Ceramic-Metal Brazed Sensing Assembly for Aggressive Media
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Solution Overview
Problem
Sensing assemblies in hostile environments, such as high-temperature reactors, face contamination and corrosion issues due to aggressive gases permeating through protective layers, leading to reduced measurement precision and shortened device lifespan.
Innovation Solution
A sensing assembly with a ceramic protective tube and a metallic connecting sleeve joined using an active brazing alloy, specifically titanium-copper-silver, to create a robust and gas-tight connection, minimizing gas permeation and enhancing durability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple protective layers (metals and ceramics) are used to protect the sensing element, then the protection against aggressive gases is improved, but the gases can still permeate through the layers at high temperature causing corrosion
Solution Approach 1:
The invention uses a composite structure consisting of a ceramic protective tube (alumina, silicon carbide, or silicon nitride) combined with a metallic connecting sleeve (stainless steel or nickel-based alloy). This composite material approach creates a barrier that combines the high-temperature stability and chemical resistance of ceramics with the mechanical strength and gas-tight sealing capability of metals, effectively preventing aggressive gas permeation that single materials cannot withstand
Solution Approach 2:
The invention applies different material properties to different parts of the protective structure: the ceramic portion faces the aggressive process environment where chemical resistance is critical, while the metallic sleeve portion provides mechanical support and gas-tight sealing at the connection area. This localized material assignment optimizes protection where needed most
2Ease of manufacture
If cement is used to connect the ceramic protective tube and metallic connecting sleeve, then the assembly is easy to manufacture, but the connection is not gas-tight allowing aggressive gases to enter
Solution Approach 1:
The invention changes the connection method from cement bonding to active brazing, which involves heating the assembly to high temperature (above the melting point of the brazing alloy) to create a metallurgical bond. This parameter change in the joining process transforms the connection from porous and gas-permeable to gas-tight and reliable, while still maintaining manufacturing feasibility through a standardized brazing procedure
3Temperature
If the sensing assembly is designed for high-temperature resistance, then it can withstand harsh environments, but the materials become porous at high temperature allowing gas diffusion
Solution Approach 1:
The invention selects specific ceramic materials (alumina, silicon carbide, or silicon nitride) that maintain structural integrity and low porosity at high temperatures up to 1800°C. The ceramic protective tube is designed with appropriate density and microstructure to prevent gas diffusion even when exposed to prolonged high-temperature conditions, while the metallic sleeve compensates for any minor thermal expansion or structural changes
4Reliability
If a braze connection is used between ceramic and metal, then gas-tight connection is achieved, but the manufacturing process becomes more complex
Solution Approach 1:
The invention uses an active brazing alloy (containing titanium, copper, and silver) as an intermediary material that facilitates bonding between the ceramic protective tube and metallic connecting sleeve. The active brazing alloy contains elements that react with both ceramic and metal surfaces, creating strong chemical bonds and ensuring gas-tight sealing. This intermediary approach simplifies the overall manufacturing process compared to attempting direct ceramic-metal bonding, while achieving reliable gas-tight connections
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 significantly increases the lifespan of sensing assemblies by preventing aggressive gas ingress and maintaining measurement accuracy in harsh conditions, withstanding high pressures and temperatures up to 1800°C.
Implementation Method 1
The protective tube and the connecting sleeve are joint together at least in a predefined area by means of a braze produced via a brazing component arranged in the predefined area
Data Source
Figure 1a~1
Figure 2~3
Figure 4a~4
AI summary
The present invention refers to a sensing assembly (1) for sensing a process variable of a medium, preferably for sensing a temperature, comprising one measuring insert (6) for sensing the process variable, preferably for sensing a temperature, one protective tube (11) at least partially surrounding the measuring insert (11), which protective tube (11) at least partially consists of a ceramic material and which protective tube (11) is embodied to protect at least the measuring insert (6) against at least one process condition, and a connecting sleeve (14), which connecting sleeve (14) at least partially consists of a metallic material, and which connection sleeve (14) is at least surrounding the protective tube (11) in an area facing away from the medium. According to the present invention, the protective tube (11) and the connecting sleeve (14) are joint together at least in a predefined area (A) by means of a braze produced via a brazing component (17) arranged in the predefined area (A). The present invention as well relates to a method for manufacturing a sensing assembly (1) according to the present invention.