Ceramic Pressure Cell Brazing with Graded Active Solder
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Solution Overview
Problem
State-of-the-art pressure measuring cells face issues due to thermal expansion mismatches between ceramic components and active brazing materials, leading to stress and potential failure, as well as high melting points of suitable brazing alloys that are not compatible with ceramic components.
Innovation Solution
A pressure measuring cell assembly and method using an active hard solder with a core volume composition of Zr, Ni, and Ti, where the active braze has a cohesive core volume with a specific coefficient of thermal expansion, and boundary layers with a lower liquidus temperature, allowing for a pressure-tight joint with reduced thermal stress and compatible melting points.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If an active brazing alloy with a coefficient of thermal expansion matching the ceramic material is selected, then thermal stress between the ceramic components and the joint is reduced, but the melting point of the brazing alloy becomes too high for the ceramic electrodes to withstand
Solution Approach 1:
The active brazing alloy joint is segmented into two distinct compositional regions: a first region adjacent to the ceramic bodies with composition optimized for thermal expansion matching, and a second region away from the ceramic bodies with composition optimized for lower melting point. This segmentation allows each region to fulfill its specific functional requirement independently.
Solution Approach 2:
Different regions of the active brazing alloy are assigned different compositions with locally optimized properties. The first region has composition designed to match the thermal expansion coefficient of aluminum oxide ceramic, while the second region has composition designed for reduced melting temperature, enabling each local area to perform its specific function optimally.
2Reliability
If the ceramic components are joined using a conventional active brazing alloy, then a pressure-tight joint is achieved, but significant thermal stresses develop during cooling due to differing coefficients of thermal expansion
Solution Approach 1:
The brazing alloy is divided into functional regions with distinct compositions. The first region provides strong bonding to ceramic with matched thermal expansion properties, while the second region accommodates thermal stress through its lower melting point composition, thereby maintaining joint integrity while reducing thermal stress.
Solution Approach 2:
The active brazing alloy functions as a composite material system with two distinct compositional phases. This composite structure combines the benefits of thermal expansion matching near the ceramic interface with the advantages of lower melting point in the bulk region, resolving the stress-integrity contradiction.
3Strength
If the active brazing alloy is heated to a high temperature to melt the alloy, then the ceramic bodies are successfully joined, but the electrodes prepared on the ceramic bodies cannot withstand these temperatures
Solution Approach 1:
The brazing alloy composition is segmented such that the second region has a lower melting point than the first region. This allows the brazing process to occur at a temperature that is sufficient to melt the second region and form a strong joint, while remaining below the degradation temperature of the ceramic electrodes.
Solution Approach 2:
The composition parameters of the active brazing alloy are changed across different regions. By adjusting the compositional parameters, the melting point is reduced in the second region while maintaining adequate joint strength, enabling brazing at lower temperatures compatible with electrode materials.
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 solution effectively reduces thermal stress and maintains the mechanical properties of the ceramic components by using a core volume composition with minimal thermal expansion mismatch and boundary layers that solidify at lower temperatures, ensuring a durable and reliable joint.
Implementation Method 1
heated in a high vacuum to a temperature that allows the active brazing alloy to melt
Implementation Method 2
Upon cooling, the active brazing alloy solidifies
Implementation Method 3
the ceramic bodies with an intermediate solder mold are heated in a high vacuum to a temperature that allows the active brazing alloy to melt
Implementation Method 4
differing coefficients of thermal expansion of the ceramic body material on the one hand and the active brazing alloy on the other can lead to significant stresses between the ceramic components and the joint
Data Source
Figure 1
Figure 2
AI summary
Assembly, comprising: two ceramic bodies that are connected by means of a joint, which has an active hard solder, wherein the active hard solder has over a contiguous pore volume, which is respectively at a distance of at least 1 µm from the ceramic bodies, an average composition CK with a liquidus temperature T1(CK), wherein the composition CK has a coefficient of thermal expansion a(CK), for which a(CK) = m.a(K), where m ≤ 1.5, in particular m ≤ 1.3 and preferably m ≤ 1.2, where a(K) is the average coefficient of thermal expansion of the ceramic material of the ceramic bodies, wherein the joint has boundary layers which bound the ceramic bodies, wherein at least one of the boundary layers that lies outside the core volume has an average composition CG with a liquidus temperature T1(CG) that is not less than 50 K, preferably not less than 100 K, and particularly preferably not less than 200 K below the liquidus temperature T1(CK) of the average composition CK of the core volume.