Electrical conductivity of the sintered specimens of all the compositions sintered by various methods, namely, microwave sintering, vacuum sintering and conventional sintering were measured and compared with the conductivity as per the International Annealed Copper Standard (IACS) (100%) and listed in the Table 4.4 below. The conductivity was measured using 4-probe method in SIGMATEST D 2.068, FOERSTER instrument.
(a) (b)
59
It can be noted from the Table 4.4 the electrical conductivity gradually increases from the range of 44–54% to 66–68.6% when the sintering method was changed from conventional to microwave sintering techniques. Moreover, when sintering temperature was increased, e.g., 800°C to 900°C in case of microwave sintering, electrical conductivity was found to be higher (65.5–68.6%) than that of the specimens (62–66%) sintered at lower temperature. This is because of the recovery and relaxation of grains, and grain growth, which reduces grain boundary area (Raghavan, 1992). Moreover, at higher temperature a little higher densification was obtained by elimination of porosity. This also helps in improving bonding among particles. These make easier to flow electron and achieve higher conductivity (Rajkovic et al., 2008). But with increasing Cr content from 1 to 6 at.%, it decreased the conductivity slightly and further decreased with the addition of 4 wt.% of nanosize SiC particles with the same composition, although the relative density almost remain in the similar level (Table 4.1). This is expected as because of the electrical conductivity of Cr and SiC is much lower than that of Cu. But, amount of these elements added in present investigation is very low. But after mechanical milling for 50 h, the particle size reduced to a much finer size. Therefore, for a given volume fraction of reinforcement finer size particles would generate more interface area. These interfaces possibly act as source of additional electron scattering. This might be a significant factor in reduction of electrical conductivity. Moreover, matrix (Cu) grain size was found to be finer in case of composition containing higher amount of Cr for the 50 h milled samples. Addition of SiC again refined the matrix grains further for the same duration of milling (50 h) and SiC. It is know that smaller size particles/crystallite size contributes less electrical conductivity. Because grain boundaries acts as a centre of electron scattering (Murty et al., 2007). So the reduction
of electrical conductivity in case of Cu99Cr1–4 wt.% SiC is higher than that of
Cu94Cr6–4 wt.% SiC compacts for a particular sintering temperature in a particular
sintering method.
In present investigation, the highest level of conductivity obtained was in the range of 65–69%, which is much lower if compared with IACS. This could be further increased by improving the sintered density of the materials by employing proper
60
sintering techniques, like, spark plasma sintering, where, suitable sintering parameters (temperature, pressure, time) can be combined to achieve desired level of densification without much deteriorated the mechanical properties (Angerer et al., 2005).
However, the material developed in the present investigation showed hardness values in the range (200–240) Hv and electrical conductivity of (65–70)% with a densification level of 90–95%. Therefore this material could be successfully used in field of electro-technology, where the devices require high electrical conductivity as well as high strength like electrodes for electrical spot welding, relay blades and contact supports, sliding electrical contacts, and so forth (Lopez et al., 2007).
Table 4.4: Electrical conductivity of different specimens sintered by conventional, vacuum and microwave sintering techniques at different temperatures.
Blend Composition
Electrical Conductivity of sintered pellets (%)
Conventional sintering Vacuum sintering Microwave sintering 900°C 1000°C 900°C 1000°C 800°C 900°C Cu99Cr1 54 58 53.4 60 66 68.6 Cu99Cr1–4 wt.% SiC 46.4 54.8 46.2 54 63 66 Cu94Cr6 53 55.8 53.3 58 65 67 Cu94Cr6–4 wt.% SiC 44 53.4 45.5 53 62 65.5
61
CHAPTER 5
CCCooonnncccllluuusssiiiooonnnsss aaaSSSnnntttddduuu ddFdFFyyyuuu tttuuurrreee SSScccooopppeee ooofff
5.1 Conclusions
a) In the present investigation attempts have been made to synthesize bulk Cu-based
nanocomposites of Cu99Cr1, Cu94Cr6, Cu99Cr1–4 wt.% SiC and Cu94Cr6–4 wt.%
SiC compositions reinforcing with a small amount of Cr and/or SiC particles developed by mechanical milling (milled for 50 h) followed by microwave sintering (800°C and 900°C, 30 min), vacuum sintering (900°C and 1000°C, 60 min) and conventional pressure-less sintering (900°C and 1000°C, 60 min) methods.
b) Formation of Cu-Cr solid solution by mechanical milling detected to be very small (< 1 at.%). This could be noticed from the XRD data and variation of lattice parameters of Cu-based solid solution with the progress of milling. The solubility of Cr in Cu also is very small as per the equilibrium phase formation of Cu-Cr alloys (<0.03 at.% at 400°C).
c) Dilatation measurement indicated that sintering of the mechanically milled specimens above 700°C could generate bulk consolidated specimens.
d) XRD and AFM analysis of the sintered samples revealed that the crystallite size
was in the range of 65–125 nm and average particle size was < 200 nm in size,
respectively. Crystallite/particles size was found to be much finer in SiC reinforced composites.
e) Enhancement in the relative sintered density (with respect to theoretical density) was observed by microwave sintering method (90.8–95%) compared to vacuum sintering (84.2–89%) and conventional pressure-less sintering (82.3–87%) methods.
f) Vickers hardness of microwave sintered compacts were found to be in the range of 190–240 Hv depending on variation of composition, compared to 173–218 Hv for vacuum sintered and 170–216 Hv for conventionally sintered specimens of same compositions.
g) Wear resistance of the microwave sintered specimens was found be increased with in Cr content and also addition of nanosize SiC particles.
62
h) The microwave sintered compacts showed greatly superior mechanical properties compared to the vacuum sintered and pressure-less sintered compacts, possibly due to the presence of much finer microstructural features and better densification. i) Electrical conductivity of the microwave sintered specimens showed 60–70% conductivity of IACS compared to 40–50% for conventional and vacuum sintered compacts.
j) Therefore, microwave sintering is found to be most effective in consolidation of Cu-Cr or Cu-Cr–SiC nanocomposite powders compared to the other methods used in the present investigation as suitable combination of mechanical and electrical properties were obtained using this technique. Therefore the material developed in the present investigation could be successfully used for thermo-electric applications.