DOI: 10.1002/adfm.200700902
By
and
*
With the aim of preparing a high performance conductive ink, we sought to control the surface chemistry of Cu nanoparticles so as to minimize surface oxidation. Specifically, the surface oxide layer on Cu nanoparticles synthesized in ambient atmosphere was minimized by adjusting the molecular weight of poly(N-vinylpyrrolidone) capping molecules, as confirmed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses. In addition, we demonstrate that by minimizing the thickness of the surface oxide layer, Cu granular films with good conductivity could be obtained by sintering nanoparticle assembles. Finally, we fabricated highly conductive Cu patterns on a plastic substrate by ink-jet printing.
In recent years, increasing attention has been devoted to the development of convenient and low-cost processing techniques to fabricate conductive features such as electrodes, conductive lines, and interconnects for use in various electronic applications. Although photolithography has been widely adopted in the microfabrication of conductive features, it is time consuming, complicated, and expensive. Therefore, there is a need for direct digital printing techniques that are simpler and less expensive. In this context, ink jet printing is a promising alternative to traditional lithography.
[1–8]
[ * ] Prof. J. Moon, Dr. S. Jeong, K. Woo, D. Kim
Department of Materials Science and Engineering, Yonsei University
Seoul 120-749, (Korea)
E-mail: jmoon@yonsei.ac.kr
Dr. S. Lim, Dr. J. S. Kim
LCD R&D Center, Samsung Electronics Co. LTD
Gyeonggi-Do 449-711 (Korea)
Prof. H. Shin
School of Advanced Materials Engineering, Kookmin University
Seoul 136-702 (Korea)
Prof. Y. Xia [+]
Department of Chemistry, University of Washington
Seattle, WA 98195-1700 (USA)
[+] Present address: Department of Biomedical Engineering, Washington
University, St. Louis, MO 63130-4899, USA.
[ ** ] This work was supported by the Korea Science and Engineering
Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Science and Technology (No.
R0A-2005-000-10011-0 and R0A-2007-000-20105-0). It was also partially supported by the Second Stage of Brain Korea 21 Project in
2007.
Most previous works on the fabrication of conductive patterns using ink-jet printing technology have focused primarily on doped conjugated polymers such as polyanilines,
[9] polypyrroles,
[10] and PEDOT;
[11] however, these materials are characterized by low electrical conductivity
(typically 10
2
S cm
1
) and poor electrical and thermal stabilities. On the other hand, metals that possess high conductivity (typically 10
5
S cm
1
) and operational stability can be applied via ink-jet printing in the form of nanoparticles in conductive inks. The metal nanoparticles contained in the printed pattern are then converted to conductive granular metal films via post-printing thermal annealing at much lower temperatures than the melting points of the corresponding bulk metals. To date, noble metals such as gold and silver have mainly been utilized in printing highly conductive elements in electronic devices, but the high cost of these metals has hindered the use of this appealing approach.
In this regard, copper is a good alternative material as it is highly conductive but significantly cheaper than Au and Ag.
Several methods have been developed for the preparation of copper nanoparticles, including thermal reduction,
[12] sonochemical reduction,
[13] sion techniques.
[15–17] chemical reduction,
[14] and microemul-
In our previous study, we synthesized Cu nanoparticles stabilized with poly( N -vinylpyrrolidone) and fabricated conductive lines using ink-jet printing method.
[6]
However, the surface oxide layer on the Cu particles was not controlled, which resulted in a high resistive conductive film.
When Cu nanoparticles are synthesized on a large scale in ambient atmosphere, the formation of a surface oxide layer on the Cu is inevitable because the oxide phases are thermodynamically more stable. The presence of copper oxides on the surfaces of nanoparticles has two negative consequences: it increases the annealing temperature required, and reduces the electrical conductivity. Here we present an approach that reduces surface oxidation during Cu nanoparticle synthesis
Adv. Funct. Mater.
2008 , 18 , 679–686 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim 679

S. Jeong et al. / Ink-Jet Printed Conductive Structures
Table 1.
The conductivity of Cu granular films, prepared from different Cu nanoparticles of varying particle size and surface oxide layer thickness, as a function of heat-treatment temperature. The resistivity of bulk copper is 1.7
mV cm. The exothermic peak positions, indicating the onset of sintering, also depend on the particle size.
Particle size (nm)
Oxide layer thickness (nm)
Resistivity at 275 8 C ( mV cm)
Resistivity at 325 8 C ( mV cm)
Exothermic Peak ( 8 C)
10000
35 5
3.1
92
34
173
PVP molecular weight [g/mol]
29000
45 8
2.5
27.8
17.2
165
40000
60 8
1.6
11.6
11.5
158 through the use of polymeric capping agents, and show that the thickness of the oxide layer depends on the molecular weight of the polymer used. We demonstrate that the use of Cu nanoparticles with reduced surface oxidation improves the electrical properties of granular films formed by annealing such nanoparticles. In addition, we show that complex conductive patterns can be successfully ink-jet printed on plastic substrates using an ink based on Cu nanoparticles.
Copper nanoparticles 35–60 nm in size were synthesized in ambient atmosphere by the polyol method. Poly( N vinylpyrrolidone) (PVP, M w
¼ 10000, 29000, and 40000 g mol 1 ) was used as a capping agent to hinder surface oxidation and diethylene glycol (DEG) was utilized as a reaction medium. Polyol synthesis at 140 8 C for 1 h gave rise to relatively uniform PVP-coated Cu nanoparticles. The average particle size increased with PVP molecular weight, as summarized in Table 1. It was reported that higher molecular weight PVP in the polyol synthesis of metal nanoparticles usually results in larger particle size.
[18] Microstructural observation by high resolution transmission electron microscopy (HRTEM) combined with X-ray photoelectron spectroscopy (XPS) analysis suggested that the surface of each copper nanoparticle was covered with amorphous copper oxide on which PVP was adsorbed. However, X-ray diffraction analysis showed only features associated with Cu metal.
[19] Surface oxidation occurs via reactions between Cu and oxygen atoms originating from either O
2 or H
2
O. The copper oxide phase formed under the reaction conditions will be CuO, which preferentially forms under ambient atmosphere below 600 8 C as predicted by a calculated thermodynamic phase diagram
(Fig. 1a).
The HRTEM images of the synthesized Cu nanoparticles
(Fig. 1b) show that regardless of the PVP molecular weight, an amorphous layer is present on the surface of each nanoparticle.
It is interesting that the thickness of the surface amorphous layer decreases with increasing PVP molecular weight, from an average of 3.1 nm to 1.6 nm as the PVP molecular weight is increased from 10000 to 40000 g mol
1
(see Table 1). Given that the lengths of the PVP chains increase with molecular weight, and that PVP is generally invisible under an electron microscope, we consider that the surface amorphous layer
Figure 1.
a) Thermodynamic phase transition behavior from Cu to Cu
2
O or CuO as a function of oxygen partial pressure and temperature; b) HRTEM image of Cu nanoparticles synthesized using PVP of molecular weight 10000 g mol 1 , 29000 g mol 1 , or 40000 g mol 1 . The dotted lines indicate the borders of the oxide layer. The oxide layer thicknesses of the Cu nanoparticles were measured to be 3.1, 2.5, and 1.6 nm, respectively. The synthetic conditions, other than the capping polymer molecular weight, were identical for the three types of Cu nanoparticles. Scale bar ¼ 5 nm.
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S. Jeong et al./ Ink-Jet Printed Conductive Structures
Figure 2.
XPS spectra of Cu nanoparticles synthesized using capping molecules with different molecular weights: a) Cu 2p
3/2 nanoparticles synthesized using PVP with a molecular weight of 10000 g mol
1
, 29000 g mol
1
, or 40000 g mol and b) O 1s of Cu
1
. The Au was deposited onto the Cu nanoparticles as a reference material for calibrating the binding energy shift.
solely represents the surface layer of CuO. Prior to the nucleation and growth of the nanoparticles, the Cu ions dissolved in the polyol medium bond to the PVP; specifically lone pairs of electrons on the oxygen atoms in the polar groups of the PVP mer unit are donated to an sp hybrid orbital of the Cu
2 þ ions to form a Cu–O coordination.
[20–22]
This bonding of the Cu ions to the PVP reduces the susceptibility of the Cu ions to oxidation during the particle synthesis and prevents particle agglomeration; both of these functions depend on the molecular weight of the PVP.
To gain an in-depth understanding of the interaction between Cu and PVP, we used XPS to analyze the surfaces of Cu nanoparticles synthesized with PVP of three different molecular weights (Fig. 2); the detailed peak assignments are
Table 2.
Binding energy values of Cu 2p
3/2 and O 1s for Cu nanoparticles synthesized with PVP of different molecular weight.
PVP molecular weight [g/mol]
10000
29000
40000
Cu 2p
3/2 in Cu
932.1
932.1
932.0
Cu 2p
3/2 in CuO
934.2
934.3
934.3
Binding Energy [eV]
O 1s in PVP (Non-interacting)
529.7
529.7
–
O 1s in PVP (Interacting)
531.2
531.2
531.2
O 1s in DEG
533.3
533.3
533.2
Adv. Funct. Mater.
2008 , 18 , 679–686 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim www.afm-journal.de
681

S. Jeong et al. / Ink-Jet Printed Conductive Structures
Figure 3.
Schematic diagram representing the different conformations of the capping molecules chemisorbed on the surfaces of Cu nanoparticles synthesized using a) lower molecular weight PVP and b) higher molecular weight PVP. Also shown is the microstructural evolution of each particle type during sintering.
listed in Table 2. XPS spectral analyses in the Cu 2p
3/2 region confirm that the surface oxide layer becomes thinner as the
PVP molecular weight increases. The peak area around
934.3 eV resulting from CuO decreases with increasing PVP molecular weight. The peak area ratios of CuO to (Cu þ CuO) obtained from the Cu nanoparticles synthesized using PVP with molecular weights of 10000, 20000, and 40000 g mol
1 are
0.47, 0.40, and 0.27, respectively. On the other hand, the Cu 2p
3/
2 binding energies for Cu and CuO are shifted towards lower energy in all three samples of Cu nanoparticles compared to those of pure Cu and CuO, respectively. (The Cu 2p
3/2 binding energies for pure Cu and CuO are 932.6 and 934.6 eV, respectively.) The strong interaction between an oxygen atom on the PVP and the Cu core induces an image dipole, resulting in partial positive charges on the surface of the Cu particle.
This phenomenon causes a lowering of the Cu 2p
3/2 binding energy and an upward shift of the O 1s binding energy.
[23–25]
However, the binding energy shifts (i.e., binding energy values) of the Cu 2p
3/2 and O 1s peaks were the same for all of the Cu nanoparticles, regardless of the PVP molecular weight.
This indicates that the electrostatic interaction between the
Cu core and PVP is not affected by the PVP molecular weight, and is not a factor determining the surface oxide layer thickness.
The trend in the O 1s XPS peaks as a function of PVP molecular weight is quite different from what was observed for the Cu 2p
3/2 peaks (Fig. 2b). Curve fitting deconvolutes the O
1s region into three peaks at 529.7, 531.2, and 533.3 eV; these repeating units that are not interacting with the Cu core, carboxyl oxygen atoms of the PVP repeating units that are interacting with the Cu core, and hydroxyl oxygen atoms
(C–OH) of the DEG solvent molecules that are hydrogenbonded to PVP, respectively. The binding energy of each of the three peaks remains the same in spite of variations in the PVP molecular weight. However, the peak intensity at
529.7 eV increases with PVP molecular weight, indicating that the fraction of PVP repeating units that are not interacting with the Cu core (i.e., unattached, free repeating units) increases with decreasing PVP molecular weight. The fraction of PVP non-interacting with Cu is calculated using the peak area ratio of PVP non-interacting to total adsorbed PVP on Cu from XPS spectra, which are 0.202, 0.045, and 0 for the molecular weight of 10000, 20000, and 40000 g mol
1
, respectively. The amount of PVP repeating units that are not interacting with each Cu particle can be determined by surface area, adsorption density, and fraction of PVP non-interacting with Cu. The particle size varies from 35 to 60 nm with increasing the molecular weight of
PVP. Even taken into consideration the differences in the corresponding surface area of the particle, however, the amount of PVP repeating units that are not interacting with each Cu particle increases with decreasing its molecular weight since the difference in surface area is insignificant and the fraction of PVP non-interacting is a dominant factor. This finding suggests that the capping molecules on the Cu surface take on different conformations depending on the PVP molecular weight, as depicted in Figure 3. The longer PVP chains are more effectively adsorbed on the Cu surface compared to shorter chains. In other words, the density of sites at which PVP attaches to the Cu surface increases with
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S. Jeong et al./ Ink-Jet Printed Conductive Structures
Figure 4.
Resistivity of the Cu conductive film as a function of heattreatment temperature. The ink-jet printed Cu nanoparticulate films were heat-treated at various temperatures between 200 8 C and 325 8 C under vacuum (10
3
Torr).
increasing PVP chain length. Hence PVP with a higher molecular weight is more densely packed on the Cu surface
(Fig. 3b), whereas PVP with a lower molecular weight is loosely packed on the Cu surface due to a lower density of attachment sites (Fig. 3a). These findings suggest that a densely packed layer of adsorbed PVP on the Cu surface can hinder access of the oxygen atoms to the metal during the particle synthesis, resulting in reduced surface oxidation and thus to a thinner oxide layer. The lack of the O 1s peak at 529.7 eV in the XPS spectrum of the Cu nanoparticles synthesized using PVP with a molecular weight of 40000 g mol
1 may suggest that all of the
PVP repeating units are attached to the Cu surface. If this were the case, it would indicate that the surface oxide layer of thickness 1.6 nm is the minimum that can be achieved using PVP capping molecules. In support of this idea, Cu nanoparticles synthesized using PVP with an even higher molecular weight, 55000 g mol
1
, exhibited a surface thickness and XPS peak area ratio similar to those obtained with the
40000 g mol 1 PVP.
The thickness of surface oxide layer on individual Cu nanoparticles significantly influences the microstructure and physical properties of the resulting consolidated particulate film. In the present work, we monitored the evolution of the conductivity of Cu granular films prepared from Cu nanoparticles with different surface oxide layer thicknesses as a function of heat-treatment temperature; the results are listed in
Figure 4 and Table 1. For each annealing temperature, the resistivity of the film prepared using Cu nanoparticles with a higher molecular weight PVP is lower than that of the film prepared using Cu nanoparticles with a lower molecular weight
PVP. The development of a more conductive film when Cu particles prepared using a higher molecular weight PVP are annealed at the same temperature as those prepared using a lower molecular weight PVP indicates that the particles with a thinner surface oxide layer undergo coalescence to a greater extent during a given annealing time. The amount and type of
PVP chemisorbed on the Cu particles could potentially affect the sintering process by interrupting the migration of Cu atoms
Figure 5.
a) Thermal gravimetric analysis curves of Cu nanoparticles synthesized using PVP with different molecular weights. The weight loss below 200 8 C is attributed to the thermal decomposition of adsorbed water and chemisorbed solvent molecules, and the significant weight loss above 200 8 C is assigned to the thermal decomposition of PVP. The amount of adsorbed PVP was calculated by weight loss above 200 8 C; b) SEM image showing the microstructure of the Cu film heat-treated at 325 8 C. The insets show high-magnification SEM images of the as-synthesized Cu nanoparticles prior to sintering. All scale bars ¼ 500 nm; c) DSC analysis of the Cu nanoparticles synthesized using different molecular weights of PVP.
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2008 , 18 , 679–686 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim www.afm-journal.de
683

S. Jeong et al. / Ink-Jet Printed Conductive Structures
Figure 6.
a) Cu conductive patterns ink-jet printed on polyimide substrates using Cu nanoparticles with the thinnest oxide layer; b) confocal images of a single ink droplet after drying and a single printed line.
into neck regions. However, thermal gravimetric analysis revealed that the amounts of adsorbed PVP were nearly identical for the three types of Cu nanoparticles ( 1.5 wt %), and that thermal decomposition of the PVP commenced at around 200 8 C regardless of the polymer molecular weight
(Fig. 5a); these findings indicate that the differences in the sintering behavior are not due to variations in the amount and type of PVP chemisorbed on the Cu nanoparticles.
The distinct sintering kinetics of the Cu nanoparticles with different surface oxide layer thicknesses can be attributed to the characteristics of the particles, such as their size and melting point ( T m
). Smaller particles are expected to show faster sintering kinetics due to a reduced mass diffusion length.
However, the experimental results are not in line with this expectation, as shown in Table 1. Specifically, the Cu nanoparticles prepared using a higher molecular weight
PVP (i.e., a thinner surface layer) have the largest particle size ( 60 nm) but the fastest sintering kinetics. This finding leads us to conclude that the observed differences in the sintering kinetics are associated with variations in the thickness of surface oxide layer rather than the particle size. It is generally recognized that the sintering process occurs at a temperature that is around 80% of the melting point. The melting point of bulk CuO ( T m
1330 8 C) is higher than that of bulk Cu ( T m
830 8 C). The scanning electron microscopy
(SEM) microstructural analysis results (Fig. 5b) indicate that, compared to the Cu film prepared from Cu nanoparticles with a thicker surface oxide layer, the Cu film prepared from nanoparticles with a thinner surface oxide layer is much denser, forming more particle–particle junctions when heat-treated at the same temperature. This enhanced sintering process is also supported by exothermic peaks in the differential scanning calorimetry (DSC) analysis (Fig. 5c and Table 1). The formation of necks between particles involves surface diffusion of unstable surface atoms, which is observed as the exothermic peaks.
[4,26]
The exothermic peak position shifts toward lower temperature as the molecular weight of PVP increases (i.e., the surface oxide layer becomes thinner), indicating that the sintering process for the Cu nanoparticle with a thinner oxide layer is initiated at a lower heat-treatment temperature.
The thickness of surface oxide layer thus plays a critical role in determining the resistivity of the Cu granular film, predominating over the particle size effect.
To demonstrate the applicability of the Cu nanoparticles with controlled surface chemistry, we prepared the ink-jet printable conductive inks. Figure 6 shows complex conductive patterns printed on flexible polyimide substrate using the Cu nanoparticles prepared with PVP of 40000 g mol
1 in molecular weight. Solvent evaporation from the printed single ink droplet produces a spherical dot pattern of diameter
120 m m. The line pattern is generated by reducing the dot-to-dot distance. The separated dots begin to merge together at a distance of 80 m m, and printing at an interdot distance of 60 m m results in a continuous line with a 130 m m linewidth and relatively smooth edge definition. To our knowledge, this represents the first attempt to directly write a conductive Cu pattern on a plastic substrate via ink-jet printing of an ink containing Cu nanoparticles. The conductive features exhibited relatively low electrical resistivity ( 11 mV cm) after heat-treatment at 275 8 C for 60 min, as shown in
Table 1. Thus, the Cu nanoparticle-based conductive ink used here provides a convenient and low-cost method for fabricating
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S. Jeong et al./ Ink-Jet Printed Conductive Structures conductive features. Especially, the ability to directly write conductive Cu patterns provides new opportunities for the fabrication of flexible printed electronics such as organic light emitting diodes, organic thin-film transistors, and solar cells.
We have controlled the thickness of the surface oxide layer on Cu nanoparticles by varying the molecular weight of
PVP added to the reaction mixture used to generate the nanoparticles. The minimum thickness of the amorphous CuO and chemisorbed PVP layers was determined to be 1.6 nm, and was obtained using PVP with a molecular weight of 40000 g mol
1 or higher. The molecular weight of the PVP capping agent determines their conformation when chemisorbed on the
Cu surface, which significantly influences the formation of the surface oxide layer. In addition, our results confirm that the surface oxide layer thickness is the predominant factor determining the electrical conductivity of the Cu nanoparticle granular film obtained by annealing nanoparticle assembles.
Importantly, we established that the synthesized Cu nanoparticles with controlled surface chemistry could be used to directly write conductive features on a plastic substrate.
Copper nanoparticles 35–60 nm in size were synthesized in ambient atmosphere by the polyol method. Details of the synthesis procedure were described previously.
29000, and 40000 g mol
[19]
Poly( N -vinylpyrrolidone) ( M w
¼ 10000,
1
, Sigma–Aldrich), acting as a capping molecule to hinder the formation of an oxide layer, was dissolved in diethyleneglycol (99 %, Sigma–Aldrich). Sodium phosphinate monohydrate (NaH
2
PO
2
H
2
O; Junsei), used as a reducing agent, was added to the DEG solution and the solution was heated to a reaction temperature of 140 8 C. An aqueous solution of copper (II) sulfate pentahydrate (98%; Sigma–Aldrich) was then injected into the hot reaction medium via a syringe pump at a rate of 2 ml/min. In the synthesis step, the volume ratio of DEG to water was 7.3:1. After 1 h of reaction, the solution was cooled to room temperature and the particles were separated by centrifugation and then washed with methanol.
Conductive ink incorporating the synthesized Cu nanoparticles was prepared using a mixed solvent comprised of ethylene glycol (EG),
2-methoxyethanol (MeEtOH), and methyl alcohol (MeOH). The weight ratio of EG:MeEtOH:MeOH was 5:4:1. The copper nanoparticles were dispersed in the premixed solvent, and the mixture was subjected to ball milling for 12 h. The resulting ink was filtered through
5 m m nylon mesh prior to use. The solid loading of the ink was 20 wt %.
The Cu conductive ink was printed onto a polyimide substrate
(Kapton, Dupont) in ambient atmosphere using an ink-jet printer. The printer set up consisted of a drop-on-demand piezoelectric ink-jet nozzle manufactured by Microfab Technologies, Inc. (Plano, TX) and the diameter of the nozzle was 30 m m. Uniform ejection of the droplets was achieved by applying a 25 V pulse lasting 10 m s at a frequency of
400 Hz. The diameter and velocity of the ejected droplets were about
50 m m and 3 m s
1
, respectively. The ink-jet printed Cu nanoparticulate films were heat-treated at various temperatures between 200 8 C and
325 8 C under vacuum (10
3
Torr) for 60 min to form interparticular connections for developing electrical conductivity.
The surface oxide layer on the Cu particles was observed by high resolution transmission electron microscopy (JEM-4010, JEOL) and
X-ray photoelectron spectroscopy (ESCALAB 22i-XL, VG Scientific
Instrument). Differential scanning calorimetry (STA409C, Netzsch) was used for thermal profile measurement, and the thermal decomposition behavior of the PVP bonded on the particles was monitored using thermal gravimetric analysis (STA409C, Netzsch).
Thermal analysis was conducted under inert gas (Ar) atmosphere. The sheet resistivities of the Cu printed films as a function of annealing temperature were measured in air by a 4-point probe (CMT-SR200N,
Chang Min Co., Ltd.), and the film thickness was measured using a surface profiler (AS500, KLA-Tencor Co.). The size of printed pad for measuring the resistivity was 1 1 cm
2
. The three-dimensional morphology and microstructure of the ink-jet printed Cu conductive patterns were observed by confocal microscopy (LEXT OLS3000,
Olympus) and scanning electron microscopy (JSM-6500F, JEOL), respectively. The particle size was evaluated by both laser light scattering and image analysis based on the SEM image.
Phase diagram for copper oxides can be constructed by thermodynamic calculations starting from the equilibrium among Cu
2
O, Cu, and O
2
:
4Cu ð s Þ þ O
2
ð g Þ !
2Cu
2
O ð s Þ ð 1 Þ where (s) indicates a solid phase and (g) is a gas phase. The standard Gibbs free energy for this reaction is represented as follows:
D G o ¼ RT ln K a
¼ RT ln a 2
Cu
2
O a 4
Cu
P
O
2
ð 2 Þ
D G o ¼ 339 ; 000 14 : 24 T ln T þ 247 T ð 3 Þ where D G o is expressed in joules.
The existence of another copper oxide, cupric oxide (CuO) should be also considered. The reaction for the formation of cupric oxide from Cu
2
O and O
2 can be written as follows:
2Cu
2
O ð s Þ þ O
2
ð g Þ !
4CuO ð s Þ ð 4 Þ
The standard Gibbs free energy for above reaction is expressed as follows:
D G o ¼ RT ln K a
¼ RT ln a 4
CuO a 2
Cu
2
O
P
O
2
D G o ¼ 292 ; 460 22 : 16 T ln T þ 370 : 7 T
ð 5 Þ
ð 6 Þ
We can calculate the oxygen pressures for the equilibrium between the two copper compounds. When both the phase diagrams are plotted on the same axes, we construct a diagram illustrating the stability ranges for the oxides of copper and metallic copper as a function of the temperature and the oxygen pressure.
Received: August 8, 2007
Revised: October 25, 2007
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