Metal Mandrel Temperature Control for Silica Soot OVD
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Solution Overview
Problem
The use of metal mandrels in the Outside Vapour Deposition (OVD) process for producing hollow cylindrical bodies of synthetic vitreous silica is limited due to issues such as differential thermal expansion, which leads to cracking of the silica soot, and corrosion, resulting in contamination and high costs, especially for large diameter applications.
Innovation Solution
The process involves careful control of the mandrel temperature to maintain it within narrow bounds throughout the deposition, using metal or metal alloy mandrels with appropriate coatings, and employing chlorine-free silica precursors to prevent contamination and cracking, allowing for the successful production of large diameter silica soot bodies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If metal mandrels are used in the OVD process, then costs are reduced and large diameter deposition is enabled, but differential thermal expansion causes cracking of the silica soot and corrosion leads to contamination
Solution Approach 1:
A boron nitride coating layer is applied to the metal mandrel surface to act as an intermediary barrier. This coating prevents direct contact between the metal mandrel and silica soot, eliminating corrosion and contamination while also providing thermal insulation to reduce differential thermal expansion effects. The coating enables metal mandrels to be used successfully for large diameter OVD processes.
Solution Approach 2:
The deposition temperature is carefully controlled and maintained within a limited range (600-1100°C) to minimize thermal expansion differences between the metal mandrel and silica soot. By limiting the temperature range and using a thermally insulating coating, the patent reduces thermal stress that would otherwise cause cracking, enabling successful deposition on metal mandrels.
2Length of stationary object
If large diameter ceramic mandrels are used, then large diameter silica bodies can be produced, but the mandrels are fragile and at high risk of breakage
Solution Approach 1:
The patent uses a composite structure consisting of a metal mandrel core providing mechanical strength and durability, combined with a boron nitride coating layer providing chemical inertness and thermal insulation. This composite approach combines the advantages of both materials: the metal provides reliability and resistance to breakage, while the coating protects the silica soot from contamination.
Solution Approach 2:
Metal mandrels with boron nitride coating are more durable and can be reused multiple times compared to traditional ceramic mandrels that are fragile and at high risk of breakage. The enhanced reliability allows for repeated use in large diameter OVD processes, reducing overall manufacturing costs.
3Productivity
If deposition temperature is increased to improve deposition rate, then productivity increases, but thermal expansion difference increases causing more cracking
Solution Approach 1:
The boron nitride coating serves as a thermal barrier between the metal mandrel and silica soot, reducing heat transfer and minimizing thermal expansion differences. This allows the deposition process to proceed at higher temperatures with reduced risk of cracking, thereby improving productivity while maintaining silica soot integrity.
Solution Approach 2:
The patent optimizes the deposition temperature range (600-1100°C) to balance productivity and silica soot integrity. By using the thermally insulating boron nitride coating, the process can operate at the upper end of this range with reduced thermal stress, achieving higher deposition rates without excessive cracking.
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 enables the use of metal mandrels for producing large diameter hollow cylindrical porous bodies of synthetic vitreous silica, reducing costs and minimizing contamination, while maintaining the integrity of the silica soot, and allows for the manufacture of hollow ingots with diameters greater than 50 mm, preferably greater than 100 mm, or more preferably greater than 300 mm.
Implementation Method 1
The precursor is converted in the flame by oxidation or hydrolysis into a stream of nanoparticles of silica
Implementation Method 2
The precursor is converted in the flame by oxidation or hydrolysis into a stream of nanoparticles of silica
Implementation Method 3
this stream is directed at a rotating substrate or mandrel on which the particles collect as a porous silica soot body
Data Source
AI summary
Processes for the manufacture of a hollow cylindrical porous body of synthetic vitreous silica soot by outside vapour deposition on a mandrel are described, in which the temperature of the mandrel is controlled to be substantially constant throughout the deposition process. In preferred embodiments, the mandrel is composed of metal or metal alloy. Hollow ingots of pure or doped synthetic vitreous silica glass manufactured by such processes are also described.