Electro-vacuum devices (magnetrons, traveling wave tubes, vacuum interrupters, X-ray tubes, waveguides, sealing leads, vacuum anodes, etc.) impose stringent requirements on oxygen-free copper strips, including low outgassing, high gas tightness, immunity to hydrogen embrittlement, low vaporization rate, reliable brazing performance and high-temperature stability. Failures of oxygen-free copper strips can be categorized into six types: inherent metallurgical defects of raw materials, forming defects from stamping, failures induced by heat treatment/hydrogen brazing, performance degradation under vacuum service conditions, surface contamination & corrosion, and failures caused by thermal/electrical loads.

I. Inherent Metallurgical Defects of Raw Materials (Root-Cause Hidden Hazards Triggered in Subsequent Processes)

1. Excessive Oxygen Content & Cu₂O Inclusions at Grain Boundaries (Most Critical Cause of Vacuum Failure)

• Phenomenon: Oxygen content of copper strip exceeds 30 ppm (TU1 standard ≤10 ppm), with cuprite eutectic distributed along grain boundaries. During hydrogen brazing at 850–950°C, the chemical reaction occurs: \(\ce{Cu2O + H2 = 2Cu + H2O(g)}\). Water vapor generates high internal pressure along grain boundaries, resulting in intergranular microcracks (hydrogen embrittlement / hydrogen disease).

• Consequences for devices: Sharp rise in cavity leak rate, air leakage at sealing joints, complete device rejection; crack propagation under operation of high-power tubes leads to rapid vacuum breakdown.

• Inducing factors: Inadequate shielding during melting, oxygen absorption by ingots, oxygen traces in furnaces for rolling & annealing, counterfeit oxygen-free copper substituted by ordinary T2 copper.

2. Excessive Harmful Impurities (Pb, Bi, Sb, P, S, Fe)

• Low-melting-point impurities Pb & Bi: Melt along grain boundaries during high-temperature brazing, causing liquid embrittlement cracks and loss of hermetic sealing; drastically reduce high-temperature mechanical strength of copper strip.

• Residual excess phosphorus: Deoxidizer residues severely impair electrical & thermal conductivity, generate dense pores in brazed welds and drastically boost outgassing volume.

• Sulfur & iron: Form sulfide/iron oxide inclusions, continuously release CO and H₂S under high vacuum & temperature, degrade vacuum degree and contaminate cathode filaments.

3. Internal Metallurgical Defects Introduced During Copper Strip Rolling

• Internal pores, porosity & pipe shrinkage: Micropores inherited from ingot extrusion get exposed after stamping; sustained outgassing under vacuum prevents high vacuum maintenance, and micropores trigger electrical breakdown under high voltage.

• Delamination, peeling & foreign inclusion: Oxide scale trapped and pressed into strip during rolling; cracking occurs upon bending or deep drawing, forming gaps at welding interfaces that cause air leakage.

• Overgrown/non-uniform grains: Lack of vacuum bright annealing leads to inconsistent thermal deformation under high-temperature service, generating stress cracks at metal-glass sealing interfaces.

II. Forming Defects from Stamping, Bending & Deep Drawing

1. Excessive Residual Stress from Cold WorkingHard-temper copper strips directly used for sealing warp and deform due to stress relief during high-temperature baking or brazing, separating metal-glass sealing interfaces and causing leakage; dimensional drift of device anodes and waveguide cavities increases RF loss.

2. Microcracks from Bending/Deep Drawing & Edge BurrsCommon thin gauge copper strips (0.05–0.5 mm thickness) develop surface microcracks with overly small bending radii. Burrs induce field emission, micro-arcing and RF breakdown under high vacuum & high voltage; gaps beneath burrs trap cutting fluids, leading to massive outgassing in follow-up processes.

3. Machining oil contamination, fingerprint salts & embedded polishing powderStamping lubricants, salt residues from human fingerprints and abrasive dust infiltrate micro-scratches on surfaces. These contaminants decompose under high vacuum & temperature, releasing H₂, hydrocarbons and water vapor, which degrade vacuum degree, poison cathodes and attenuate electron emission performance.

III. High-Temperature Process Failures: Typical Defects from Hydrogen Brazing, Vacuum Baking & Sealing

1. Hydrogen Embrittlement Cracks (Directly Caused by Excessive Oxygen)

As stated above, this is the most typical failure mode for electro-vacuum oxygen-free copper, pushing leak rates beyond acceptable thresholds of \(10^{-9}–10^{-11}\ \mathrm{Pa·m^3/s}\) for hermetic requirements.

2. Brazed Weld Defects

• Unremoved surface oxide film on copper strip: Poor wetting of brazing filler metal, cold solder joints and leakage gaps.

• Overgrown grains & segregated impurities in copper strip: Weld porosity and incomplete fusion, developing micro-leaks after long-term thermal cycling.

• Mismatched thermal expansion coefficients: Differential expansion between copper strip and Kovar/ceramic sealing components generates cyclic thermal stress, leading to fatigue cracking of welds after repeated temperature fluctuations.

3. Excessive Outgassing During Vacuum Baking (Core Pain Point for Ultra-High-Vacuum Devices)

• Poor cleanliness, abundant micropores or high impurity content of copper strip: Continuous release of H₂, CO and H₂O after high-temperature degassing fails to maintain target vacuum, resulting in high noise and unstable beam current of microwave tubes.

• Over-pickling and rough uneven surfaces: Copper vaporization rate rises sharply under high temperature; deposited copper films contaminate cathodes and insulating ceramics, reducing insulation resistance and triggering frequent arcing.

4. High-Temperature Softening & Creep Deformation

Oxygen-free copper melts at 1083°C and loses most mechanical strength at typical brazing temperatures (~900°C). Under long-term high-temperature operation of high-power devices, copper strip electrodes and support structures deform via creep, creating tensile stress on sealing joints (cracking) and electrode displacement (short circuits).

IV. Surface Oxidation & Corrosion During Storage and Assembly

1. Ambient-Temperature Oxidation & Verdigris FormationCopper strips stored in hot, humid or sulfur/chloride-containing atmospheres form CuO and basic copper carbonate (\(\ce{Cu2(OH)2CO3}\)) on surfaces. Oxide films bring the following risks:

• Impede brazing filler wetting, causing cold solder joints and leakage;

• Decompose under high vacuum & temperature to continuously release oxygen and water vapor;

• Raise surface thermal emissivity, worsening heat dissipation of high-power anodes and leading to excessive temperature rise.

2. Local Galvanic CorrosionStacked contact between copper strips and dissimilar metals (steel, Kovar) forms micro-galvanic cells under damp conditions, generating pitting micropores that act as permanent micro-leakage channels under vacuum.

V. Performance Degradation During Vacuum Service of Finished Devices

1. High-Temperature Copper Vaporization & Material Loss

High-power beam bombardment and RF heat loss raise copper strip temperatures, sustaining continuous copper atom vaporization:

• Deposited copper films cover insulating ceramics and cathodes, lowering insulation resistance and inducing frequent high-voltage arcing;

• Progressive thinning of anode copper strips reduces structural strength and causes deformation, shortening device service life;

• Thickness variation of copper films inside resonant cavities shifts resonant frequencies and increases signal loss.

2. Field Emission & Vacuum Arcing

Micro-tips formed by surface scratches, burrs and oxide protrusions trigger electron field emission under high electric fields, generating micro-discharges and arcing. Spattered copper particles contaminate internal components and may cause catastrophic device breakdown in severe cases.

3. Thermal Cycling Fatigue Cracking

Frequent startup-shutdown cycles subject copper strips to repeated thermal expansion and contraction:

• Cyclic stress accumulates at copper-ceramic/metal sealing interfaces, propagating fatigue microcracks and slow air leakage;

• Crack expansion initiates at stress-concentrated bending and stamping locations.

4. Degraded Electrical & Thermal Conductivity with Local Overheating

Surface oxide films and internal impurities raise electrical resistivity. Heat concentrates locally on anodes and bus copper strips under high-power operation, accelerating creep and vaporization to form a self-amplifying vicious cycle.

5. Residual Gas Release & Cathode Contamination

Micropores, internal impurities and surface contaminants continuously release residual gases, which adsorb onto electron-emitting cathodes and cause cathode poisoning. This reduces emission current, cuts output power and drastically shortens device lifespan.

VI. Application-Specific Failures Under Special Operating Conditions

1. Sputtering Loss from Ion BombardmentPlasma bombardment inside ion sources, accelerators and microwave tubes strips surface copper via sputtering, increasing cavity impurities and destabilizing beam current.

2. Failure to Meet Non-Magnetic RequirementsExcessive ferromagnetic impurities in copper strips distort magnetic field distribution in magnetrons and particle accelerators, leading to beam deflection and abnormal device performance.

3. Insufficient Rigidity & Resonance Deformation of Thin-Wall Copper StripsThin-gauge oxygen-free copper strip electrodes and waveguide walls undergo mechanical resonance under RF excitation, exacerbating fatigue cracking and arcing risks.

Brief Categorized Summary for On-Site Troubleshooting

1. Hermetic Leakage: Hydrogen-induced intergranular cracks, cold solder welds, stamping microcracks, corrosion micropores, stress cracks at sealing joints

2. Vacuum Outgassing & Contamination: Excessive impurities, machining oil residues, rough pickled surfaces, oxide films, internal micropores

3. High-Voltage Arcing & RF Malfunction: Burrs, surface micro-protrusions, vaporized copper film deposition, structural displacement & deformation

4. High-Temperature Mechanical Failures: Creep softening, thermal cycle fatigue, warpage from residual cold-working stress

5. Cathode Performance Degradation: Sustained residual gas release, cathode contamination by vaporized metal

6. Inherent Raw Material Defects: Excessive oxygen, harmful impurity overload, strip delamination/peeling, abnormal grain structure

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