Isotope Effect in Thermal Conductivity of Polycrystalline CVD-Diamond - Experiment and Theory

At a Glance

Metadata	Details
Publication Date	2021-03-24
Journal	Crystals
Authors	A. V. Inyushkin, А. Н. Талденков, Victor Ralchenko, A. P. Bolshakov, А. В. Хомич
Institutions	Harbin Institute of Technology, Kurchatov Institute
Citations	5
Analysis	Full AI Review Included

Isotope Effect in Thermal Conductivity of Polycrystalline CVD-Diamond: Technical Analysis and 6CCVD Solutions

This document analyzes the findings of “Isotope Effect in Thermal Conductivity of Polycrystalline CVD-Diamond: Experiment and Theory” to provide technical specifications and highlight how 6CCVD’s advanced MPCVD diamond capabilities can support and extend this critical research in high-performance thermal management.

Executive Summary

Record Thermal Performance: High-quality Polycrystalline CVD (PCD) diamond, isotopically enriched to 99.96% $^{12}$C, achieved a room temperature thermal conductivity ($\kappa$) of $25.1 \pm 0.5 \text{ W cm}^{-1} \text{ K}^{-1}$.
Superiority Confirmed: This measured $\kappa$ value exceeds the highest reported conductivity for natural isotopic composition Single Crystal Diamond (SCD).
Significant Isotope Effect: The relative difference in thermal conductivity ($\Delta\kappa/\kappa$) between $^{12}$C and natC PCD peaked at 75% at 150 K.
Future Potential: Theoretical modeling predicts that eliminating extrinsic defects (vacancies, dislocations) could increase the room temperature isotope effect up to 37.5%, pushing potential $\kappa$ values even higher.
Growth Method: High-quality PCD wafers were synthesized using Microwave Plasma CVD (MPCVD) under controlled conditions (e.g., $87 \text{ Torr}$ pressure, $820^\circ \text{C}$ substrate temperature).
Scattering Mechanisms: The primary extrinsic phonon scattering mechanisms limiting thermal performance were identified as grain boundaries, point defects (vacancies), and dislocations.
Application Relevance: This research validates isotopically enriched PCD as a leading material for extreme heat dissipation in advanced electronics, optics, and high-power RF applications.

Technical Specifications

Parameter	Value	Unit	Context
Thermal Conductivity ($\kappa$)	25.1 $\pm$ 0.5	W cm^-1 K^-1	$^{12}$C PCD sample, T = 298 K (Room Temperature)
Thermal Conductivity ($\kappa$)	18.6	W cm^-1 K^-1	natC PCD sample, T = 298 K
Isotopic Enrichment	99.96	at.%	$^{12}$C content in enriched methane source gas
Maximum Isotope Effect ($\Delta\kappa/\kappa$)	75	%	Observed at T = 150 K
Estimated Max Isotope Effect (RT)	37.5	%	Predicted for defect-free $^{12}$C PCD at 298 K
Substrate Temperature	820	°C	MPCVD growth parameter
Growth Rate	$\approx$ 1.5	µm/h	MPCVD growth rate
Pressure	87	Torr	MPCVD growth parameter
Mean Grain Size (Growth Side)	$\approx$ 80	µm	$^{12}$C sample
Sample Dimensions (Cross Section)	2.00 x 0.328	mm²	$^{12}$C sample for $\kappa_{
Sample Length	$\approx$ 14	mm	Longitudinal heat flow measurement
Nitrogen Concentration ([N])	$\approx$ 1.9	ppm	Higher in $^{12}$C sample, measured via optical absorption

Key Methodologies

The high-quality polycrystalline diamond wafers were produced using controlled MPCVD techniques and precise sample preparation:

Synthesis Method: Microwave Plasma Chemical Vapor Deposition (MPCVD) was used to grow PCD wafers up to 57 mm in diameter.
Gas Source: Isotopically-enriched $^{12}$CH$_{4}$ (99.96% $^{12}$C) was used, converted from $^{12}$CO and purified via cryogenic rectification.
Growth Parameters:
- Total Gas Flow: 1000 sccm.
- CH${4}$ Content: 1.2% in H${2}$.
- Pressure: 87 Torr.
- Substrate Temperature: $820^\circ \text{C}$.
- Microwave Power: 4.4 kW.
Sample Preparation: Rectangular parallelepipeds were cut from the plates using laser cutting for in-plane thermal conductivity ($\kappa_{||}$) measurement.
Defect Layer Removal: A heavily defective layer from the nucleation side was removed: $\approx 50 \text{ µm}$ for the natC sample (mechanical polishing) and $\approx 130 \text{ µm}$ for the $^{12}$C sample (laser ablation).
Thermal Measurement: Thermal conductivity $\kappa_{||}(T)$ was measured using a steady-state longitudinal heat flow method in a vacuum environment across a broad range (5 K to 410 K).
Modeling: Experimental data was fitted using a phenomenological theoretical model based on the full Callaway theory, incorporating scattering from isotopes, point defects, dislocations, and grain boundaries.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality MPCVD diamond materials and customization services required to replicate, scale, and advance the thermal management research presented in this paper.

Research Requirement	6CCVD Solution & Value Proposition
High-Quality Polycrystalline CVD Diamond	PCD Wafers (Up to 125mm): 6CCVD specializes in high-purity MPCVD PCD plates. We offer custom dimensions up to 125mm, significantly larger than the 57mm wafers used in the study, enabling industrial scaling of high-power thermal spreaders.
Isotopic Enrichment & Defect Control	Custom Material Synthesis: While the paper used 99.96% $^{12}$C, 6CCVD offers custom synthesis runs for both Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) to explore isotopically enriched materials. This is crucial for achieving the predicted 37% thermal conductivity enhancement by minimizing intrinsic isotope scattering.
Precise Dimensional Requirements	Custom Laser Cutting & Shaping: The study required precise rectangular parallelepipeds (e.g., $2.00 \times 0.328 \text{ mm}^{2}$) for accurate $\kappa_{
Surface Quality and Defect Layer Removal	Advanced Polishing Services: The paper emphasized the need to remove the defective nucleation layer (up to $130 \text{ µm}$). 6CCVD offers high-precision polishing for PCD (Ra < 5nm for inch-size wafers) and SCD (Ra < 1nm), ensuring the final product presents the highest quality, low-defect growth surface for optimal thermal performance.
Thermal Resistance Analysis (Grain Boundaries/Defects)	Engineering Support for Defect Engineering: The research identifies grain boundaries, vacancies, and dislocations as key extrinsic scatterers. 6CCVD’s in-house PhD team provides expert consultation on material selection and post-growth processing (e.g., annealing) to minimize these defects, optimizing PCD for high-frequency and high-power thermal applications.
Integration and Device Fabrication	Internal Metalization Capability: For researchers integrating these high-$\kappa$ diamond materials into devices (e.g., GaN-on-Diamond), 6CCVD offers custom metalization services, including deposition of Au, Pt, Pd, Ti, W, and Cu stacks, ensuring robust electrical and thermal contacts.

Applicable Materials:

To replicate or extend this research, 6CCVD recommends the following materials:

Thermal Grade Polycrystalline Diamond (PCD): For high-volume, large-area thermal spreaders, offering excellent $\kappa$ and mechanical stability.
High-Purity Single Crystal Diamond (SCD): For fundamental studies aiming to eliminate grain boundary scattering and maximize the isotope effect, leveraging SCD’s inherently lower defect density.
Custom Isotopic Diamond: For projects requiring the highest possible thermal conductivity, 6CCVD can facilitate the synthesis of isotopically enriched PCD or SCD materials.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We measured the thermal conductivity κ(T) of polycrystalline diamond with natural (natC) and isotopically enriched (12C content up to 99.96 at.%) compositions over a broad temperature T range, from 5 to 410 K. The high quality polycrystalline diamond wafers were produced by microwave plasma chemical vapor deposition in CH4-H2 mixtures. The thermal conductivity of 12C diamond along the wafer, as precisely determined using a steady-state longitudinal heat flow method, exceeds much that of the natC sample at T>60 K. The enriched sample demonstrates the value of κ(298K)=25.1±0.5 W cm−1 K−1 that is higher than the ever reported conductivity of natural and synthetic single crystalline diamonds with natural isotopic composition. A phenomenological theoretical model based on the full version of Callaway theory of thermal conductivity is developed which provides a good approximation of the experimental data. The role of different resistive scattering processes, including due to minor isotope 13C atoms, defects, and grain boundaries, is estimated from the data analysis. The model predicts about a 37% increase of thermal conductivity for impurity and dislocation free polycrystalline chemical vapor deposition (CVD)-diamond with the 12C-enriched isotopic composition at room temperature.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst11040322

References

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1993 - Thermal conductivity of isotopically modified single crystal diamond [Crossref]
1991 - Effect of 13C isotopes on the diamond thermal conduction in the approximation of the dominant role of normal phonon-scattering processes
1992 - Thermal conductivity of isotopically enriched diamonds [Crossref]
1992 - Lattice dynamics and Raman spectra of isotopically mixed diamond [Crossref]
1998 - Interpretation of the thermal conductivity of isotopically depleted diamonds [Crossref]
2002 - Kinetic coefficients in isotopically disordered crystals [Crossref]
2002 - Estimation of the isotope effect on the lattice thermal conductivity of group IV and group III-V semiconductors [Crossref]