Pulsed Reactant Gas Flow for Diffusion-Limited CVI/CVD Densification
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Solution Overview
Problem
Conventional CVI/CVD processes face challenges with pore occlusion due to soot and tar accumulation, leading to inefficient densification and increased manufacturing costs, as pores become blocked, requiring multiple cycles and costly machining to reopen them.
Innovation Solution
A method and system that pulsate the flow rate of reactant gases during the CVI/CVD process to enhance mass transfer, using a mass flow controller to alter flow rates and heat the gas before introduction, thereby disturbing the boundary layer and improving infiltration efficiency within a single processing cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional isothermal CVI/CVD process is used, then the process is simple to operate, but pore occlusion occurs due to soot and tar accumulation leading to inefficient densification
Solution Approach 1:
The patent applies periodic action by implementing cyclic temperature variations during the CVI/CVD process. The temperature is periodically increased above the deposition temperature to combust soot and tar deposits, then reduced to allow controlled deposition. This periodic heating and cooling cycle prevents pore occlusion while maintaining process simplicity, resolving the contradiction between ease of operation and densification efficiency.
2Ease of manufacture
If conventional CVI/CVD process is used, then manufacturing costs are controlled, but multiple cycles and machining are required to reopen occluded pores
Solution Approach 1:
The patent implements continuity of useful action by maintaining the CVI/CVD process in continuous operation with periodic temperature cycles rather than stopping for machining interventions. The periodic combustion phases continuously clear pores of soot and tar accumulation, allowing the densification process to proceed uninterrupted through multiple deposition cycles without requiring removal and machining of the workpiece, thus reducing manufacturing time while controlling costs.
3Stability of the object's composition
If reactant gas flows at nominal rate, then the process is stable, but boundary layer formation reduces mass transfer efficiency
Solution Approach 1:
The patent applies dynamics by transitioning from static nominal gas flow to dynamic pulsed gas flow. The gas flow rate is periodically varied between nominal conditions and enhanced pulse conditions that temporarily disrupt the boundary layer. This dynamic flow variation maintains overall process stability while significantly enhancing mass transfer efficiency during pulse phases, allowing reactants to penetrate deeper into the porous structure.
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
This approach allows for densification of carbon/carbon parts to a commercially viable density in a single cycle, reducing manufacturing time and costs by preventing pore occlusion and enhancing reactant infiltration, thus improving the quality and efficiency of the CVI/CVD process.
Implementation Method 1
The reactant gas may form a boundary layer in proximity to an axially top surface and an axially bottom surface of the annular porous structure
Implementation Method 2
pulsing the reactant gas into the ID volume to disturb the boundary layer and enhance a mass transfer process
Implementation Method 3
The preheater may heat the reactant gas to a defined temperature before flowing the reactant gas into the ID volume
Implementation Method 4
The gas diffuses into the stack of porous materials, driven by concentration gradients, and undergoes a CVD reaction such as thermal decomposition, hydrogen reduction, co-reduction, oxidation, carbidization, or nitridation to deposit a binding matrix
Implementation Method 5
undergoes a CVD reaction such as thermal decomposition, hydrogen reduction, co-reduction, oxidation, carbidization, or nitridation to deposit a binding matrix
Data Source
AI summary
A system and method for enhancing a diffusion limited CVI/CVD process is provided. The system may densify a porous structure by flowing a reactant gas around the porous structure. A mass flow controller may be configured to pulse the flow rate of the reactant gas around the porous structure. The mass flow controller may pulse the flow rate from a nominal flow rate to a first flow rate. The mass flow controller may pulse the first flow rate back to the nominal flow rate or to a second flow rate. The mass flow controller may pulse the flow rate between the nominal flow rate, the first flow rate, and the second flow rate, as desired.