Fabrication of Al 2 O 3-Cu Nanocomposites Using Rotary Chemical Vapor Deposition and Spark Plasma Sintering

A two-step rotary chemical vapor deposition techniquewas developed to uniformlymixCunanoparticles with the γAl 2 O 3 powders, and then the as-obtained powders were consolidated toAl 2 O 3 -Cu nanocomposites by spark plasma sintering. In the RCVDprocess, the metal-organic precursor of copper dipivaloylmethanate (Cu(DPM) 2 ) reacted with O 2 and then was reduced by H 2 in order to erase the contamination of carbon. At 1473K, the relative density of Al 2 O 3 -Cu increased with increasing CCu and the maximum value was 97.7% at CCu = 5.2 mass%. The maximum fracture toughness of Al2O3-Cu was 4.1MPam 1/2 at CCu = 3.8 mass%, and 1MPam higher than that of monolithic Al 2 O 3 , validating the beneficial effects of Cu nanoparticles.


Introduction
The incorporation of small amounts of metal nanoparticles, such as Cu [1], Ni [2][3][4], W [5], and Cr [6], has been proved to be beneficial for improving the densification, mechanical properties, and conductivity of Al 2 O 3 .Particularly, the incorporation of Cu nanoparticles has attracted much attention due to its high ductility and good electrical conductivity [1,7,8].Kim et al. fabricated Al 2 O 3 -Cu nanopowders by ball milling for 30 h and then sintered at 1100-1400 ∘ C for 5 min in vacuum under a pressure of 50 MPa by the pulse electric current sintering (PECS) [7].The composites sintered at 1250 ∘ C for 5 min showed a relative density above 97% and enhanced fracture toughness of 4.5 MPa m 1/2 .Oh and Yoon fabricated Al 2 O 3 -Cu nanocomposites by hydrogen reduction and hot pressing of Al 2 O 3 /CuO powders.The composite prepared from Al 2 O 3 -Cu nitrate mixture exhibited maximum strength of 953 MPa [8].
In general, that the wet methods such as coprecipitation usually produces Al 2 O 3 phase, which would transform to Al 2 O 3 during high temperature sintering.Thus the uniform distribution of Cu would be quite necessary for high mechanical properties.However, the agglomeration of Cu nanoparticles in sol-gel process and ball milling is usually unavoidable [9].Compared to the ball milling and wet sol-gel, the dry CVD technique seems more promising due to its easy operation and low pollution to the environments.Fluidized bed CVD (FBCVD) is one of the most efficient CVD techniques to coat each individual particle of a powder from gaseous species [10].However, the application of FBCVD on powder is limited depending mainly on density and particle size of powder [11,12].The use of a novel rotary CVD (RCVD) technique to prepare Ni nanoparticles on ceramic powders has been reported in our previous work.Ni nanoparticles 13.9-84.5nm and 10-100 nm in diameter were precipitated on the surface of hBN [13] and cBN [14], respectively.The relative density and hardness of hBN were found to be increased by the incorporation of Ni nanoparticles.The hardness of Al 2 O 3 -30 vol% cBN/Ni was 27 GPa, about 1 GPa and 5 GPa higher than that of Al 2 O 3 -30 vol% cBN and monolithic Al 2 O 3 .
In the present study, Cu nanoparticle was precipitated on Al 2 O 3 for uniform mixture by RCVD using copper dipivaloylmethanate (Cu(DPM) 2 ) as a precursor.In order to erase the carbon contamination, the powder was firstly oxidized by oxygen and then reduced by hydrogen into the metal state.

Journal of Nanomaterials
Then the Al 2 O 3 -Cu powders were consolidated by spark plasma sintering at 1373 to 1573 K for 0.6 ks.The effects of Cu nanoparticles on the sintering behavior, microstructure, and mechanical properties of Al 2 O 3 were investigated.

Experimental Procedures
The detailed description about RCVD can be found elsewhere [13].In the present study, Cu nanoparticles were precipitated on Al 2 O 3 powders by RCVD at 573 to 873 K for 30 min using Cu(DPM) 2 as a precursor.The Cu(DPM) 2 precursor in the evaporator was heated at 393 to 423 K and carried into the reaction zone by Ar at a flow rate of 1.67 × 10 −6 m 3 ⋅s −1 .The supply rate (  ) of Cu(DPM) 2 was set at 0.56 × 10 −6 kg⋅s −1 , and O 2 was also filled in to erase the carbon contamination from Cu(DPM) 2 .The Al 2 O 3 powder, <100 nm in average diameter and 2 g in weight, was fed into the reactor and preheated at 793 to 823 K.The total inner pressure of the RCVD apparatus was kept at 800 Pa with a partial pressure of 240 Pa for O 2 and 560 Pa for Ar.The deposition time was fixed at 1.8 ks.After deposition, the O 2 was cut off and H 2 was used to reduce the CuO to Cu.The Al 2 O 3 -Cu powder was consolidated by spark plasma sintering (SPS, model SPS-210LX, SPS Syntex Inc., Japan) at 1373 to 1673 K for 0.6 ks.The heating rate was 3.3 K⋅s −1 and the pressure was 100 MPa loaded in the whole sintering process.The temperature was measured by an optical pyrometer focused on a hole ( 2 × 5 mm) in the graphite die.
The crystal structure and phase of the Al 2 O 3 -Cu powder and the sintered bodies were identified by X-ray diffraction (XRD) with CuK radiation.The microstructure was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The Cu mass content (hereafter,  Cu ) in the Al 2 O 3 -Cu powder was estimated by energy-dispersive X-ray spectroscopy (EDS) and was averaged by five measurements at different areas.The specific surface area, a total surface area per unit of mass, was measured by the BET method and calculated using [15] Here   is the area occupied by a molecule of the adsorbate,  is the Avogadro's number, and   is the molar volume of the analysis gas at standard temperature and pressure (STP).  is the monolayer adsorbed gas quantity, which could be obtained from Here  and  0 are the equilibrium and saturation pressure of adsorbates at the temperature of adsorption,  is the adsorbed gas volume quantity, and  is the BET constant.
The bulk density of the sintered body was determined by an Archimedes' method and converted to relative density using the theoretical density of Al 2 O 3 (3.99 × 10 3 kg⋅m −3 ) [16] and Cu (8.99 × 10 3 kg⋅m −3 ) [17].The hardness of Al 2 O 3 -Cu nanocomposites was measured using Vicker's indentation test under a load of 19.6 N [18] and the fracture toughness was evaluated by measuring the crack length generated by Vicker's indentation [19].Ten points were averaged for each value of hardness and fracture toughness.

Results and Discussion
Figure 1 shows the effects of precipitation temperature (  ) on phase structure of the RCVD-treated Al 2 O 3 powders.At   = 573 K, no Cu peaks were identified due to the low precipitation temperature.At 673 to 873 K, the diffraction peaks at 2 = 43.3∘ (111) and 50.5 ∘ (200) were indexed to Cu, and no CuO and C peaks were observed.On the other hand, the intensity of Cu peaks varied with the precipitating temperature and showed the highest at 773 K. Figure 2 shows the TEM image and EDS spectrum of Cu precipitated Al 2 O 3 powders at 773 K.The EDS spectrum showed clearly the presence of Cu peaks.The Mo peaks were also observed due to the Mo mesh grids used to support the powder for TEM observation.However, it is difficult to distinguish the Cu nanoparticles from Al 2 O 3 by the TEM images due to the small grain size of Al 2 O 3 and Cu.The presence of carbon and oxygen peaks in the EDS spectra was ascribed to the thin organic film coated on the Mo mesh grid to support the powders for TEM observation.for the strongest peak using the diffraction angle of the peak and half width of the peak area of Figure 1.The grain size of Cu increased from 18 to 28 nm with increasing the precipitating temperature from 673 to 873 K.The maximum  Cu reached 5.2 mass% at 773 K. Figure 4 shows the effect of  Cu on the relative density of Al 2 O 3 -Cu nanocomposites.At  SPS = 1373, 1473, and 1573 K, the relative density of Al 2 O 3 was 93.6%, 98.4%, and 97.6%, respectively.The maximum value of 98.4% at  SPS = 1473 K was a little higher than Kim's results at  SPS = 1523 K for 0.3 ks (97%) [7], indicating that RCVD is better in precipitating Cu nanoparticles for the densification of Al 2 O 3 .Hotta and Goto have consolidated Al 2 O 3 by spark plasma sintering from the raw powder of Al 2 O 3 of ∼200 nm in diameter [20].The relative density at  SPS = 1473 K reached above 99%.In our present work, however, the maximum relative density was only 98.4% at  SPS = 1473 K using the Al 2 O 3 of 80 nm in grain size, lower than Hotta's result using the Al 2 O 3 starting powder at the same sintering temperature.This could be attributed to the phase transformation of the maximum value was 97.7% at  Cu = 5.2 mass%.But at  SPS = 1373 and 1573 K, the relative density of  Cu decreased with increasing  Cu .At  SPS = 1573 K and  Cu = 5.2 mass%, the relative density was only 90.6%, about 7% lower than that of monolithic Al 2 O 3 at the same sintering temperature.The decrease of the relative density at  SPS = 1573 K was explained from the microstructural observation of Figure 5.  [21].In the present study, however, although the grain size of Al 2 O 3 was not apparently inhibited, the relative density was increased due to the decrease of the pores.The reason might be that the Cu was in situ coated on the surface of Al 2 O 3 (80 nm in grain size) instead of ball-mill mixing.At  SPS = 1473 and 1573 K, the pores were both observed in Al 2 O 3 and Al 2 O 3 -Cu.By incorporating pores among grains, the grain size increase of Al 2 O 3 -Cu at  SPS = 1573 K induced the lower relative density as shown in Figure 4. On the other hand, at 1373 K, the nanocomposite is less porous than monolithic Al 2 O 3 , due to the filling of the Cu nanoparticles in the pores of bulk Al 2 O 3 .However, the opposite is true at  SPS = 1573 K as Cu might retard the diffusion of the densification process at a high temperature.
Figure 6 shows the effect of  Cu on the Vickers' hardness and fracture toughness at a load of 19.6 N. The maximum hardness was 20.7 ± 1.4 GPa at 2.1 mass% Cu, whereas the maximum fracture toughness was 4.1 ± 0.3 MPa m 1/2 at  Cu = 3.8 mass%.Compared to those values of 19.8 GPa and 3.1 MPa m 1/2 of the monolithic Al 2 O 3 , the hardness and fracture toughness of Al 2 O 3 -Cu nanocomposites were about 4% and 32% higher at 3.8 mass%, respectively.The reason that the hardness and toughness were achieved at different compositions is that the concentration of Cu determined these two quantities differently.The Cu nanoparticle would like to fill the pores of Al 2 O 3 and thus increased the relative density and the hardness of Al 2 O 3 .But the fracture toughness was increased mainly by crack deflecting and bridging.Thus the maximum hardness and toughness were achieved at different compositions as shown in Figure 6.
The incorporation of second particle phases, such as Cu, Cr, and Ni, would often induce the crack deflection and bridging by absorbing the energy and thus increase the fracture toughness related to the indentation crack length [6,7,22].In the present study, the optimum  Cu was 3.8 mass%, with which the crack length is the lowest and then the fracture toughness was calculated to be the largest value according to the method as described in the experimental part [19].On the other hand, the Al 2 O 3 -Cu nanocomposites in [7] has a fracture toughness of 4.5 MPa m 1/2 and 0.4 MPa m 1/2 higher than the value in the present study, but the hardness of the nanocomposites was not mentioned [7].The higher value of the fracture toughness of the nanocomposite in [7] might be due to its lower hardness and the different testing techniques.When the  Cu was higher than 4 mass%, the fracture toughness and hardness of Al 2 O 3 -Cu nanocomposites both decreased, which might be due to the agglomeration of Cu nanoparticles at a higher content.
Figure 7 shows a comparison of the hardness and fracture toughness with Al

Conclusions
Cu nanoparticles were precipitated on Al 2 O 3 by rotary chemical vapor deposition at  SPS = 673 to 873 K for 30 min,

Figure 3
shows the effect of sintering temperature ( SPS ) on the phase structure of Cu precipitated Al 2 O 3 powders at  SPS = 773 K.The phase transformation of Al 2 O 3 to Al 2 O 3 started at  SPS = 1273 K, where three Al 2 O 3 peaks appeared together with Al 2 O 3 and Cu peaks.At  SPS = 1373 K, the Al 2 O 3 transformed completely to Al 2 O 3 .

Figure 4 :
Figure 4: The effect of Cu content on the relative density of Al 2 O 3 -Cu composites at  SPS = 1373, 1473, and 1573 K, respectively.

Figure 5 shows
SEM images of the fracture surface of Al 2 O 3 and Al 2 O 3 -Cu sintered at  SPS = 1373 to 1573 K.At  SPS = 1373 K, some pores were observed in Al 2 O 3 fracture surfaces.In contrast, almost no pores were identified in Al 2 O 3 -Cu at the same sintering temperature, indicating Cu nanoparticles improved the densification of Al 2 O 3 .Adding a small amount of nanoparticles could usually inhibit the grain growth of Al 2 O 3 .For example, Ji and Yeomans found that 5 vol% Cr apparently decreased the grain growth of Al 2 O 3 hot pressed at 1723 K and thus increased the strength and fracture toughness of Al 2 O 3 [6].Zhang et al. also found that Ni could decrease the grain growth of Al 2 O 3

Figure 6 : 2 )
Figure 6: The dependence of Vickers hardness and fracture toughness on the mass content of Cu.

Figure 7 :
Figure 7: Comparison of microhardness and fracture toughness of Al 2 O 3 -3.8mass% Cu nanocomposite with those of the Al 2 O 3 -based composites reported in literature.

Table 1
shows the specific surface area, Cu grain size, and  Cu of Al 2 O 3 and Al 2 O 3 -Cu powders.The specific surface area of monolithic Al 2 O 3 was 49.3 m 2 ⋅g −1 .The addition of Cu decreased the specific surface area of Al 2 O 3 slightly from 41.8 to 45.4 m 2 ⋅g −1 due to the high theoretical density of Cu (kg⋅m −3 ).The crystalline grain size of the Cu nanoparticles was obtained by Scherrer's Formula

Table 1 :
Specific surface area, grain size of Cu and content of Cu in Al 2 O 3 and Al 2 O 3 -Cu powders.
*The crystalline grain size of the Cu nanoparticles was obtained by Scherrer's formula from the XRD patterns.
[26]3 -based nanocomposites in literature mixed by various methods, typically ball milling.The hardness of Al 2 O 3 body, Al 2 O 3 -Nb, and Al 2 O 3 -Nb-CNTs composites ranged from 16 to 22 GPa, with the low fracture toughness of 2.8-3.6 MPa m 1/2 [6, 23].The incorporation of soft phases, such as Ti 3 SiC 2 , would increase the fracture toughness of Al 2 O 3 body with a decrease of hardness [24, 25].Yao et al. incorporated Ni to increase the fracture toughness of Al 2 O 3 to 4.3 MPa m 1/2 , while the hardness was 14.2 GPa[26].It is common that the higher hardness, the lower fracture toughness.The reported values ranged in the hatched area in Figure7.In the present study, the fracture toughness increased to 4.1 MPa m 1/2 , about 40% higher than that of Al 2 O 3 body with the hardness of 20.4 GPa, higher than most of reported values for the Al 2 O 3 body and many Al 2 O 3 -based composites in literature.