The effect of powder sintering on the palladium ... - Semantic Scholar
CARBON
4 8 ( 2 0 1 0 ) 1 9 3 2 –1 9 3 8
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The effect of powder sintering on the palladium-catalyzed formation of carbon nanofibers from ethylene–oxygen mixtures Mark A. Atwater a, Jonathan Phillips a b
a,b
, Zayd C. Leseman
a,*
University of New Mexico, MSC01 1150, Albuquerque, NM 87131, USA Los Alamos National Labs, MSE549, Los Alamos, NM 87545, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon nanofiber growth on palladium particles from ethylene–oxygen mixtures was
Received 17 December 2009
investigated with respect to thermal history. Electron microscopy, combined with focused
Accepted 29 January 2010
ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addi-
Available online 4 February 2010
tion of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate. 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanofibers, because of their low weight and high strength [1], are desirable in applications such as composite materials [2–4], but they can also be beneficial in electronic components, catalyst support, and gas storage [1,5,6]. The properties are very size dependent, and control of the fiber morphology will determine the applicability to various functions. Consequently, the production process is vital in the commercialization of carbon nanofibers. Many methods for producing carbon nanofibers have been demonstrated. By far, the most common method is catalytic chemical vapor deposition (CCVD) at, or near, atmospheric pressure. Many procedures use metal particles, usually nanoscale, for the catalyst. Formation is commonly achieved by either depositing a thin layer of metal (e.g. by sputtering) and then annealing it to form discrete, supported particles
[7], or by precipitation of a metal precursor (e.g. nickel nitrate) or coprecipitation of multiple metals, again followed by an annealing process to encourage particle growth and separation [8–10]. Recently, ethylene–oxygen mixtures were found to generate carbon nanofibers over a wide range of temperatures when using commercially-purchased palladium powder as the catalyst [11], but fiber yields from bulk powders can be difficult to predict due to sintering effects. The present work is intended to develop additional means to optimize carbon nanofiber growth. Protocols for improving deposition rate and sintering resistance are desirable since catalyst particle size correlates with carbon fiber size [5,6,12,13], and powder sintering is a significant obstacle in maintaining consistent particle size. Although keeping temperatures low will reduce sintering, it is important to grow fibers at relatively high temperatures since growth rates are strongly temperature dependent. Therefore, a balance
* Corresponding author: Fax: +1 505 665 5548. E-mail address: [email protected] (Z.C. Leseman). 0008-6223/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.01.060
CARBON
4 8 ( 20 1 0 ) 1 9 3 2–19 3 8
between deposition rate and sintering extent must be found for optimization. Ideally, an optimized protocol should maintain the growth rate of a higher temperature while minimizing sintering leading to increased fiber size disparity. Even though soak times at high temperature are typically short for catalytic carbon deposition (
4 8 ( 2 0 1 0 ) 1 9 3 2 –1 9 3 8
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The effect of powder sintering on the palladium-catalyzed formation of carbon nanofibers from ethylene–oxygen mixtures Mark A. Atwater a, Jonathan Phillips a b
a,b
, Zayd C. Leseman
a,*
University of New Mexico, MSC01 1150, Albuquerque, NM 87131, USA Los Alamos National Labs, MSE549, Los Alamos, NM 87545, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon nanofiber growth on palladium particles from ethylene–oxygen mixtures was
Received 17 December 2009
investigated with respect to thermal history. Electron microscopy, combined with focused
Accepted 29 January 2010
ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addi-
Available online 4 February 2010
tion of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate. 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanofibers, because of their low weight and high strength [1], are desirable in applications such as composite materials [2–4], but they can also be beneficial in electronic components, catalyst support, and gas storage [1,5,6]. The properties are very size dependent, and control of the fiber morphology will determine the applicability to various functions. Consequently, the production process is vital in the commercialization of carbon nanofibers. Many methods for producing carbon nanofibers have been demonstrated. By far, the most common method is catalytic chemical vapor deposition (CCVD) at, or near, atmospheric pressure. Many procedures use metal particles, usually nanoscale, for the catalyst. Formation is commonly achieved by either depositing a thin layer of metal (e.g. by sputtering) and then annealing it to form discrete, supported particles
[7], or by precipitation of a metal precursor (e.g. nickel nitrate) or coprecipitation of multiple metals, again followed by an annealing process to encourage particle growth and separation [8–10]. Recently, ethylene–oxygen mixtures were found to generate carbon nanofibers over a wide range of temperatures when using commercially-purchased palladium powder as the catalyst [11], but fiber yields from bulk powders can be difficult to predict due to sintering effects. The present work is intended to develop additional means to optimize carbon nanofiber growth. Protocols for improving deposition rate and sintering resistance are desirable since catalyst particle size correlates with carbon fiber size [5,6,12,13], and powder sintering is a significant obstacle in maintaining consistent particle size. Although keeping temperatures low will reduce sintering, it is important to grow fibers at relatively high temperatures since growth rates are strongly temperature dependent. Therefore, a balance
* Corresponding author: Fax: +1 505 665 5548. E-mail address: [email protected] (Z.C. Leseman). 0008-6223/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.01.060
CARBON
4 8 ( 20 1 0 ) 1 9 3 2–19 3 8
between deposition rate and sintering extent must be found for optimization. Ideally, an optimized protocol should maintain the growth rate of a higher temperature while minimizing sintering leading to increased fiber size disparity. Even though soak times at high temperature are typically short for catalytic carbon deposition (
