Copper-Carbon Nanotube System

Much work has been devoted to developing copper-CNT composites. These composites are excellent candidates for thermal management applications due to the high conductivity of Cu (-400 W nr1 К _l) as well as of CNTs (-3000 W nr1 K_l). Most of the researchers have utilized the powder metallurgy technique. A few studies are on developing sensors where Cu particles are deposited on the CNTs or they are elec- trochemically deposited. Most of the studies have used ball milling to disperse CNTs in Cu powder. Ni-coated CNTs synthesized by electro-deposition or electroless deposition are better reinforcement because they lead to better bonding between the CNT and the Cu matrix. The molecular level mixing (MLM) method was carried out in a Cu-CNT system and excellent dispersion was obtained in the powder. In this method, CNTs are dispersed in a Cu-salt solution followed by drying, calcinations, and H2

reduction to get the powders. Consolidation has been carried out by pressing and sintering, rolling, equal channel angular pressing, SPS. sandwich processing, and high-pressure torsion. Laser Engineered Near-net shaping has also been used effectively for synthesizing macro-scale composites from Cu-CNT composite powders with up to 10 vol.% of reinforcement content [20]. Micro-scale composites for thermal management have been prepared by electrochemically depositing Cu on aligned CNT arrays. Copper-CNT composites have also been prepared by electrodeposition and electroless deposition of Cu on CNTs. They have also been prepared by mixing in mineral oil and making paste for sensor applications. Formation of carbides or any interfacial products has not been reported in any of the studies mentioned previously. Table 5.2 shows the various aspects of the work carried out on Cu-CNT composites.

Reports on Cu-CNT systems are focused equally on the improvement in mechanical and electrical properties. Mechanical properties of Cu-CNT composites clearly show the effects of processing techniques on their improvement. Conventional powder metallurgy techniques, comprising compaction and sintering, help increase the hardness up to 20% with 15 vol.% CNT addition [21]. An electroless coating of the CNTs with Ni improves their bonding with the Cu matrix and increases the hardness by ~80 to 100% for even 9-12 vol.% CNT addition [22-24]. SPS improves the hardness by 79% with 10 vol.% CNT addition [25]. Further deformation by rolling leads to improvement on dispersion and alignment of CNT clusters and improved the hardness by 207% and the elastic modulus up to 95% [26]. Molecular-level mixing leads to excellent dispersion and elimination of CNT clusters in SPS composites [27]. This causes an extraordinary strengthening of the composite with a 200% increase in the yield strength and 70% increase in the elastic modulus. Molecular level mixing, followed by cold pressing and vacuum sintering, has been found to increase the flexural strength of the composites also by 31% [28]. The strengthening was explained by improved load transfer to the CNTs due to the presence of Cu and О atoms at the CNT interface that helped in bonding as well as dispersion. Samples prepared by shock wave consolidation of molecular level mixed powders showed increased hardness (51%) than that predicted from the Hall-Petch relation [29]. A similar increase was observed in 1 wt.% CNT composite prepared by high pressure torsion of ball milled powders [30]. Out of the total 640 MPa increase in the hardness, 280 MPa was attributed to the grain refinement and the balance of 360 MPa was attributed to CNT addition. For coatings deposited by electrodeposition technique, a 36.4% increase in hardness was reported for a 10 vol.% SWNT composite, which might be good improvement considering the inherent porous nature of electrodeposited coatings [31]. Cu-CNT composite, processed by cold rolling of sandwiched layers of metal and CNT, showed an 8% increase in tensile strength and a 12.8% increase in the elastic modulus [32]. Cu-CNT composites prepared by SPS of electroless Cu-plated CNTs showed an excellent increase in elastic modulus by 100% and yield strength by 183% for a 15 vol.% CNT addition [33]. However, further increase in CNT content to 20 vol.% led to lowering of the yield strength compared to 15 vol.% CNT (~96%), although the elastic modulus increased slightly (112%). These results again indicate the significance of dispersion and bonding between CNTs and the

CNT Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

0-25 vol.%

Cu powder and CNT mixed by ball milling (30 min), isostatically pressed (350 MPa. 5 min), isothermally sintered(850°C, 2 h. vacuum), cold rolled, annealed (600°C. 3 h)

Cu-15 vol.% CNT H- I18VHN

H - 98 VHN

Homogeneous distribution of CNT is reported

Coefficient of friction decreases with CNT addition, wear loss decreases up to 12 vol.% CNT and then slightly increases

[21]

4-16 vol.%

Nickel-coated Cu powder and CNT. ball milled (30 min), isostatically pressed (100°C, 600 MPa, 10 min), isothermally sintered (800°C, 2 h)

Cu - 12 vol.% CNT H-21.5 HRB* (* Rockwell hardness with В scale)

H - 10.2 HRB

Coefficient of friction decreases with increasing CNT content. Wear volume decreases with increase in CNT content up to 12 vol.%

[22]

4.8, 12, and 16 vol.%

Nickel-coated Cu powder and CNT. ball milled (30 min), isostatically pressed (100°C, 600 MPa, 10 min), isothermally sintered (800°C, 2 h)

Cu - 12 vol.% CNT H-21.5 HRB* (* Rockwell hardness with В scale)

H - 10.2 HRB

Coefficient of friction decreases with increasing CNT content. Wear volume decreases with increase in CNT content up to 12 vol.%

[23]

SWNT

Cu nanoparticles are electrodeposited on SWNTs

Glucose detecting sensitivity increases four times

[131]

CNT Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

0-15 vol.%

Ball milled(24 h). SPS (750°C, 40 MPa. 1 min)

Cu - 10 vol.% CNT H ~ 100 MPA

H -56 MPa

Better dispersion of CNT in composites with Cu-nano powders is reported

[132]

CNTs grown in arrays and in between places were filled with Cu by electrodeposition

Cu-CNT films show lower thermal resistance than only CNTs

[133]

Cu-CNT-mineral oil hand mixed and put in fused silica tube with a Cu wire for maintaining electrical contact

Better sensitivity of microchips with CNT for carbohydrate detection

[134]

5 and 10

vol.%

CNT and Cu ion are suspended in a solvent, dried, calcination, reduction, performed for composite powder (molecular level mixing), pre-compacted (10 MPa). SPS (550°C, 50 MPa. 1 min. vacuum)

Cu - 10 vol.% CNT CTysic,' 455 MPa

Oysio ~ 150Mpa E -80 GPa

Homogeneous distribution of CNT is reported with high interfacial strength

[27]

0-5.25 vol.%

CNTs coated with Ni by electroless deposition, mixed with Ni powder, ball milled (30 min), hot pressed (1100°C, 32 MPa. 1 h)

Cu - 2.25 vol.% CNT: H -280 VHN

H-155 VHN

CNTs are found to get agglomerated at higher concentrations

Coefficient of friction reduces drastically for 2.25 vol.% CNT and then reduction is lowered, wear loss is minimum at 2.25 vol.% CNT

[24]

Dispersion of CNT and Cu microparticles in mineral oil, used as electrode

Excellent performance with detection limits in micro-molar levels for non-electroactive amino acids for composite electrode

[135]

Cu nanoparticle added to SWNTs in Nation solution and sonicated, solution is dropped on polished electrode and dried

Good dispersion of CNT is observed in dried film forming network

Cu-SW'NT Composite gives most synergistic signal effect

[136]

5 vol.%

Cu powder, CNT-mixed, ultrasonicated (1 h). dried, equichannel angular pressed through 8 passes

Hardness increases - with number of passes, i.e., amount of deformation H ~

115VHN (after 8 passes)

Reduction in agglomerates observed along with improved distribution; improves upon deformation

[137]

5 and 10

vol.%

Spray-dried Cu powder and CNT. ball milled (24 h, 150 rpm), precompacted (10 MPa). SPS (700°C, 50 MPa. 1 min. vacuum), rolled (50% reduction), annealed (650°C. 3 h)

ASTM-E8M Dogboneshaped sample Gauge length - 9 mm Width - 2.5 mm

Cu-10 vol.% CNT E E - 70 GPa oYSIT)

  • - 137 GPa VS(T) -135МРаоте
  • - 197 MPa oTS -175 MPa H -281 MPa -0.57 GPa H- 1.75 GPa

CNT-rich and CNT-free regions are observed distinctly in matrix

[26]

30 and 55 vol.% SWNT

Thick network of SWNT, prepared by suspension of CNT dropped on substrate and dried, Cu is electrochemically deposited on CNT network

Electrical conductivity and thermal expansion coefficient of composite is same as Cu

[138]

CNT Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

Cu-CNT composite particle, precipitated from dispersion of CuS04 and CNT in distilled water by adding NaOH and KBH4 '

CNTs are found covered by Cu particles

Catalytic activity increases in presence of CNT

[139]

Cu - electroless deposited on CNT from CuS04 solution

Cu particles deposited on surface and inside of CNTs, Cu particle on surface of CNTs are found uniform in size and distribution

Composite possesses fine electron conductivity

[140]

SWNT

Cu - argon sputtered on SWNT bundles at high vacuum

Formation of 1D array of nanoclusters prefereably at the groove of CNTs in the bundle is reported

[141]

Cu nanoclusters - electrochemically deposited on CNT (on electrode)

Some aggregation of nanoclusters is observed

High sensitivity, good reproducibility, and fast response

[142]

CVD-grown aligned CNT - copper deposited by electrochemical plating

Cu fillings are found occupying the voids between CNTs forming compact channels

Composite shows better electrical and thermal conductivity than only aligned CNTs

[143]

1 vol.%

Cu powder and CNT. mixed, ultrasonicated (1 h). filled in a sheathe, cold ECAP through 8

passes

Hardness increases with number of passes, i.e., amount of deformation H - 115VHN (after 8 passes)

Reduction of agglomeration and better dispersion with greater number of ECAP passes is reported

[144]

0-10 vol.%

CNT and Cu ion is ssuspended in a solvent, dried, calcination, reduction, performed for composite powder (molecular level mixing), SPS (550°C. 50 MPa. 1 min. vacuum)

Cu - 10 vol.% CNT H- 1.1 GPa

H - 0.8 GPa

Homogeneous dispersion of CNT is found with good inter-facial bonding (CNTs embedded in Cu powders)

Wear loss reduces with addition of CNT

[25]

1 vol.%

Cu powder and CNT - mixed, ultrasonicated (1 h), dried, filled in a sheathe. ECAP for 8

passes

Hardness increases with number of passes, i.e., amount of deformation H - 115VHN (after 8 passes)

Homogeneous distribution of CNT in matrix is obsrved

[145]

SWNT

CVD grown SWNT films (19). sandwiched with Cu films (20), cold rolled, annealed (1050°C, 10 h), cold rolled

Length - 4.9 -5.2 mm Width - .8 - .09 mm Thickness -0.025 mm

оте-361 MPa E- 132 GPa

От*-334 MPa E - 117 GPa

Good interfacial adhesion reported

[32]

CNT-dispersed. ultrasonicated. CuS04 added. NaOH added, dried, reduced (molecular level mixing)

CNTs are found homogeneously implanted in the Cu spheres

[146]

CNT Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

CNT and Cu nano powder, mixed in mineral to prepare a paste electrode

Detection sensitivity is sufficient with reasonable repeatability and operational stability

[147]

CNT and Cu micro powder, mixed in mineral to prepare a paste electrode

Highly sensitive and fast detector

[148]

7-10 vol.% SWNT

CNT dispersed in CuS04 electrolyte, electrochemically deposited under ultrasonic field

Cu-10 vol.% CNT H- 1.61 GPa

H-1.18 GPa

Interface is found wettable and in good adhesion

Electrical conductivity is comparable to pure Cu for low to high temperatures

[31]

5 and 10 vol.%

Composite powder, prepared by oxidation-reduction process (molecular level mixing), compacted. SPS (550°C . 50 MPa. 1 min, vacuum)

Not

mentioned

°ys(C) - 455MPa E (N0-138 GPa

oVS|c, - 150 MPa E (NO - 100 GPa

Uniform dispersion and good reinforcement of CNT with matrix is reported

[149]

1 wt.%

Ball milled (5 h), isostatically compacted (500 MPa), consolidation by high pressure torsion (6 GPa. 5 revolutions)

H - 3.5 GPa

H - 2.8 GPa

Homogeneous and good dispersion of CNT in matrix is reported

[30]

10 vol.%

Composite powder, prepared by oxidation-reduction process (molecular level mixing), shock wave consolidated using propellant gun system fixture

H - 1.19 GPa

H - 0.80 GPa

Homogeneous and good dispersion of CNT in matrix is reported

[29]

1 wt.%

Ball milled (5 h. argon atmosphere), isostatically compacted, consolidation by high pressure torsion (6 GPa)

Micro-pillar

compression

testing

Diameter

  • - 5 pm Length
  • - 11pm

VSlCl - 1125 MPa

oYS)c, -738 MPa

Homogeneous distribution of CNT in matrix is found

[150]

0-20 vol.%

Composite powder prepared by electroless deposition of Cu on CNT. SPS (550°C, 50 MPa. 1 min)

Gaige length - 9 mm Width - 2.5 mm

CTvsm - 350 MPa E

  • - 105.9 GPa H
  • - 1.4 GPa

°vsn i- 120 MPa E -51.6 GPa H - 0.7 GPa

Uniform distribution of CNT in matrix is found

Electrical conductivity decreases with increasing CNT content

[33]

0.5 vol.%

Mechanically mixed, hot pressed (600°C, 45 MPa. 30 min)

Uniform distribution of CNT in Cu-Ni matrix is reported

No improvement is recorded in electrical and thermal conductivity, initial coefficient of Friction decreased for Cu, reinforced with Ni-coated CNTs

[151]

Pulse reversed electrochemical deposited

Gauge length

- 4 mm Width - 0.4 mm

Thickness

- 0.04 mm

For CNT diameter of 1.5 - 3 nm - 670 MPa

o.rs - 230 MPa

Uniform dispersion of CNT in Cu matrix is reported

[152]

CNT Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

5, 10.and 15 vol.%

Dry impact blended of Cu and CNT powder (40 min. 5000 rpm), SPS (600°C . 50 MPa. 5 min)

Distribution of CNT is uniform up to 10 vol.% and at 15 vol% clustering starts

No improvement in thermal conductivity with 5 and 10 vol.% CNT addition; decrease in the same with 15 vol.%- CNT - due to interface resistance, CNT clustering porosity

[34]

Note: E - elastic modulus; H - hardness; cxs - tensile strength; oys(T) - yield strength in tension; cYS(C, “ yield stress in compression; — Data not available or not applicable.

matrix in Cu-CNT composites. Cu-CNT composites prepared by SPS of dry- impact-blended powders showed similar thermal conductivity as copper up to 15 vol.% reinforcement and a decrease thereafter [34].

 
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