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Thermal response of Cyberwood by Joule heat?
Cyberwood thought to enhance air temperature measurements by significant reductions in electrical resistance may be superseded by the increased Ca2+ ions produced at higher sample temperatures caused by Joule heat
Recently, Physics Today  reported treating living plants with solutions of CNTs significantly increased the electrical response of living plants to air temperatures. CNTs stand for carbon nanotubes. The CNTs  were non-modified type 3100 Multi Walled from Nanocyl™ having a typical diameter of 10 nm and length > 100 nm. Aradopis thaliana, a mustard green, was shown to take up the CNTs from a solution through the leaf surface and increase photosynthetic activity 30% because of increased near IR absorption not found with untreated leaves.
Although living plants are known sensitive to changes in air temperature by increased electrical conductivity, the sensitivity disappears when the plant dies. However, CNT treated plants were found  to retain electrical conductivity sensitivity to Ca2+ ions after dying, the CNT composite called Cyberwood. Importantly, the electrical conductivity of the Cyberwood increased significantly compared to dead untreated leaves. Indeed, electrical resistance was reduced almost 3 orders of magnitude as the temperature increased from 35 to 75 C. Hence, the TCR of Cyberwood is, TCR = (ΔR/Ro)/ΔT, where TCR stands for has a temperature coefficient of resistance. The change in resistance is, ΔR = Rmax – Ro, where Rmax and Ro are the maximum and minimum resistance while ΔT is the corresponding temperature change. The TCR is significant for Cyberwood, i.e., for slow temperature changes from 35 to 75 C, ΔT = 40 K and ΔR/Ro = 693, giving TCR = 17.5 K-1, or 1750 % K-1. In contrast, vanadium oxide having the greatest TCR known today of about 6 % K-1 is significantly lower than the TCR for Cyberwood.
Why the CNTs increase the TCR is thought caused by the release of calcium Ca2+ ions upon changes in air temperature, the increased ions reducing the electrical resistance of the Cyberwood. Indeed, similar thermal response was found  for CNTs in dried yeast cells of Candida albicans. Again, the CNT composite samples were subjected to slow temperature cycles in an electronically regulated oven. The sample temperature was measured using a thin-film thermo element positioned close to, but not in contact with the sample. The thumbnail shows the monitoring of the oven temperature (blue solid line) and of the resulting sample current (red dashed lines) when applying a constant voltage of 1 V for about 600 μW of electrical power dissipated in the sample.
In the C albicans/CNT measurements, air temperature near the sample is not the same as the temperature of the sample. Joule heat increases the temperature of the sample above that of the adjacent air and may give rise to higher TCR than given by the oven temperature alone. Both oven temperature and Joule heat decrease resistance, but Joule heat appears more significant as the thumbnail shows the current upon reaching 550 μA never returns to its initial value of 450 μA even though the oven temperature returns to 25 C. Other explanations for the monotonic increase in current include the removal of moisture during the first day or so. But after 10 days the current is observed stabilized at which time the current should vanish at temperatures of 25 C if air temperature alone is causing the change in resistance. But this does not occur. Since the sample temperature most likely exceeds the 100 C air temperature shown in the thumbnail, the number of Ca2+ ions liberated in the sample may be significantly increased thereby overstating the TCR of Cyberwood to changes in air temperature.
QED induced EM radiation from CNTs in both Aradopis thaliana and C. albicans is proposed as the thermal mechanism in CNT composites. QED stands for quantum electrodynamics and EM for electromagnetic. The Joule heat absorbed in the CNTs having 10 nm diameters produces QED induced EM radiation at EUV levels that photoactivate the liberation of Ca2+ ions to decrease the electrical resistance of the CNT composite. EUV stands for extreme ultraviolet radiation.
QED radiation is produced in CNTs because QM precludes atoms in CNTs from having the heat capacity to allow absorbed thermal energy from either oven air temperature or Joule heat to be conserved by the usual increase in temperature. QM stands for quantum mechanics. But CNTs have high surface to volume ratios, and therefore the absorbed thermal energy is temporarily confined in the CNT surfaces. Spontaneously, QED converts the EM energy in the surfaces into standing wave EM radiation within the CNT, the EM radiation having wavelength λ = 2 nd, where n and d are the CNT refractive index and diameter. Since the standing EM radiation between the CNT surfaces is created from the absorbed thermal energy confined in the surfaces, the QED confinement eventually vanishes allowing the standing EM radiation in the EUV to be emitted from the CNT and absorbed in the surrounding CNT composite, the EUV producing the Ca2+ ions that reduce the resistance of the sample. QED radiation is continually produced in CNTs provided the oven temperature exceeds 25 C and Joule heat is produced in the sample. See numerous QED applications at http://www.nanoqed.org/
QED radiation requires the refractive index n of the CNTs to be greater than that of the CNT composite. For CNTs as graphite, n = 2.5. The Cyberwood index n is not known, but is likely < 2.5. Hence, the QED radiation for CNTs having diameter d = 10 nm, is emitted at wavelength λ = 50 nm which is in the EUV. The EUV radiation from the CNTs is promptly absorbed at the Cyberwood interface to produce the Ca+ ions that reduce the sample resistance.
The giant TCRs reported for Cyberwood may be overstated as Joule heat dissipated in the sample may increase the sample temperature to significantly increase the number of Ca2+ ions released above that by air temperature alone.
The CNTs are induced by QED to liberate Ca2+ ions from the Cyberwood that reduce the resistance of the sample, but experiments are required to determine the significance of the EUV photoactivation of Ca2+ ions in CNT composites.
To confirm the suggested significance of Joule heat on air temperature measurements, the TCRs for Aradopis thaliana/CNT and C. albicans/CNT should be measured again using the temperature of the sample itself instead of the air near the sample.
 M. Wilson, “Tobacco cells infused with carbon nanotubes feel the heat,” Physics Today, 15-17, June 2015.
 J. P. Giraldo, et al., “Plant nanobionics approach to augment photosynthesis and biochemical sensing,” Nat. Mater., 13, 4000, 2014
 R. Di Giacomo, et al., “Plant nanobionic materials with a giant temperature response mediated by pectin-Ca2+,”
 R. Di Giacomo, et al.,” Candida albicans/MWCNTs: a Stable Conductive Bio-Nano-Composite and its Temperature Sensing Properties,”