Production and Heat Properties of an X-ray Reflective Anode Based on a Diamond Heat Buffer Layer

At a Glance

Metadata	Details
Publication Date	2020-01-06
Journal	Materials
Authors	Xinwei Li, Xin Wang, Ye Li, Yanyang Liu
Institutions	Changchun University of Science and Technology
Citations	6
Analysis	Full AI Review Included

Executive Summary

This research details the design, production, and thermal validation of a novel X-ray reflective anode incorporating a diamond heat buffer layer (DHBL) to significantly enhance heat dissipation in micro-focus X-ray sources.

Core Achievement: The DHBL composite anode successfully increased the working power limit by approximately two times compared to conventional tungsten anodes (73 W vs. 37.9 W).
Thermal Performance: Utilizing diamond’s high thermal conductivity, the anode rapidly conducts and removes the high heat flux concentrated in the micro-focus spot, ensuring thermal stability and preventing target surface destruction.
Structural Integrity: Cross-sectional analysis via Scanning Electron Microscopy (SEM) confirmed robust, smooth, and crack-free bonding between all layers (Tungsten, Diamond, Solder, Copper Substratum), validating the metallization and vacuum soldering processes.
Metallization Strategy: A Titanium Carbide (TiC) layer was formed via high-temperature annealing of a vacuum-evaporated Ti film, providing the necessary stable Ohmic contact and adhesion between the diamond and the solder layer.
Validation Methodology: A reliable hybrid method combining Finite Element Analysis (FEA) modeling with indirect infrared radiation spectral measurements was established to accurately determine the vertical temperature distribution within the complex multi-layer structure.
Heat Transfer Mechanism: The diamond layer facilitates superior vertical heat transfer from the tungsten target to the copper substratum, minimizing temperature rise at the focal spot surface (up to 463 °C lower than copper-only anodes at 300 s).

Technical Specifications

Parameter	Value	Unit	Context
Diamond Thermal Conductivity (TC)	1893	W/mK	Measured property of the DHBL.
Diamond Thickness	0.5	mm	Thickness of the prepared heat buffer layer.
Thermal Stress Coefficient (Diamond)	1.2 x 10^-6	/°C	Measured property of the DHBL.
Tungsten Target Film Thickness	2.2	µm	Deposited via magnetron sputtering.
Solder Layer Thickness	15	µm	Layer bonding DHBL to Copper Substratum.
Reference Focal Spot Size	20	µm	Standard size used for testing and simulation.
DHBL Anode Power Limit	73	W	Limited by Tungsten melting point (3400 °C).
Conventional Tungsten Anode Power Limit	37.9	W	Limited by Tungsten melting point (3400 °C).
Silver Buffer Anode Power Limit	37.2	W	Limited by Silver melting point (962 °C).
Max Surface Temp Difference (300 s)	463	°C	DHBL anode surface vs. Only Copper anode surface (under constant power).
FEA Validation Error	< 9.2	%	Average error between simulated and measured surface temperature values.
Infrared Detection Range	900-2500	nm	Wavelength range used for indirect surface temperature measurement.
Diamond Raman Peak	1330	cm^-1	Characteristic absorption peak confirming diamond structure.

Key Methodologies

The production and validation of the diamond composite anode involved specialized material processing and advanced thermal analysis techniques:

Anode Production Process

Diamond Heat Buffer Layer (DHBL) Preparation:
- Method: Plasma Enhanced Chemical Vapor Deposition (PECVD).
- Substrate: Mo slice (used temporarily).
- Goal: Achieve uniform homogeneity and high thermal conductivity (1893 W/mK).
Surface Metallization (Adhesion Layer):
- Material: Titanium (Ti).
- Process: Vacuum-evaporation coating followed by high-temperature annealing.
- Result: Ti reacted with diamond to produce a stable Titanium Carbide (TiC) structure, ensuring high adhesion and Ohmic contact necessary for subsequent soldering.
Substratum Soldering:
- Conditions: High temperature and vacuum environment.
- Materials: Metallized DHBL, special proportion solder alloy (15 µm thick), and copper substratum.
- Control: Adopted a gradient temperature profile during heating and cooling to mitigate stress and prevent cracking caused by material thermal expansion mismatch.
Tungsten Target Plating:
- Method: Magnetron Sputtering (utilized for high-quality film deposition).
- Material: General tungsten slice used as the target.
- Result: Deposition of a 2.2 µm thick tungsten film on the exposed diamond surface.

Thermal and Structural Testing

Inter-Layer Bonding Test:
- Technique: Cross-section characterization using Scanning Electron Microscopy (SEM).
- Focus: Observation of smoothness, coverage, and absence of gaps or cracks between the Tungsten, Diamond, Solder, and Copper layers.
Surface Temperature Measurement (Indirect):
- Technique: Infrared Imaging Spectrography (900-2500 nm range) combined with spectral analysis.
- Process: Infrared images were obtained, and the temperature at the focal spot was calculated indirectly by analyzing the spectral curves corresponding to effective CCD pixels.
Vertical Temperature Distribution Analysis:
- Technique: Finite Element Method (FEM) using ANSYS software.
- Model: Two-dimensional unsteady heat transfer model, approximating the electron beam bombardment area as heating up evenly.
- Validation: FEM results were verified against the experimentally measured surface temperatures (average error < 9.2%).

Commercial Applications

The diamond composite anode technology is critical for applications requiring high-intensity, high-resolution X-ray generation, where thermal management is the primary limiting factor for performance.

High-Resolution X-ray Imaging: Enables the use of smaller focal spots (down to 20 µm) at higher power levels, directly improving the spatial resolution of images in accordance with projection imaging principles.
Non-Destructive Testing (NDT): Used in industrial inspection systems for quality control of complex components, such as microelectronics, printed circuit boards (PCBs), and advanced materials.
Micro-focus Computed Tomography (CT): Provides the necessary X-ray intensity and stability for high signal-to-noise ratio (SNR) imaging in 3D reconstruction, particularly for small samples.
Advanced X-ray Sources: Applicable in next-generation X-ray tubes (reflective sources) where maximizing X-ray intensity is necessary while minimizing structural complexity and vibration (unlike rotating or liquid-metal anodes).
Materials Science Research: Supports high-power X-ray diffraction and scattering experiments requiring stable, high-flux sources.

View Original Abstract

This paper introduces an X-ray reflective anode with a diamond heat buffer layer, so as to improve heat dissipation of micro-focus X-ray sources. This also aids in avoiding the destruction of the anode target surface caused by the accumulation of heat generated by the electron beam bombardment in the focal spot area. In addition to the description of the production process of the new reflective anode, this study focuses more on the research of the thermal conductivity and compounding ability. This paper also introduces a method that combines finite element analysis (FEA) in conjunction with thermal conductivity experiments, and subsequently demonstrates the credibility of this method. It was found that due to diamonds having a high thermal conductivity and melting point, high heat flux produced in the micro-focus spot region of the anode could be conducted and removed rapidly, which ensured the thermal stability of the anode. Experiments with the power parameters of the radiation source were also completed and showed an improvement in the power limit twice that of the original.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13010241

References

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