Carbon nanotubes (CNTs) can sustain temperatures up to approximately 500°C in air before they begin to oxidize and degrade. This temperature threshold is crucial for the preparation and application of CNTs, especially in environments where exposure to air is unavoidable.
Detailed Explanation:
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Oxidation of Graphite and CNTs: Graphite, the material from which CNTs are derived, is sensitive to oxygen and begins to oxidize when exposed to air at temperatures around 500°C (932°F). This oxidation process can lead to a rapid loss of mass, up to 1% per day under certain conditions. Continuous exposure to elevated temperatures in air will cause the material to lose thickness and eventually fail structurally. This characteristic of graphite directly affects the stability and longevity of CNTs when exposed to similar conditions.
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Operational Temperatures and Environments: The operational temperatures for graphite, and by extension CNTs, can be significantly higher in controlled environments with reduced oxygen levels or under vacuum. For instance, graphite can be used up to 2450°C (4442°F) at pressures down to 10-2 torr, and up to 2150°C (3902°F) at 10-4 torr. These conditions are not typical for most practical applications of CNTs in air, but they highlight the potential for higher temperature operations under controlled conditions.
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Preparation Techniques and Temperature Control: The preparation of CNTs often involves techniques like plasma-enhanced chemical vapor deposition (PECVD), which can lower the required substrate temperatures for growing high-quality CNTs. Traditionally, temperatures above 800°C were needed, but advancements in PECVD have enabled the growth of CNTs at temperatures as low as 120°C. This reduction in temperature not only eases the constraints on the materials and substrates that can be used but also helps in maintaining the integrity of the CNTs by avoiding the high-temperature oxidation zone.
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Implications for Applications: Understanding the temperature limits of CNTs in air is vital for their integration into various applications, such as nanoelectronic devices and field emission devices. By ensuring that the operational temperatures remain below the oxidation threshold, the longevity and performance of CNT-based devices can be preserved. This is particularly important for in-situ preparation of CNTs where the materials are directly integrated into devices at lower temperatures, thus avoiding the risk of oxidation and degradation.
In conclusion, while CNTs can technically withstand high temperatures under specific conditions (like in vacuum or inert atmospheres), their practical operational limit in air is around 500°C due to the onset of oxidation. This understanding is crucial for the design and implementation of CNT-based technologies in various industries.
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