Scientists Confirm Elusive Transverse Thomson Effect After 100 Years
A Historic Thermoelectric Discovery
In a recent publication in Nature Physics, scientists detail the inaugural experimental detection of the transverse Thomson effect—a fundamental thermoelectric phenomenon that has remained unobserved since its theoretical proposal over 100 years ago.
For more than a hundred years, thermoelectric effects have underpinned physicists understanding of the relationship between heat and electricity, grounded in the Seebeck, Peltier and Thomson effects, all discovered in the 19th century.
The Thomson effect refers to the generation of heat or cooling within a conductor when both electric current and a temperature gradient move concurrently in the same direction.
Researchers have long postulated the existence of a transverse form of this effect, expected to arise when electric current, thermal gradient and magnetic field are oriented perpendicularly within a conductor.
Landmark Experiment Led by Japanese Researchers
A team of researchers, headed by Atsushi Takahagi of Nagoya University and Ken-ichi Uchida of the University of Tokyo, hs now confirmed the existence of the transverse thermoelectric effect—a higher-order phenomenon.
"The thermoelectric effect has long inspired my work, particularly given its promise for energy harvesting and thermal regulation," Takahagi remarked in an interview with publishers website.
Identifying the Relevant Signal
Overcoming Experimental Challenges
Observing the transverse Thomson effect in a laboratory setting has long posed a challenge, largely owing to interference from other thermal phenomena such as the Peltier and Ettingshausen effects.
Innovative Thermoelectric Imaging
To address the difficulty in isolating the signal, the team employed sophisticated thermoelectric imaging methods, underpinned by lock-in thermography to detect and study the effect.
"We conducted the experiment using an infrared camera to capture how the sample responded thermally to a pulsing electric current," Uchida stated.
"By isolating the temperature fluctuation matching the frequency of the applied current from the thermal images, we distinguished the thermoelectric signals from Joule heating."
Isolating the Effect
The breakthrough arose upon realizing that the spatial pattern of the transverse Thomson effect diverged from that of competing phenomena. Measurements were conducted with and without a thermal gradient and the results were subtracted to isolate the pure signal.
The initial measurement, taken with a temperature gradient, encompasses both the transverse Thomson and Ettingshausen effects. The subsequent measurement, conducted without the gradient, reflects solely the latter—enabling isolation of the transverse Thomson effect through subtraction.
Selecting the Most Appropriate Material
Why Bi₈₈Sb₁₂?
The research team opted for a bismuth-antimony alloy (Bi₈₈ Sb₁₂) in their experiments, owing to its pronounced Nernst effect at ambient temperature.
Applying a temperature gradient and magnetic field at right angles to a material induces an electric field orthogonal to both—a phenomenon referred to as the Nernst effect.
In contrast, the Ettingshausen effect operates in reverse: when an electric current and magnetic field are applied at right angles, a transverse temperature gradient emerges.
What Makes Bi₈₈Sb ₁₂ Ideal
Takahagi explained, "We discovered that the magnitude of the heat source linked to the transverse Thomson effect is dictated by the Nernst coefficient as well as its temperature derivative."
"Bi₈₈ Sb₁₂ has long been recognized for possessing a substantial Nernst coefficient, rendering it a strong contender for demonstrating the transverse Thomson effect."
"This marks a fundamental departure from the traditional Thomson effect, which relies exclusively on the temperature derivative of the Seebeck coefficient."
Shifting from Warm to Cool Conditions
Perhaps most remarkable was the discovery that altering the direction of the magnetic field alone could toggle the system between heating and cooling. The observed behaviour was intricately field-dependent, even demonstrating a full sign reversal at specific field intensities.
The team found that the transverse Thomson coefficient is governed by two opposing factors:
- One linked to the temperature gradient of the Nernst coefficient, which typically results in heating.
- The other to its magnitude, which generally induces cooling.
The interplay between these components results in the field-dependent reversal of sign detected in their experimental findings.
Simulation and Validation
The numerical simulations mirrored the experimental results with precision, reinforcing the theoretical insight and verifying their measurement approach.
Prospective Applications
This discovery offers fresh prospects for improving thermal control solutions, notably in settings where accurate localized heat modulation is crucial.
"Evidence has emerged in recent literature that the Thomson effect, in its conventional form, enhances Peltier cooling," said Takahagi.
"In the same vein, the transverse Thomson effect is projected to facilitate improvements in transverse thermoelectric cooling applications."
Toward Advanced Materials
This study suggests potential avenues for engineering more efficient materials. In the examined Bi₈₈Sb₁₂ alloy, partial cancellation between the two components of the transverse Thomson coefficient constrains the effect's overall strength.
"Uncovering materials where these two factors act in concert may pave the way for high-performing candidates for the transverse Thomson effect, offering a promising direction for future inquiry," said Uchida.
Explore More Groundbreaking Discoveries!
The long-theorized transverse Thomson effect has finally been confirmed after over a century—an achievement that opens new frontiers in thermoelectric science and energy technology.
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