the most conductive commercial metals available.
Q: Can oxygen-free copper be welded?
Yes. It is welded and brazed with the standard copper processes, and its freedom from oxide particles makes sound joints easier to achieve than in tough-pitch copper.
Q: Where is TU3 C10200 copper used?
In vacuum and electron-device components, high-conductivity busbar and conductor parts, electrical joints, photovoltaic and battery connections, and brazed assemblies that demand a clean, low-oxygen substrate.
