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Preparation and Characterization of Graphite Powder Covered with CeO2
JOURNAL OF RARE EARTHS Vo1.24, Spec. Issue, Dec. 2006, p.57
Preparation and Characterization of Graphite Powder Covered with CeO, Dong Zhiguo (fa& El ) , Yao Guangchun (?d’bf &) * , Liu Yihan ( i t d & K ) , Luo Tiaojiao ( Li Hongbin ($&&) ( School of Material & Metallurgy , Northeastern University, Shenyang 110004 , China )
A%),
Abstract: In order to improve the wetting properties of graphite with A1 melt and reduce the oxidation of the graphite, by which the segregation of components during the liquid-stir-casting process could be prevented. In this paper, a uniform thin nano-film of CeOZ, about 20 nm thick, was successfully prepared onto graphite powder surface by heterogeneous nucleation process. The results show that an obvious chemical reaction did exit between CeOz film and graphite with the formation of ce-0-C bond, leading to a shift of the binding energy of C and c e . The cover with CeOz film illustrates a distinct change of surface state of graphite with a decrease of angle of contact.
Key words : CeOz; graphite ; covered ; composite ; rare earths CLC number: TB383 ; TF123.2 5 Document code : A +
Metal-matrix composites ( MMCS) reinforced with graphite for their good performances, such as high specific strength, high specific modulus, and high ductility high wear resistance etc. have the most potential application as structural and functional materials in the fields of aviation, astronavigation , biological materials and civilian industry in the future“s21. Liquid fabrication techniques of MMCs have advantages over other techniques ( i . e . , powder metallurgy), some of these advantages are: ease of production of components with complex shapes, flexibility and lower cost, so the liquid fabrication technique is concerned very much by many r e ~ e a r c h e r s ‘ ~ - ~But ’. during the preparation of carbon fiber reinforced a h minum alloy matrix composites by liquid-stir-casting , the lamination of carbon fibers and high porosity would appear in the aluminum alloy matrix composites, which is very harmful to increasing mechanical properties of composites. This could be attributed to some disadvantages of graphite, such as lower density, lamination, and its hydrophobic nature, resulting in the segregation of components during the liquid-stir-casting process. Refs. [ 8 , 9 ] show that, in aluminum alloy, some rare earth can refine metal grain, decrease the segregation of components, lower the porosity and reduce the content of impurities, with a certain content, which can improve mechanical properties, physical properties, process ability and integrated service properties.
Article ID: 1002 - 0721(2006) - 0057 - 04
So in this study, in order to improve the wetting properties of graphite with A1 melt then reduce the oxidation of the graphite, a thin film of CeOz was covered onto the graphite successfully by heterogeneous nucleation process to prepare the composite powder. The SEM, IR, XRD and XPS were used to character the surface characteristic of the composite powder.
1 Experimental The crude scale-like graphite employed in this study was provided by Shandong Nanshu Company. The graphite powder was treated by HF for 12 h then rinsed with distilled water. Then the powder was immersed in a boiled solution of H2S04 and HN03 mixture for 1 h . Cerium sol were prepared by dissolving Ce( N 0 3 ) 3
*6H20in distilled water and adding an appropriate of citric acid, heating to 70 “c with constant stirring. During the mixing process of graphite powder with the solution, some surface active reagent was added into the sol. The sol was dried in vacuum at 135 ‘T to form gel. And the pH should be controlled about 5.0 by a diluted ammonia solution. After filtration, the treated graphite powder was dried under 250 “c in muffle. X-ray powder diffraction ( XRD ) patterns were recorded at a scanning rate of 0.1 s with a Japan Riga DMA-IIIC X-ray diffractometer using monochromatic high-intensity Co Ka radiation (0.17889 nm) .
Received date: 2006 - 06 - 26; revised date: 2006 - 09 - 16 Foundation item: Project supported by the National ‘973’ Project (619990649) Biography: Dong Zhiguo ( I977 - 1, Male, Doctoral candidate * Corresponding author (E-mail : gcyao@ m i l . neu .edu .cn )
JOURNAL OF RARE EARTHS, Vol. 2 4 , spec. Issue, Dec . 2006
58
The chemical compositions of composite films were measured by the monochromatic XPS (VG Scientific, ESCARLAB 220-IXL) using an A1 Ka X-ray with energy 1486.6 eV and the take-off angle of 90". Narrow scan spectra of C l s , O l s , and Ce 3d were recorded with 20 eV pass energy for the quantification of composition.
2 Results and Discussion 2.1 SEM character of CeOJgraphite composite powder Though there showed no obvious changes of graphite powder after treatment, a uniform layer of Ce02 grain was observed (Fig. 1) . The sediment of Ce02 was controlled by the rate of the reaction though the concentration of Ce ( N 0 3 ) 3 , pH and temperature are shown influence on the sediment process. A lower rate of the reaction was needed to obtain the uniform thin film on graphite surface. The reaction of the CeOz covered on graphite can be denoted with the equation : Ce( N 0 3 ) 3+ H202+ C+HN03 + CeOJC + H20 It is well known that the metal hydroxide can be precipitated from solution by heterogeneous nucleation"O1. To form a coating using heterogeneous nucleation, the pH of suspension of the particles to be coated is controlled at a certain regime with a fit pH solution to control precipitation of the coating materials on the surface of coated particles by heterogeneous nucleation. Fig. 2 shows a schematic of the nucleation and growth behavior of a precipitate from solution with increasing solution pH["]. As the pH of suspension rises, the solution becomes more saturated. The ideal path would edge over the heterogeneous nucleation limit without touching the homogeneous nucleation regime, the coating materials will precipitate by heterogeneous nucleation and not by homogeneous nucleation. Moreover, the thickness of coating layer can be
Schematic of nucleation and growth behavior of a precipitate from solution with increasing solution pH
Fig.2
controlled by controlling the amount (concentration) of coating materials.
2.2 XRD analysis Fig. 3 is XRD patterns of cerium oxide coated graphite powder. An intense Ce02(111) reflection was present at 28.68' in all scans, indicating that Ce02 (111) was the majority plane. Other major peaks appear at 47.68' and 56.58' for all samples and these peaks could be indexed as Ce02(220), CeO, ( 3 11 ) . But some amorphous character was observed with some broader peak due to the nano-effect of the CeOz film. It suggested that the structure of graphite was intact because peak at 26. 12" was the strongest one. The CeOz exhibits a preferred ( 111) orientation. The diffraction peaks of ( l l l ) , (200), ( 2 2 0 ) , (311) correspond to the cubic Ce02 phase. Using Schemer formula ( D = a/\ //?cosO) , the size of the grain in Ce02 film was about 21.6 nm.
2 . 3 XPS analysis Two peaks could be accepted after peak analysis of element C ( F i g . 4 ) . The left one is 284.11 eV and the right one is 285.47 eV . An obvious chemical reaction did exit between CeOz film and graphite because
CeO, ( 1 1 1 )
:/coo2(220)
Ce0,(311)
ri_ I
.
,
.
,
.
,
M
3
0
4
0
5
l
2
Fig. 1 SEM graph of CeOJgraphite composite powder
Fig.3
8
11
J(
0
7
0
8
0
9
')
XRD patterns of CeOz films on graphite
0
Dong 2 G et a1 .
Preparation and Characterkzion of Graphite Powder Covered with CeO,
a 1.37 eV increase. After bombardment of argon ion for 20 min, only slight elevation of C peak was observed, which showed that the thickness of the film was about 20 nm. An obvious shoulder-peak was observed in the XPS spectra of 01s , and the first one was 529.88 eV , which was Corresponding to the 01s in G O 2 , and the second was 532.13 eV , which was corresponding to the adsorbed 01s on the graphite surface (Fig. 5 ) . Both the two value of binding energy are higher than the standard value. This could be attributed to the formation of Ce - 0 - C bond which leading to a shift of the binding energy due to the difference in the electron attractability of C and Ce[l2].
Fig. 4
59
ed. But to the graphite powder covered with CeOn. a strongly hydrophilic bond of Ce-0 of the film led to a higher energy surface, so a decrease of the angle was illustrated.
Binding enerey/eV XPS pattern of element C in CeO, coated graphite Fig.6 II-
Angle of contact of H,O/graphite ( a ) and HZO/CeO2/ graphite (b)
100
-
3 Conclusion
X-
A uniform thin nano-film of Ce02, about 20 nm
7-
in thickness , was successfully prepared onto graphite powder surface by heterogeneous nucleation process . The graphite covered with CeO, film shows a distinct change of surface state of graphite with a decrease of angle of contact.
6-
5-
5-0
sis
5i0
sjs
54,
545
Binding cncrjp3':cV
Fig.5
XPS pattern of element C in CeO, coated oxide
2.4 Angle of contact analysis A distinct change in angle of contact measurement was illustrated in Fig. 6. After being covered by CeOz, the angle of contact decreased from 90" to 30", It is suggested that the film of Ce02 on the surface of graphite was uniform and improved the wetting character of the graphite. The decrease of the angle of contact can be interpreted by the change of character and the arrangement of the surface atom or atom cluster. As to the graphite, an unpolarized hexangular C net led to a lower energy surface so a hydrophobic character was illustrat-
References : [ 1] Ju C P , Chen K I , Lin J H.
Process, microstructure and properties of squeeze-cast short-carbon-fibre reinforced aluminium-matrix composites [ J ] . Journal of Materials Science, 1994, 29 : 5127.
[ 21 Wu R J . The present condition and prospects on metal matrix composites [ J 1. Aeronautical Manufacturing Technology, 2001, 3: 19. [ 31 Evans A G , Hutchinson J W , NcNeejubg R N . Stressstrain behavior of matrix composites with discontinuous reinforcements [ J ] . Scripta Metallurgica, 1991, ( 2 5 ) :
3. [4] Latanish R M . Corrosion science corrosion engineeringan advanced technologies [J ] . Corrosion Science, 1995, 51(4): 270.
60
JOURNAL OF RARE EARTHS, Vol. 2 4 , Spec. Issue, Dee. 2006
[5] Eliosson J , Somclstrom R . Applications of aluminum matrix composites [ J 1 . Key Engineering Materials, 1995, (13) : 104. [6] Martin A , Martinez M A , Llorca J. Wear of Sic reinforced AI-matrix composites in the temperature range 20-2OOOC [ J ] . Wear, 1996, 193: 169. [ 71 Lee C S. Wear behavior of aluminum matrix composite materials [J] . Mater . Sci . , 1992, 27: 793. [8] Wang S H , Zhou H P , Kang Y P . The influence of rare earth elements on microstmctures and properties of 6061 aluminum alloy vacuum-brazed joints [ J ] . Journal of Alloys and Compounds, 2003, (352) : 79. [9] Sun Yangshan, Yao Zhengjun, Zhang Zhonghua, et al. Mechanical properties of F q Al-based alloys with cerium
[ LO]
[ 111
[ 121
[ 131
addition [ J ] . Scripta Metallurgica et Materiala, 1995, 33(5): 811. Blendell J E , Bowen H K , Coble R L. High purity alumina by controlled precipitation from aluminum sulfate solutions [J]. J . Am. Ceram. SOC.Bull., 1984, (63): 1797. B a n g Juxian , Gao Longqiao . Nanocomposite powders from coating with heterogeneous nucleation processing [ J ] . Ceramics International, 2001, 27 : 143. De Keijser Th H . , Langford J L, Miuerneijer E . Appl. Cryst., 1982, 15: 308. Turner M R , Duguet L C . Suf. Interface. Anal. , 1997, 25: 917.
Preparation and Characterization of Graphite Powder Covered with CeO, Dong Zhiguo (fa& El ) , Yao Guangchun (?d’bf &) * , Liu Yihan ( i t d & K ) , Luo Tiaojiao ( Li Hongbin ($&&) ( School of Material & Metallurgy , Northeastern University, Shenyang 110004 , China )
A%),
Abstract: In order to improve the wetting properties of graphite with A1 melt and reduce the oxidation of the graphite, by which the segregation of components during the liquid-stir-casting process could be prevented. In this paper, a uniform thin nano-film of CeOZ, about 20 nm thick, was successfully prepared onto graphite powder surface by heterogeneous nucleation process. The results show that an obvious chemical reaction did exit between CeOz film and graphite with the formation of ce-0-C bond, leading to a shift of the binding energy of C and c e . The cover with CeOz film illustrates a distinct change of surface state of graphite with a decrease of angle of contact.
Key words : CeOz; graphite ; covered ; composite ; rare earths CLC number: TB383 ; TF123.2 5 Document code : A +
Metal-matrix composites ( MMCS) reinforced with graphite for their good performances, such as high specific strength, high specific modulus, and high ductility high wear resistance etc. have the most potential application as structural and functional materials in the fields of aviation, astronavigation , biological materials and civilian industry in the future“s21. Liquid fabrication techniques of MMCs have advantages over other techniques ( i . e . , powder metallurgy), some of these advantages are: ease of production of components with complex shapes, flexibility and lower cost, so the liquid fabrication technique is concerned very much by many r e ~ e a r c h e r s ‘ ~ - ~But ’. during the preparation of carbon fiber reinforced a h minum alloy matrix composites by liquid-stir-casting , the lamination of carbon fibers and high porosity would appear in the aluminum alloy matrix composites, which is very harmful to increasing mechanical properties of composites. This could be attributed to some disadvantages of graphite, such as lower density, lamination, and its hydrophobic nature, resulting in the segregation of components during the liquid-stir-casting process. Refs. [ 8 , 9 ] show that, in aluminum alloy, some rare earth can refine metal grain, decrease the segregation of components, lower the porosity and reduce the content of impurities, with a certain content, which can improve mechanical properties, physical properties, process ability and integrated service properties.
Article ID: 1002 - 0721(2006) - 0057 - 04
So in this study, in order to improve the wetting properties of graphite with A1 melt then reduce the oxidation of the graphite, a thin film of CeOz was covered onto the graphite successfully by heterogeneous nucleation process to prepare the composite powder. The SEM, IR, XRD and XPS were used to character the surface characteristic of the composite powder.
1 Experimental The crude scale-like graphite employed in this study was provided by Shandong Nanshu Company. The graphite powder was treated by HF for 12 h then rinsed with distilled water. Then the powder was immersed in a boiled solution of H2S04 and HN03 mixture for 1 h . Cerium sol were prepared by dissolving Ce( N 0 3 ) 3
*6H20in distilled water and adding an appropriate of citric acid, heating to 70 “c with constant stirring. During the mixing process of graphite powder with the solution, some surface active reagent was added into the sol. The sol was dried in vacuum at 135 ‘T to form gel. And the pH should be controlled about 5.0 by a diluted ammonia solution. After filtration, the treated graphite powder was dried under 250 “c in muffle. X-ray powder diffraction ( XRD ) patterns were recorded at a scanning rate of 0.1 s with a Japan Riga DMA-IIIC X-ray diffractometer using monochromatic high-intensity Co Ka radiation (0.17889 nm) .
Received date: 2006 - 06 - 26; revised date: 2006 - 09 - 16 Foundation item: Project supported by the National ‘973’ Project (619990649) Biography: Dong Zhiguo ( I977 - 1, Male, Doctoral candidate * Corresponding author (E-mail : gcyao@ m i l . neu .edu .cn )
JOURNAL OF RARE EARTHS, Vol. 2 4 , spec. Issue, Dec . 2006
58
The chemical compositions of composite films were measured by the monochromatic XPS (VG Scientific, ESCARLAB 220-IXL) using an A1 Ka X-ray with energy 1486.6 eV and the take-off angle of 90". Narrow scan spectra of C l s , O l s , and Ce 3d were recorded with 20 eV pass energy for the quantification of composition.
2 Results and Discussion 2.1 SEM character of CeOJgraphite composite powder Though there showed no obvious changes of graphite powder after treatment, a uniform layer of Ce02 grain was observed (Fig. 1) . The sediment of Ce02 was controlled by the rate of the reaction though the concentration of Ce ( N 0 3 ) 3 , pH and temperature are shown influence on the sediment process. A lower rate of the reaction was needed to obtain the uniform thin film on graphite surface. The reaction of the CeOz covered on graphite can be denoted with the equation : Ce( N 0 3 ) 3+ H202+ C+HN03 + CeOJC + H20 It is well known that the metal hydroxide can be precipitated from solution by heterogeneous nucleation"O1. To form a coating using heterogeneous nucleation, the pH of suspension of the particles to be coated is controlled at a certain regime with a fit pH solution to control precipitation of the coating materials on the surface of coated particles by heterogeneous nucleation. Fig. 2 shows a schematic of the nucleation and growth behavior of a precipitate from solution with increasing solution pH["]. As the pH of suspension rises, the solution becomes more saturated. The ideal path would edge over the heterogeneous nucleation limit without touching the homogeneous nucleation regime, the coating materials will precipitate by heterogeneous nucleation and not by homogeneous nucleation. Moreover, the thickness of coating layer can be
Schematic of nucleation and growth behavior of a precipitate from solution with increasing solution pH
Fig.2
controlled by controlling the amount (concentration) of coating materials.
2.2 XRD analysis Fig. 3 is XRD patterns of cerium oxide coated graphite powder. An intense Ce02(111) reflection was present at 28.68' in all scans, indicating that Ce02 (111) was the majority plane. Other major peaks appear at 47.68' and 56.58' for all samples and these peaks could be indexed as Ce02(220), CeO, ( 3 11 ) . But some amorphous character was observed with some broader peak due to the nano-effect of the CeOz film. It suggested that the structure of graphite was intact because peak at 26. 12" was the strongest one. The CeOz exhibits a preferred ( 111) orientation. The diffraction peaks of ( l l l ) , (200), ( 2 2 0 ) , (311) correspond to the cubic Ce02 phase. Using Schemer formula ( D = a/\ //?cosO) , the size of the grain in Ce02 film was about 21.6 nm.
2 . 3 XPS analysis Two peaks could be accepted after peak analysis of element C ( F i g . 4 ) . The left one is 284.11 eV and the right one is 285.47 eV . An obvious chemical reaction did exit between CeOz film and graphite because
CeO, ( 1 1 1 )
:/coo2(220)
Ce0,(311)
ri_ I
.
,
.
,
.
,
M
3
0
4
0
5
l
2
Fig. 1 SEM graph of CeOJgraphite composite powder
Fig.3
8
11
J(
0
7
0
8
0
9
')
XRD patterns of CeOz films on graphite
0
Dong 2 G et a1 .
Preparation and Characterkzion of Graphite Powder Covered with CeO,
a 1.37 eV increase. After bombardment of argon ion for 20 min, only slight elevation of C peak was observed, which showed that the thickness of the film was about 20 nm. An obvious shoulder-peak was observed in the XPS spectra of 01s , and the first one was 529.88 eV , which was Corresponding to the 01s in G O 2 , and the second was 532.13 eV , which was corresponding to the adsorbed 01s on the graphite surface (Fig. 5 ) . Both the two value of binding energy are higher than the standard value. This could be attributed to the formation of Ce - 0 - C bond which leading to a shift of the binding energy due to the difference in the electron attractability of C and Ce[l2].
Fig. 4
59
ed. But to the graphite powder covered with CeOn. a strongly hydrophilic bond of Ce-0 of the film led to a higher energy surface, so a decrease of the angle was illustrated.
Binding enerey/eV XPS pattern of element C in CeO, coated graphite Fig.6 II-
Angle of contact of H,O/graphite ( a ) and HZO/CeO2/ graphite (b)
100
-
3 Conclusion
X-
A uniform thin nano-film of Ce02, about 20 nm
7-
in thickness , was successfully prepared onto graphite powder surface by heterogeneous nucleation process . The graphite covered with CeO, film shows a distinct change of surface state of graphite with a decrease of angle of contact.
6-
5-
5-0
sis
5i0
sjs
54,
545
Binding cncrjp3':cV
Fig.5
XPS pattern of element C in CeO, coated oxide
2.4 Angle of contact analysis A distinct change in angle of contact measurement was illustrated in Fig. 6. After being covered by CeOz, the angle of contact decreased from 90" to 30", It is suggested that the film of Ce02 on the surface of graphite was uniform and improved the wetting character of the graphite. The decrease of the angle of contact can be interpreted by the change of character and the arrangement of the surface atom or atom cluster. As to the graphite, an unpolarized hexangular C net led to a lower energy surface so a hydrophobic character was illustrat-
References : [ 1] Ju C P , Chen K I , Lin J H.
Process, microstructure and properties of squeeze-cast short-carbon-fibre reinforced aluminium-matrix composites [ J ] . Journal of Materials Science, 1994, 29 : 5127.
[ 21 Wu R J . The present condition and prospects on metal matrix composites [ J 1. Aeronautical Manufacturing Technology, 2001, 3: 19. [ 31 Evans A G , Hutchinson J W , NcNeejubg R N . Stressstrain behavior of matrix composites with discontinuous reinforcements [ J ] . Scripta Metallurgica, 1991, ( 2 5 ) :
3. [4] Latanish R M . Corrosion science corrosion engineeringan advanced technologies [J ] . Corrosion Science, 1995, 51(4): 270.
60
JOURNAL OF RARE EARTHS, Vol. 2 4 , Spec. Issue, Dee. 2006
[5] Eliosson J , Somclstrom R . Applications of aluminum matrix composites [ J 1 . Key Engineering Materials, 1995, (13) : 104. [6] Martin A , Martinez M A , Llorca J. Wear of Sic reinforced AI-matrix composites in the temperature range 20-2OOOC [ J ] . Wear, 1996, 193: 169. [ 71 Lee C S. Wear behavior of aluminum matrix composite materials [J] . Mater . Sci . , 1992, 27: 793. [8] Wang S H , Zhou H P , Kang Y P . The influence of rare earth elements on microstmctures and properties of 6061 aluminum alloy vacuum-brazed joints [ J ] . Journal of Alloys and Compounds, 2003, (352) : 79. [9] Sun Yangshan, Yao Zhengjun, Zhang Zhonghua, et al. Mechanical properties of F q Al-based alloys with cerium
[ LO]
[ 111
[ 121
[ 131
addition [ J ] . Scripta Metallurgica et Materiala, 1995, 33(5): 811. Blendell J E , Bowen H K , Coble R L. High purity alumina by controlled precipitation from aluminum sulfate solutions [J]. J . Am. Ceram. SOC.Bull., 1984, (63): 1797. B a n g Juxian , Gao Longqiao . Nanocomposite powders from coating with heterogeneous nucleation processing [ J ] . Ceramics International, 2001, 27 : 143. De Keijser Th H . , Langford J L, Miuerneijer E . Appl. Cryst., 1982, 15: 308. Turner M R , Duguet L C . Suf. Interface. Anal. , 1997, 25: 917.