Microwave Spectroscopy as a Potential Tool for Color Grading Diamonds

At a Glance

Metadata	Details
Publication Date	2021-06-12
Journal	Energies
Authors	Yossi Rabinowitz, Ariel Etinger, Asher Yahalom, Haim Cohen, Yosef Pinhasi
Institutions	Ariel University, Ben-Gurion University of the Negev
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Microwave Spectroscopy for Diamond Characterization

Research Paper Analyzed: Microwave Spectroscopy as a Potential Tool for Color Grading Diamonds (Rabinowitz et al., Energies 2021, 14, 3507)

Executive Summary

This research validates a novel, non-destructive method for determining diamond purity and color grade by correlating microwave (MW) dielectric properties with nitrogen (N) contamination levels. This technique is highly relevant for engineers requiring precise material characterization.

Core Achievement: Established a strong, linear correlation between the S12 transmission parameter (or resonant peak frequency) and the nitrogen concentration (0 to 2960 ppm) in diamonds.
Methodological Advantage: MW spectroscopy (3.95-26.5 GHz) overcomes the limitations of traditional IR/UV-Vis methods, providing accurate color grading for both rough and polished stones, independent of surface geometry effects.
Material Sensitivity: The dielectric permittivity ($\epsilon_r$) of diamond is shown to be highly sensitive to N concentration, making MW measurement a reliable proxy for Type IIa purity assessment.
Key Frequencies: Optimal correlation was demonstrated using transmission measurements at 26.025 GHz and 7.0375 GHz, and resonant peak shifts in the 3.15-5.85 GHz range.
6CCVD Relevance: This research underscores the critical need for ultra-high purity, low-nitrogen Single Crystal Diamond (SCD) materials (Type IIa equivalent) for baseline measurements and high-frequency electronic applications. 6CCVD specializes in delivering SCD with guaranteed low N content.

Technical Specifications

The following hard data points were extracted from the experimental results, demonstrating the sensitivity and reliability of the MW spectroscopy method.

Parameter	Value	Unit	Context
Frequency Range (Tested)	3.95 - 26.5	GHz	Full range of MW spectroscopy
Nitrogen Concentration (Range)	0 - 2960	ppm	Master Diamonds (2a to L1 grades)
Optimal Transmission Frequency	26.025	GHz	Highest correlation linearity (WR42 waveguide)
S12 Average (2a, 26.025 GHz)	-23.01	dB	Lowest N concentration (0 ppm)
S12 Average (L1, 26.025 GHz)	-23.46	dB	Highest N concentration (2960 ppm)
S12 Standard Deviation (26 GHz)	0.06 - 0.09	dB	Indicates high measurement reliability
Resonator Frequency Range	3.15 - 5.85	GHz	Used WR187 waveguide with mirrors
Peak Shift Correlation	Decreases	GHz	Peak frequency decreases as N concentration increases
Sample Dimensions (Nominal)	4.32 x 4.33 x 2.68	mm	Round Brilliant Cut Master Diamonds
Sample Holder Material	Polyethylene	N/A	Low permittivity ($\epsilon_r \approx 2.2$) for high contrast

Key Methodologies

The experiment utilized standard microwave characterization techniques adapted for small diamond samples, focusing on precise S-parameter measurement across multiple frequency bands.

Sample Preparation: Four GIA-graded master diamonds (2a, D, H, L1) with known nitrogen concentrations (0 to 2960 ppm) were selected. All samples were brilliant cut and measured at a constant temperature of 24 °C.
Infrared (IR) Validation: A Bruker ALPHA II Fourier-transform infrared (FT-IR) spectrometer was used to validate the absence of plastic deformation and to quantitatively estimate nitrogen content via absorption peaks at 1095 cm^-1.
Transmission/Reflection Method: Complex scattering parameters (S11 and S12) were measured using a Keysight N5230 Vector Network Analyzer (VNA).
Waveguide Setup: Four standard rectangular waveguides were used to cover the full frequency range:
- WR187 (3.95-5.85 GHz)
- WR90 (8.2-12.4 GHz)
- WR62 (12.4-18 GHz)
- WR42 (18-26.5 GHz)
Resonant Method: The WR187 waveguide was modified with mirrors at both ends to create a resonant cavity, amplifying the effect of the diamond’s electromagnetic properties on the S21 parameter (transmission).
Sample Fixturing: Custom polyethylene containers were fabricated to precisely fix the diamonds within the waveguides, minimizing container effects due to polyethylene’s low permittivity ($\epsilon_r \approx 2.2$).

6CCVD Solutions & Capabilities

This research confirms that precise control over nitrogen concentration is paramount for both gemological grading and advanced engineering applications utilizing diamond’s unique dielectric properties. 6CCVD is uniquely positioned to supply the high-purity, custom-dimension diamond materials required to replicate and extend this research into commercial applications.

Applicable Materials for Replication and Extension

Research Requirement	6CCVD Material Solution	Technical Rationale
Type IIa Baseline (2a Diamond)	Optical Grade Single Crystal Diamond (SCD)	Guaranteed ultra-low nitrogen content (N < 1 ppm) essential for establishing the zero-contamination baseline required for accurate MW correlation.
High-Purity Substrates	Electronic Grade SCD Wafers	Required for developing high-frequency devices (e.g., RF, power electronics) where N-induced dielectric loss must be minimized. Available up to 500 µm thick.
Contamination Study (N-Doping)	Controlled N-Doped SCD	6CCVD can intentionally introduce controlled levels of nitrogen during MPCVD growth to create custom samples that precisely match or exceed the 40-2960 ppm range studied.
Future Dielectric Studies (Boron)	Boron-Doped Diamond (BDD)	The paper mentions Boron (B) contamination causes a blue tint. 6CCVD supplies BDD films for researchers investigating the effect of B-doping on MW permittivity and conductivity.

Customization Potential for MW Spectroscopy

The success of the MW method relies heavily on repeatable sample geometry and precise placement within the waveguide. 6CCVD offers comprehensive services to meet these stringent requirements:

Custom Dimensions: While the paper used small, brilliant-cut stones, 6CCVD can supply SCD or PCD plates/wafers up to 125mm (PCD) cut to precise dimensions (e.g., 4.32 mm x 4.33 mm) suitable for direct insertion into standard or custom waveguides (WR42, WR62, etc.).
Precision Polishing: To ensure consistent electromagnetic coupling and minimize surface scattering effects, 6CCVD guarantees ultra-smooth surfaces:
- SCD: Roughness average (Ra) < 1 nm.
- Inch-size PCD: Roughness average (Ra) < 5 nm.
Metalization Services: For researchers extending this work into active RF devices or requiring specific boundary conditions for resonant cavities, 6CCVD offers in-house metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu deposition.

Engineering Support

6CCVD’s in-house team of PhD material scientists specializes in optimizing MPCVD growth parameters to achieve specific impurity profiles (N, B) and crystal quality.

Application Focus: Our experts can assist engineers developing high-frequency components, quantum sensors, or high-power RF devices, where precise control of the diamond’s dielectric properties (as measured by MW spectroscopy) is essential.
Global Logistics: We ensure reliable global shipping (DDU default, DDP available) for time-sensitive research projects.

Call to Action: For custom specifications or material consultation regarding high-purity diamond substrates for microwave or high-frequency applications, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

A diamond’s color grading is a dominant property that determines its market value. Its color quality is dependent on the light transmittance through the diamond and is largely influenced by nitrogen contamination, which induces a yellow/brown tint within the diamond, as well as by structural defects in the crystal (in rare cases boron contamination results in a blue tint). Generally, spectroscopic instrumentation (in the infrared or UV-visible spectral range) is used in industry to measure polished and rough diamonds, but the results are not accurate enough for precise determination of color grade. Thus, new methods should be developed to determine the color grade of diamonds at longer wavelengths, such as microwave (MV). No difference exists between rough and polished diamonds regarding stray light when the MW frequency is used. Thus, several waveguides that cover a frequency range of 3.95-26.5 GHz, as well as suitable resonator mirrors, have been developed using transmission/reflection and resonator methods. A good correlation between the S12 parameter and the nitrogen contamination content was found using the transmission/reflection method. It was concluded that electromagnetic property measurements of diamonds in the MW frequency range can be used to determine their nitrogen content and color grading. The MW technique results were in good agreement with those obtained from the infrared spectra of diamonds.

Tech Support

Original Source

DOI: https://doi.org/10.3390/en14123507

References

2020 - Natural-color D-to-Z Diamonds: A Crystal-clear perspective [Crossref]
2018 - The effect of blue fluorescence on the colour appearance of round-brilliant-cut diamonds [Crossref]
2020 - Naturally colored yellow and orange gem diamonds: The nitrogen factor [Crossref]
2018 - Natural-color blue, gray, and violet diamonds: Allure of the deep [Crossref]
2018 - Natural-color pink, purple, red, and brown diamonds: Band of many colors
2019 - Natural-color fancy white and fancy black diamonds: Where color and clarity converge
2005 - Unusually large novelty cut [Crossref]