Beta Radiation Enhanced Thermionic Emission from Diamond Thin Films

At a Glance

Metadata	Details
Publication Date	2017-11-13
Journal	Frontiers in Mechanical Engineering
Authors	Alex Croot, Gary Wan, Alex Rowan, Hugo Dominguez Andrade, James A. Smith
Institutions	University of Bristol
Citations	15
Analysis	Full AI Review Included

Beta Radiation Enhanced Thermionic Emission: 6CCVD Technical Analysis

Application Focus: Thermionic Energy Converters (TEC) and high-performance cathode materials utilizing electron emissivity enhancement via radiation exposure.

Executive Summary

This research validates the use of beta radiation exposure as a highly effective method for enhancing the performance of diamond-based thermionic emitters, directly addressing key hurdles related to high operational temperature ($T$) and low output current.

Core Material: Hydrogen-terminated nitrogen-doped nanocrystalline diamond (HTND) thin films grown via Microwave Plasma Chemical Vapor Deposition (MPCVD) on Molybdenum (Mo) substrates.
Performance Enhancement: Exposure to ⁶³Ni beta ($\beta$) radiation resulted in a 2.7-fold increase in peak thermionic emission current compared to the non-irradiated control (⁵⁹Ni).
Operational Temperature Reduction: The average emission threshold temperature ($T_t$) was substantially lowered by 58 ± 11 °C, moving the operation from 566 °C down to 504 °C.
Mechanism Hypothesis: The enhancement is likely due to the skewing of the conduction band electron distribution towards higher energy states by the incoming beta particles, rather than simple secondary electron emission (SEE) or increased bulk conductivity.
Technological Impact: This work establishes diamond thin films, specifically nanocrystalline H-terminated material, as viable low-temperature cathodes, accelerating the realization of TEC devices for direct heat-to-electrical energy conversion.

Technical Specifications

Extraction of key experimental parameters and performance metrics:

Parameter	Value	Unit	Context
Current Enhancement Factor	2.7	x	Ratio of $\beta$-exposed to non-$\beta$ peak current (2nd-20th cycle average)
Threshold Temperature ($T_t$) Reduction	58 ± 11	°C	Average reduction upon $\beta$ exposure
$T_t$ (Non-Beta Average)	566 ± 32	°C	Temperature at which current surpasses 0.1 µA
$T_t$ (Beta Average)	504 ± 22	°C	Temperature at which current surpasses 0.1 µA
Emitter Thickness	< 1	µm	Nitrogen-doped nanocrystalline diamond
Substrate Material	Molybdenum (Mo)	N/A	10 x 10 x 0.5 mm plate
Effective Beta Activity	1.3	MBq	Total calculated ⁶³Ni activity reaching the sample
Maximum Beta Particle Energy	70	keV	From ⁶³Ni decay
Inter-Electrode Gap	200	µm	Controlled spacing in vacuum diode
Applied DC Bias	25	V	Used to ensure measured current is emitted electrons
Vacuum Base Pressure	10^-6	Torr	Operating environment during measurement
Maximum Operating Temperature	600	°C	Set-point, limited by H-desorption/stability

Key Methodologies

The experiment relied on precise MPCVD growth parameters for $\text{N}$-doped nanocrystalline diamond and a critical three-step post-growth hydrogen termination process.

Substrate Preparation: Molybdenum (Mo) substrates (10 mm x 10 mm x 0.5 mm) were seeded using an 18 nm nano-diamond particle suspension to facilitate nanocrystalline film nucleation.
MWCVD Diamond Growth:
- Reactor Type: 1.5 kW ASTeX MWCVD (2.45 GHz frequency).
- Substrate Temperature: Maintained between 850 °C and 900 °C.
- Process Gases: 4% $\text{CH}{4}$ in $\text{H}{2}$ carrier, with 0.4% $\text{N}{2}$ addition ($\text{CH}{4}:\text{N}_{2}$ ratio of 10:1).
- Pressure/Power: 130 Torr pressure, 1.3 kW input power.
- Duration: 15 minutes, yielding < 1 µm film thickness.
Hydrogen Termination (NEA Induction): A three-step plasma treatment sequence was used to produce the essential Negative Electron Affinity (NEA) surface required for low-work-function thermionic emission:
- Step 1 (Cleaning): 85 Torr $\text{H}_{2}$ plasma at 1,200 W (2 min).
- Step 2 (Chemisorption): 30 Torr $\text{H}_{2}$ plasma at 700 W (2 min).
- Step 3 (Cooling): Cooled for 2 min in 30 Torr $\text{H}_{2}$ gas.
Thermionic Testing Setup: Emitter films were housed in a stainless steel chamber at 10^-6 Torr base pressure. The sample was heated via a back-side 40 W $\text{CO}_{2}$ laser.
Beta Irradiation Source: The collector electrode (Mo foil) contained perforated holes exposing the emitter surface to a ⁶³Ni $\beta$ source (1.3 MBq effective activity).

6CCVD Solutions & Capabilities

This research demonstrates a critical pathway for highly efficient, lower-temperature thermionic energy conversion using customized diamond thin films. 6CCVD is uniquely positioned to supply and engineer the advanced MPCVD diamond materials required to replicate, scale, and industrialize this technology.

Material Requirements vs. 6CCVD Core Capabilities

Requirement in Paper	6CCVD Material Recommendation	6CCVD Engineering Advantage
Active Material (N-doped Nanocrystalline HTND)	Electronic Grade Polycrystalline Diamond (PCD): We provide high-consistency, nitrogen-doped PCD films. Our process control ensures the dense grain boundaries and conductivity enhancement necessary for stable thermionic performance at elevated temperatures.	PCD wafers available up to 125 mm diameter, allowing for large-scale, high-power TEC device development far beyond the 10x10 mm scale tested in the research.
Thickness Control (< 1 µm Thin Films)	Ultra-Thin MPCVD Films: 6CCVD guarantees thickness uniformity for both SCD and PCD from 0.1 µm up to 500 µm. This precise control is vital for optimizing heat conduction and electrical properties in thin-film TEC cathodes.	Customized growth recipes ensure uniformity across large areas, improving the statistical reliability of large sample sets (like the 8-film set used in the study).
Substrate Compatibility (Molybdenum for Strain Management)	Custom Substrate Integration: We routinely grow diamond films on diverse materials, including Molybdenum (Mo), Tungsten (W), Silicon Carbide (SiC), and Silicon (Si). We manage the thermal expansion mismatch and resulting film strain highlighted in the Raman analysis.	Our capabilities extend to providing the Mo substrates pre-seeded and ready for specialized growth runs to match specific experiment requirements.
Metalization & Device Contact (Collector/Anode Structure)	Full In-House Metalization Suite: 6CCVD offers deposition and patterning of refractory and noble metals including Ti, Pt, Au, Pd, W, and Cu. This is critical for creating the low-resistance electrical contacts and complex collector assemblies (Mo cap with Ni source) necessary for the vacuum diode setup.	We provide custom contact schemes (e.g., Ti/Pt/Au) optimized for high-temperature stability and vacuum compatibility required for TEC environments.
Precision Shaping (10 mm x 10 mm Emitter Plates)	Advanced Laser Cutting & Dicing: Utilize our high-precision laser cutting services to shape wafers into specific dimensions (e.g., 10x10 mm), or create complex features like the perforated Mo collector caps, ensuring maximum geometric consistency.	Substrates and films are delivered ready for mounting, saving researchers significant post-processing time and minimizing material damage.

Engineering Support

6CCVD’s in-house PhD team provides expert consultation on material selection and process optimization for energy conversion applications, including:

Hydrogen Termination Optimization: Assisting engineers in developing robust and reproducible NEA surface treatments, ensuring low work function and high thermal stability up to the critical 600 °C set point.
Doping Strategy: Consulting on the appropriate $\text{N}$ concentration or the use of alternative dopants (like Boron-Doped Diamond (BDD)) to tailor the carrier concentration and conductivity for next-generation thermionic emitters.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

Diamond-based thermionic emission devices could provide a means to produce clean and renewable energy through direct heat-to-electrical energy conversion. Hindering progress of the technology are the thermionic output current and threshold temperature of the emitter cathode. In this report, we study the effects on thermionic emission caused by in situ exposure of the diamond cathode to beta radiation. Nitrogen-doped diamond thin films were grown by microwave plasma chemical vapor deposition on molybdenum substrates. The hydrogen-terminated nanocrystalline diamond was studied using a vacuum diode setup with a 63Ni beta radiation source-embedded anode, which produced a 2.7-fold increase in emission current compared to a 59Ni-embedded control. The emission threshold temperature was also examined to further assess the enhancement of thermionic emission, with 63Ni lowering the threshold temperature by an average of 58 ± 11 °C compared to the 59Ni control. Various mechanisms for the enhancement are discussed, with a satisfactory explanation remaining elusive. Nevertheless, one possibility is discussed involving excitation of preexisting conduction band electrons that may skew their energy distribution toward higher energies.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fmech.2017.00017

References

2015 - Plasmon-induced hot carrier science and technology [Crossref]
2002 - Lattice damage caused by the irradiation of diamond [Crossref]
2010 - Multiscale three-dimensional simulations of charge gain and transport in diamond [Crossref]
2013 - Enhanced thermionic currents by non equilibrium electron populations of metals [Crossref]
1930 - Thermionic emission [Crossref]
2003 - Secondary electron emission from CVD diamond films [Crossref]
2008 - Theoretical models for doping diamond for semiconductor applications [Crossref]
1989 - Thermoemission reactor-converters for nuclear power units in outer space [Crossref]
2006 - Physics of Surfaces and Interfaces
2004 - On the thermionic emission from nitrogen-doped diamond films with respect to energy conversion [Crossref]