本研究利用化學氣相沉積法(CVD)成長多壁奈米碳管，以乙炔做為碳源、二茂鐵和二甲苯的混合溶液作為催化劑，藉由改變成長溫度、碳源種類、反應時間、催化劑濃度、反應氣體(乙炔)流量以及大氣電漿表面處理等實驗參數，觀察不同的製程參數對奈米碳管成長之影響。使用電子顯微鏡觀察奈米碳管之形態，並以拉曼光譜儀分析奈米碳管的性質。
    實驗結果顯示：成長溫度、碳源種類、反應時間、催化劑濃度、反應氣體流量皆會在特定的範圍內，成長出濃密且排列良好的奈米碳管，在反應溫度810oC、反應時間10分鐘、乙炔流量2.5sccm、催化劑濃度10wt%的成長條件下，可成長出長度約130微米之直立奈米碳管。而經過大氣電漿表面處理後之試片，可藉由電漿活化試片表面，提高試片的表面能，增加試片表面的吸附性質，實驗證明適當的電漿表面處理可明顯改善試片表面的吸附能力，使得碳原子和催化劑能有效地沉積在試片表面。
    奈米碳管的成長參數是影響奈米碳管的外型、數量、品質和純度的考慮因素，使奈米碳管在未來能朝向高成長速率、大面積與高純度的生長仍是一項需要努力的目標。

Thermal chemical vapor deposition (CVD) was adopted in this research to synthesize multi-wall carbon nanotubes where acetylene and ferrocene-xylene mixture were used as the carbon source and catalyst respectively. Effect of the processing parameters such as growth temperature, carbon source, reaction time, concentration of catalyst, flow rate of reaction gas, and pre-treatment of the substrate on the growth of carbon nanotubes were systematically studied. The morphology and characteristics of the obtained carbon nanotubes were analyzed using SEM and Raman spectroscopy.
    Results showed that densely packed and well-aligned carbon nanotubes could grow to a length above 150μm in 10 minutes duration under the conditions of 2.5sccm of acetylene, 10wt% of ferrocene-xylene and at a growing temperature of 810oC. The atmospheric plasma was employed to activate silicon substrates and results showed that plasma surface treatment could increase the deposition catalysts on substrate and subsequently improved the CNT growth.
    It was shown in the study processing parameters had profound effects on the shape, quantity, quality and purity of the obtained carbon nanotubes. Further efforts have to be made if higher growth rate, larger area and better purity carbon nanotubes are to be produced in the future.

目錄
中文摘要	I
英文摘要	II
致謝	IV
目錄	V
圖目錄	VIII
表目錄	XI
第1章 序論	1
1-1 前言	1
1-2 研究動機	2
第2章 文獻回顧與理論基礎	3
2-1 奈米碳管的歷史發展	3
2-2 奈米碳管的結構	5
2-3 奈米碳管的成長機制	7
2-4 奈米碳管的特性與應用	9
2-4-1 場發射性質的應用	9
2-4-2 奈米碳管強化複合材料	11
2-4-3 能源儲存之應用	12
2-4-4 原子力顯微鏡之探針	13
2-4-5 其他方面的應用	15
2-5 具方向性之奈米碳管	16
2-6 奈米碳管的製備方法	17
2-6-1 電弧放電法	17
2-6-2 雷射蒸發法	18
2-6-3 化學氣相沉積法	19
2-7 奈米碳管之拉曼光譜檢測	20
2-8 電漿原理、種類與電漿表面處理	22
2-8-1 電漿原理	22
2-8-2 電漿的產生方式	25
2-8-3 電漿表面處理	27
第3章 實驗方法與設備	29
3-1 實驗規劃	29
3-2 實驗設備	30
3-2-1 化學氣相沉積設備	30
3-2-2 大氣電漿表面處理設備	31
3-2-2 檢測儀器	32
3-3 實驗步驟	34
3-4 實驗流程	36
3-5 化學氣相沉積實驗參數之控制	37
第4章 結果與討論	39
4-1 成長溫度對奈米碳管成長之影響	39
4-2 碳源種類與奈米碳管成長之關係	45
4-3 反應時間與奈米碳管成長之關係	49
4-4 催化劑濃度對奈米碳管成長之影響	54
4-5 反應氣體流量對奈米碳管成長之影響	58
4-6 電漿表面處理	63
第5章 結論	71
參考文獻	73
附錄A 電漿表面處理之奈米碳管	81 
圖目錄
圖1-1 碳的同素異形體	2
圖2-1 C60分子模型	4
圖2-2 奈米碳管的HRTEM照片	4
圖2-3 二維片狀的石墨結構圖	6
圖2-4 奈米碳管的各種結構 (a)armchair (b)zigzag (c)chiral	6
圖2-5 碳經由催化劑擴散成長機制示意圖	8
圖2-6 (a)頂端成長模式 (b)底部成長模式	8
圖2-7 韓國三星電子所製造之奈米碳管平面顯示器	10
圖2-8 奈米碳管平面顯示器內部示意圖	10
圖2-9 奈米碳管高分子複合薄膜之破裂圖	12
圖2-10 (a)傳統探針 (b)奈米碳管探針	14
圖2-11 傳統探針與奈米碳管探針觀測高深寬比特徵差異	14
圖2-12 (a)定向成長之奈米碳管(b)利用微影蝕刻製程選擇性成長之奈米碳管	16
圖2-13電弧放電法製造奈米碳管之簡圖	18
圖2-14 雷射蒸發法製造奈米碳管之簡圖	18
圖2-15 化學氣相沉積法製備奈米碳管之簡圖	19
圖2-16 各種不同碳系材料的拉曼光譜(a)高定向石墨(b)多壁奈米碳管內層(c)多壁奈米碳管外層(d)玻璃碳	21
圖2-17 平行板電極之電壓與p×d乘積關係圖	24
圖2-18 低壓直流輝光放電的電流–電壓關係圖	24
圖2-19 大氣電漿的電流–電壓關係圖	24
圖2-20各種不同的電漿源(a)電暈放電(b)介電質放電(c)電漿火焰	26
圖2-21電漿表面處理前與處理後接觸角的改變	28
圖3-1 化學氣相沉積設備	30
圖3-2 化學氣相沉積設備示意圖	30
圖3-3大氣電漿表面處理設備圖	31
圖3-4 掃描式電子顯微鏡	32
圖3-5顯微拉曼光譜儀	33
圖3-6 電漿表面處理示意圖	35
圖4-1化學氣相沉積法在(a)700oC (b)730oC (c)760oC (d)790oC(e)810oC (f)840oC (g)870oC (h)900oC所成長之奈米碳管	41
圖4-2 直立之奈米碳管(a)700oC(b)730oC(c)760oC(d)790oC(e)810oC	42
圖4-3 反應溫度810oC下所成長之奈米碳管	42
圖4-4 經過高溫區至低溫區之奈米碳管(a)900oC (b)870oC (c)840oC (d)810oC (e)790oC (f)760oC (g)730oC (h)700oC	43
圖4-5 (a)過多的碳所形成的碳顆粒 (b)SEM之放大圖	44
圖4-6 各溫度下成長之奈米碳管拉曼光譜圖	44
圖4-7 使用二甲苯作為碳源所成長之奈米碳管拉曼光譜圖	48
圖4-8 使用二甲苯做為碳源在810 oC下所成長之奈米碳管	48
圖4-9 反應溫度810 oC所成長之奈米碳管(a)5分鐘(b)10分鐘	51
圖4-10 化學氣相沉積法在(a)700oC(b)730oC(c)760oC(d)790oC(e)810oC (f)840oC(g)870oC(h)900oC所成長之奈米碳管	52
圖4-11 經過高溫區至低溫區之奈米碳管(a)900oC(b)870oC (c)840oC (d)810oC (e)790oC (f)760oC (g)730oC (h)700o	53
圖4-12 反應溫度790 oC、催化劑濃度5wt%所成長之奈米碳管	56
圖4-13化學氣相沉積法在(a)700oC(b)730oC(c)760oC(d)790oC(e)810oC (f)840oC(g)870oC(h)900oC所成長之奈米碳管	57
圖4-14 反應氣體(乙炔)流量1sccm所成長之奈米碳管拉曼光譜圖	61
圖4-15反應氣體(乙炔)流量5sccm所成長之奈米碳管拉曼光譜圖	62
圖4-16 電漿表面處理60秒之奈米碳管(a)700oC (b)730oC (c)760oC (d)790oC (e)810oC (f)840oC (g)870oC (h)900oC	65
圖4-17 電漿表面處理120秒之奈米碳管(a)700oC (b)730oC (c)760oC (d)790oC (e)810oC (f)840oC (g)870oC (h)900oC	66
圖4-18 電漿表面處理180秒之奈米碳管(a)700oC (b)730oC (c)760oC (d)790oC (e)810oC (f)840oC (g)870oC (h)900oC	67
圖4-19電漿表面處理240秒之奈米碳管(a)700oC (b)730oC (c)760oC (d)790oC (e)810oC (f)840oC (g)870oC (h)900oC	68
圖4-20電漿試片表面處理60秒所成長的奈米碳管之拉曼光譜圖	69
圖4-21電漿試片表面處理240秒所成長的奈米碳管之拉曼光譜圖	70
 
表目錄
表2-1 相關儲氫材料之比較	13
表2-2 奈米碳管可能的應用	15
表2-3 高分子材料經電漿處理後的性質改變	28
表3-1 電漿表面處理參數	35
表4-1 不同碳源所成長之奈米碳管的形態比較	46
表4-2 不同成長時間之奈米碳管長度比較	50
表4-3 不同金屬催化劑濃度所成長之奈米碳管	55
表4-4 不同反應氣體(乙炔)流量所成長之奈米碳管	59
表4-5 電漿表面處理參數	64
表4-6 經EDAX分析之試片表面的氧含量	64

【1】http://smalley.rice.edu/index.cfm, 6 January 2005
【2】M.L. Cohen, “Nanotubes, nanoscience, and   nanotechnology”, Materials Science and Engineering C 15 (2001) pp. 1-11 
【3】S. Iijima, “Helical microtubules of graphitic carbon”, Nature 354 (1991) pp. 56-58 
【4】H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, “C60: Buckminsterfullerene”, Nature 318 (1985) pp. 162-163
【5】R.C. Haddon, L.E. Brus, K. Ragahavachari, “Rehybridization and π-orbital alignment: the key to the existence of spheroidal carbon clusters”, Chem. Phy. Lett. 131 (1986) pp. 165-169
【6】http://sbchem.sunysb.edu/msl/c60e.jpg, 6 January 2005
【7】N. Hamada, S. Sawada, A. Oshiyama, “New one-dimensional conductors: Graphitic microtubules”, Phys. Rev. Lett. 68 (1992)  pp. 1579-1581
【8】R. Saito, M. Fujita, G. Dresselhaus, M.S. Dresselhaus, “ Electronic structure of chiral graphene tubules”, Appl. Phys. Lett. 60 (1992) pp. 2204-2206 
【9】L. Chico, V.H. Crespi, L.X. Benedict, S.G. Louie, M.L. Cohen, “Pure carbon nanoscale devices: Nanotube heterojunctions”, Phys. Rev. Lett. 76 (1996) pp. 971-974
【10】P.J.F Harris, “Carbon nanotubes and related structures”, Cambridge Press, Cambridge (1999)
【11】R.B. Weisman, “Simplifying carbon nanotube identification”, American Institute of Physics 10 (2004) pp. 24-27
【12】M.J. Yacaman, M.M. Yoshida, L. Rendon, J.G. Santiesteban, “Catalytic growth of carbon microtubules with fullerene structure”, Appl. Phys. Lett. 62 (1993) pp. 202-204
【13】R.T.K. Baker, M.A. Barber, P.S. Harris, F.S. Feates, R.J. Waite, “Nucleation and growth of carbon deposits from the nickel catalyzed decomposition of acetylene”, Journal of Catalysis 26 (1972) pp. 51-54
【14】M.J. Jung, K.Y. Eun, J.K. Lee, Y.J. Baik, K.R. Lee, J.W. Park, “Growth of carbon nanotubes by chemical vapor deposition”, Diamond and Related Materials 10 (2001) pp. 1235-1240
【15】Y.M. Shyu, F.C.N. Hong, “Low-temperature growth and field mission of aligned carbon nanotubes by chemical vapor deposition”, Materials Chemistry and Physics 72 (2001) pp. 223-227
【16】E.F. Kukovitsky, S.G. L’vov, N.A. Sainov, V.A. Shustov, L.A. Chernozatonskii, “Correlation between metal catalyst particle size and carbon nanotube growth”, Chem. Phys. Lett. 355 (2002) pp. 497-503
【17】H. Zhang, E. Liang, P. Ding, M. Chao, “Layered growth of aligned carbon nanotube arrays by pyrolysis”, Physica B 337 (2003) pp. 10-16
【18】J.M. Kim, “Field emission from carbon nanotubes for displays”, Diamond and Related Materials 9 (2000) pp. 1184-1189
【19】R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Qian, X. Fan, E.C. Dicky, J. Chen, “Continuous production of aligned carbon nanotubes: a step closer to commercial realization”, Chem. Phys. Lett. 303 (1999) pp. 467-474
【20】M. Su, B. Zheng, J. Liu, “A scalable CVD method for the synthesis of single-walled carbon nanotubes with high catalyst productivity”, Chem. Phys. Lett. 322 (2000) pp. 321-326
【21】J.I. Sohn, C. Nam, S. Lee, “Vertically aligned carbon nanotube growth by pulsed laser deposition and thermal chemical vapor deposition methods”, Applied Surface Science 197-198 (2002) pp. 568-573
【22】M.M.J. Treacy, T.W. Ebbesen, T.M. Gibson, “Exceptionally high Young’s modulus observed for individual carbon nanotubes”, Nature 381 (1996) pp. 678-680
【23】E.W. Wong, P.E. Sheehan, C.M. Lieber, “Nanobeam mechanics:  elasticity, strength, and toughness of nanorods and nanotubes”, Science 277 (1997) pp. 1971-1975
【24】F. Li, H.M. Cheng, S. Bai, G. Su, “Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes”, Appl. Phys. Lett. 77 (2000) pp. 3161-3163
【25】D. Qian, E.C. Dickey, R. Andrews, T. Rantell, “Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites”, Appl. Phys. Lett. 76 (2000) pp. 2868-2870
【26】R.Z. Ma, J. Wu, B.Q. Wei, J. Liang, D.H. Wu, “Processing and properties of carbon nanotubes-nano-SiC ceramic”, Journal of Materials Science 33 (1998) pp. 5243-5246
【27】E. Flahaut, A. Peigney, C. Laurent, C. Marliere, F. Chastel,A. Rousset, “Carbon nanotube-metal-oxide nanocomposites: microstructure, electrical conductivity and mechanical properties”, Acta Materialia 48 (2000) pp. 3803-3812.
【28】A. Peigney, C. Laurent, E. Flahaut, A. Rousset, “Carbon nanotubes in novel ceramic matrix nanocomposites”, Ceramics International 26 (2000) pp. 677-683
【29】A. Peigney, C. Laurent, O. Dumortier, A. Rousset, “Carbon nanotubes-Fe-alumina nanocomposites. Part I: Influence of the Fe content on the synthesis of powders”, Journal of the European Ceramic Society 18 (1998) pp. 1995-2004
【30】A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, “Storage of hydrogen in single-walled carbon nanotubes”, Nature 386 (1997) pp. 377-380
【31】A. Chambers, C. Park, R.T. Baker, N. M. Rodriguez, “Hydrogen storage in graphite nanofibers”, J. Phys. Chem. B 102 (1998) pp. 4253-4256
【32】C. Liu, Y. Fan, M. Liu, H. Cong, H. Cheng, M.S. Dresselhaus, “Hydrogen storage in single-walled carbon nanotubes at room temperature”, Science 286 (1999) pp. 1127-1129
【33】Y. Chen, D.T. Shaw, X.D. Bai, E.G. Wang, C. Lund, W.M. Lu, D.D.L. Chung, “Hydrogen storage in aligned carbon nanotubes”, Appl Phys Lett 78 (2001) pp. 2128-2130
【34】P. Chen, X. Wu, J. Lin, K. L. Tan, “High H2 uptake by alkali-doped carbon nanotubes under ambient pressure and moderate temperatures”, Science 285 (1999) pp. 91-93
【35】Y. Nakayama, “Scanning probe microscopy installed with nanotube probes and nanotube tweezers “, Ultramicroscopy 91 (2002) pp. 49-56
【36】T. Larsen, K. Moloni, F. Flack, M.A. Eriksson, M.G. Lagally, C.T. Black, “Comparison of wear characteristics of etched-silicon and carbon nanotubes atomic-force microscopy probes”, Appl. Phys. Lett. 80 (2002) pp. 1996-1998
【37】C.L. Cheung, J.H. Hafner, C.M. Lieber, “Carbon nanotube atomic force microscopy tips: Direct growth by chemical vapor deposition and application to high-resolution imaging”, PNAS 97 (2000) pp. 3809-3813
【38】R. Vajtai, B.Q. Wei, Z.J. Zhang, Y. Jung, G. Ramanath, P.M. Ajayan, “Building carbon nanotubes and their smart architectures”, Smart Mater. Struct. 11 (2002) pp. 691-698
【39】A.M. Cassell, G.C. McCool, H.T. Ng, J.E. Koehne, B. Chen, J. Li, J. Han, M. Meyyappan, “Carbon nanotube networks by chemical vapor deposition”, Appl. Phys. Lett. 82 (2003) pp. 817-819
【40】L. Huang, S.J. Wind, S.P. O’Brien, “Controlled growth of single-walled carbon nanotubes from an ordered mesoporous silica template”, Nano Lett. 3 (2003) pp. 299-303
【41】R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P. Avouris, “Single- and multi-wall carbon nanotube field-effect transistors”, Appl. Phys. Lett. 73 (1998) pp. 2447-2450
【42】Y.C. Choi, D.W. Kim, T.J. Lee, C.J. Lee, Y.H. Lee, “Growth mechanism of vertically aligned carbon nanotubes on silicon substrates”, Synthetic Metals 117 (2001) pp. 81-86
【43】G. Eres, A.A. Puretzky, D.B. Geohegan, H. Cui, “In situ control of the catalyst efficiency in chemical vapor deposition of vertically aligned carbon nanotubes on predeposited metal catalyst films”, Appl. Phys. Lett. 84 (2004) pp. 1759-1761
【44】S. Huang, A.W.H. Mau, “3D carbon nanotube architectures on glass substrate by stamp printing bimetallic Fe-Pt/polymer catalyst”, J. Phys. Chem. B 107 (2003) pp. 8285-8288
【45】S. Iijima, T. Ichlhashi, “Single-shell carbon nanotubes of 1-nm diameter”, Nature 363 (1993) pp. 603-605
【46】D.S. Bethune, C.H. Kiang, M.S. Devries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, “Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls”, Nature 363 (1993) pp. 605-607
【47】Y. Saito, K. Nishikubo, K. Kawabata, T. Matsumoto, “Carbon nanocapsules and single-layered nanotubes produced with platinum group metals (Ru, Rh, Pd, Os, Ir, Pt) by arc discharge”, J. Appl. Phys. 80 (1996) pp. 3062-3067
【48】P.G. Collins, P. Avouris, “Nanotubes for electronics”, Scientific American 283 (2000) pp. 62-69
【49】A.G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C.B. Huffman, F.J. Rodriguez-Macias, P.J. Boul, A.H. Lu, D. Heymann, D.T. Colbert, R.S. Lee, J.E. Fischer, A.M. Rao, P.C. Eklund, R.E. Smalley, “Large-scale purification of single-wall carbon nanotubes: Process, product and characterization”, Applied Physics A: Materials Science & Processing 67 (1998) pp. 29-37
【50】A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tománek, J.E. Fischer, R.E. Smalley, “Crystalline ropes of metallic carbon nanotubes”, Science 273 (1996) pp. 483-487
【51】Y. Zhang, S. Iijima, “Formation of single-wall carbon nanotubes by laser ablation of fullerenes at low temperatures”, Appl. Phys. Lett. 75 (1999) pp. 3087-3089
【52】C.J. Lee, S.C. Lyu, Y.R. Cho, J.H. Lee, K.I. Cho, “Diameter- controlled growth of carbon nanotubes using thermal chemical vapor deposition”, Chem. Phys. Lett. 341 (2001) pp. 245-249
【53】E.F. Kukovitsky, S.G. L’vov, N.A. Sainov, V.A. Shustov, L.A. Chernozatonskii, “Correlation between metal catalyst particle size and carbon nanotube growth”, Chem. Phys. Lett. 355 (2002) pp. 497-503
【54】T. Okazaki, H. Shinohara, “Synthesis and characterization of single-wall carbon nanotubes by hot-filament assisted chemical vapor deposition”, Chem. Phys. Lett. 376 (2003) pp. 606-611
【55】H. Hiura, T.W. Ebbesen, K. Tanigaki, H. Takahashi, “Raman studies of carbon nanotubes”, Chem. Phys. Lett. 202 (1993) pp. 509-512
【56】Y.J. Yoon, J.C. Bae, H.K. Baik, S.J. Cho, S.J. Lee, K.M. Song, N.S. Myung, “Growth control of single and multi-walled carbon nanotubes by thin film catalyst”, Chem. Phys. Lett. 366 (2002) pp. 109-114
【57】C.J. Lee, J. Park, J.A. Yu, “Catalyst effect on carbon nanotubes synthesized by thermal chemical vapor deposition”, Chem. Phys. Lett. 360 (2002) pp. 250-255
【58】Y. Ouyang, Y. Fang, “Temperature dependence of the Raman spectra of carbon nanotubes with 1064nm excitation”, Physica E 24 (2004) pp. 222-226
【59】K.E. Kim, K.J. Kim, W.S. Jung, S.Y. Bae, J. Park, J. Choi, J. Choo, “Investigation on the temperature-dependent growth rate of carbon nanotubes using chemical vapor deposition of ferrocene and acetylene”, Chem. Phys. Lett. 401 (2005) pp. 459-464
【60】董家齊, 陳寬任, “奇妙的物質第四態–電漿”, 科學發展 354 (2002) pp. 52-59
【61】E.E. Kunhardt, “Electrical breakdown of gases: The prebreakdown stage”, IEEE Transactions on Plasma Science PS-8 (1980) pp. 130-138
【62】A. Schutze, J.Y. Jeong, S.E. Babayan, J. Park, G.S. Selwyn,  R.F. Hicks, “The atmospheric-pressure plasma jet: A review and comparison to other plasma sources”, IEEE Transactions on Plasma Science 26 (1998) pp. 1685-1964
【63】S. Ramakrishnan, M.W. Rogozinski, “Properties of electric arc plasma for metal cutting”, J. Appl. Phys. D 30 (1997) pp. 636-644
【64】李長久, 孫波, 汪民, 韓峰, 武濤, “微束等離子噴塗工藝條件對Cu塗層組織和性能的影響”, 西安交通大學學報 36 (2002) pp. 1182-1186
【65】M. Noeske, J. Degenhardt, S. Strudthoff, U. Lommatzsch, “Plasma jet treatment of five polymers at atmospheric pressure: surface modifications and the relevance for adhesion”, International Journal of Adhesives 24 (2004) pp. 171-177
【66】H. Krump, M. Simor, I. Hudec, M. Jasso, A.S. Luy, “Adhesion strength study between plasma treated polyester fibers and a rubber matrix”, Applied Surface Science 240 (2005) pp. 268-274
【67】M.C. Kim, D.K. Song, H.S. Shin, S.H. Baeg, G.S. Kim, J.H. Boo, J.G. Han, S.H. Yang, “Surface modification for hydrophilic property of stainless steel treated by atmospheric-pressure plasma jet”, Surface and Coatings Technology 171 (2003) pp. 312-316
【68】M.C. Kim, S.H. Yang, J.H. Boo, J.G. Han, “Surface treatment of metals using an atmospheric pressure plasma jet and their surface characteristics”, Surface and Coatings Technology 174-175  (2003) pp. 839-844