Silicon Carbide-Iridium Diffusion Bonding for Crack-Free Hermetic Joints
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Solution Overview
Problem
Existing methods for bonding silicon carbide (SiC) at high temperatures are prone to cracking due to the formation of brittle carbide and silicide phases, which compromises the strength and hermeticity of the joints, and other methods are complex, time-consuming, or non-hermetic.
Innovation Solution
A method involving the use of iridium foil between SiC layers, heated to 1500°C in a vacuum with applied pressure, forming a hermetic seal with a leak rate <3×10-9 cm3/sec, where approximately ⅓ of the joint is iridium and ⅔ is a reaction zone containing iridium silicides.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If refractory metals (niobium, titanium, molybdenum, nickel, tungsten) are used for diffusion bonding of SiC, then strong bonding is achieved, but the joints become susceptible to cracking at high temperatures due to brittle carbide and silicide phase formation
Solution Approach 1:
The patent changes the material parameter from conventional refractory metals to iridium, which has different chemical reactivity characteristics with SiC. This parameter change prevents the formation of brittle carbide and silicide phases while maintaining strong bonding through diffusion mechanisms, thereby achieving both strength and crack resistance at high temperatures.
Solution Approach 2:
The patent creates a composite bonding structure consisting of iridium metal and iridium silicide phases. This composite material approach allows the bond to combine the ductility and high-temperature stability of iridium with the adhesive properties of the silicide interface, preventing crack propagation while maintaining bond strength.
2Ease of manufacture
If refractory metals are used for diffusion bonding, then bonding is achieved, but hermetic seal cannot be formed due to crack formation
Solution Approach 1:
By changing the bonding material from conventional refractory metals to iridium, the chemical reaction parameters change to form a crack-free interface. This enables the formation of a hermetic seal while maintaining ease of manufacture through diffusion bonding processes.
3Strength
If silicide and carbide phases are formed in the joint, then bonding is achieved, but different physical properties (CTE, elastic modulus, thermal conductivity) produce stresses during thermal cycling leading to cracking
Solution Approach 1:
The patent changes the material composition from forming silicide and carbide phases to forming primarily iridium metal with minimal reaction products. This parameter change results in physical properties (CTE, elastic modulus, thermal conductivity) that are closer to SiC, reducing thermal stresses during cycling while maintaining bond strength.
Solution Approach 2:
The iridium bonding layer effectively copies or matches the physical properties of the SiC base materials, creating a transition layer with compatible thermal and mechanical characteristics. This property matching reduces interfacial stresses during thermal cycling, preventing crack formation while maintaining strong bonding.
4Reliability
If other bonding methods (brazing, polymer infiltration) are used to achieve hermetic seal, then hermeticity is achieved, but processing time increases to 8-14 days and free silicon may be detrimental during irradiation
Solution Approach 1:
The patent replaces complex chemical processes (brazing, polymer infiltration) with a simpler thermal diffusion process using iridium. This substitution achieves hermetic sealing through controlled diffusion bonding in significantly reduced time while avoiding the formation of detrimental free silicon phases that could occur in alternative methods.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method achieves a strong, durable, and hermetic SiC/iridium/SiC bond resistant to thermal cycling and high-temperature exposures, with minimal iridium usage and no observable cracking, maintaining hermeticity for extended periods.
Implementation Method 1
Diffusion bonding is one method used for joining SiC to SiC, and such bonding has been achieved using refractory metals foils such as niobium, titanium, molybdenum, nickel, and tungsten
Implementation Method 2
heating the iridium foil and SiC layers at a temperature of 1500° C. in a vacuum
Implementation Method 3
The first reaction zone and the second reaction zone include iridium silicides
Data Source
AI summary
Disclosed is a hermetic bond for a joint including a first layer of silicon carbide; a second layer of silicon carbide; and a bonding layer positioned between the first layer and the second layer, wherein the bonding layer includes an iridium layer, a first reaction zone positioned between the iridium foil layer and the first layer, and a second reaction zone positioned between the iridium foil layer and the second layer, wherein the first reaction zone and the second reaction zone include iridium silicides.