Introduction to Ultra-High-Purity Copper in Modern Cryogenic Systems
In cryogenic research, where experiments routinely operate below 20 K — and often below 10 mK — even parts-per-billion impurities can introduce unacceptable electrical noise, thermal bottlenecks, or magnetic interference.
This is why leading laboratories and quantum-technology companies now specify 5N (99.999%) and 6N (99.9999%) copper, as well as oxygen-free high-conductivity (OFHC) variants, for virtually every critical thermal and electrical path.
What Happens to Copper Conductivity at Cryogenic Temperatures?
Below ~20 K, phonon scattering becomes negligible and residual resistivity is almost entirely determined by impurities (Matthiessen’s rule). NIST and IUPAC data show:
|
Purity |
Typical RRR (300 K / 4 K) |
Dominant Impurity Impact |
|
4N (99.99 %) |
100 – 250 |
Acceptable for general thermal links |
|
5N (99.999 %) |
500 – 1,500 |
Standard for superconducting magnets & cryocoolers |
|
6N (99.9999 %) |
3,000 – 15,000+ |
Required for mK platforms, qubits, and SQUID systems |
RRR = Residual Resistivity Ratio. Higher RRR = dramatically lower electrical and thermal resistance at cryogenic temperatures.
Six Critical Reasons Researchers Choose 5N–6N Copper
1. Near-Perfect Thermal Conductivity Peak thermal conductivity of 6N copper at 20 K can exceed 10,000 W/m·K — more than 25× that of commercial copper.
2. Ultra-Low Magnetic Impurities Fe, Ni, and Co below 10 ppb prevent magnetic noise in dilution refrigerators and quantum processors.
3. Exceptional Electrical Performance Reduces Johnson noise and quasiparticle generation in superconducting circuits.
4. Mechanical Stability Through Thermal Cycling High-purity copper resists grain growth and embrittlement after hundreds of cycles to 4 K.
5. Minimal Outgassing in UHV Systems Essential for 10⁻¹¹ mbar vacuum chambers.
6. Precision Machinability Without Compromising Purity Our 6N copper is supplied in annealed condition yet machines to mirror finishes when required.
Real-World Applications That Demand the Highest Purity
|
Application |
Typical Purity Required |
Common Forms Used |
|
Dilution refrigerator thermal buses |
6N or 6N+ |
Plates, rods, custom heat exchangers |
|
Superconducting magnet current leads |
5N–6N OFHC |
Large billets, forged blocks |
|
Quantum computing – qubit cavities & coaxial lines |
6N, single-crystal in some cases |
Solder-coated plates |
|
MRI gradient shields & cryostat radiation shields |
5N OFHC |
Large rolled plates |
|
Particle accelerator RF cavities |
6N (Class 1) |
Seamless tubes, spun hemispheres |
How to Choose the Right Grade for Your Project
|
Project Type |
Recommended Starting Grade |
When to Move to 6N |
|
General 4 K cryostats & pulse-tube coolers |
5N OFHC |
— |
|
Superconducting magnets & NMR |
5N–6N |
If RRR > 800 required |
|
mK dilution refrigerators |
6N |
Always |
|
Quantum processors & SQUIDs |
6N (≤10 ppb magnetic impurities) |
Always |
Why Laboratories Trust High Purity Aluminum (HPA) for Their Copper Needs
Although we built our reputation on ultra-pure aluminum for stabilization of superconducting magnets, the same vacuum distillation and zone-refining platforms now produce industry-leading 6N copper with:
- Full lot traceability
- GDMS certificates showing <0.1 ppb detection limits for critical elements
- RRR measured in-house at 4.2 K with ±5 % accuracy
- Standard stock of plates – 0.50” x 6” x 12”, and Rods up to 6” Diameter
- Rapid custom machining
Next Step: Get Material That Won’t Limit Your Experiment
If you’re tired of purity being the weak link in your cryogenic chain, let us help.
Contact Us | High Purity Aluminum
• Typical analysis for our Copper 6N - HPA Typical Analysis - Cu 6N.pdf