Technical

Design Considerations

1. Voltage, Current, Power

Understanding the electrical requirements of a seal assembly is foundational in preventing dielectric breakdown, managing current capacity, and ensuring efficient power distribution.

Voltage management is essential in ensuring continued electrical isolation. Dielectric breakdown prevention is guided by Paschen’s Law, which informs the minimum gap sizes necessary to avoid electrical arcing between conductors at different potentials. For instance, in a vacuum environment, where dielectric strength is higher, smaller gaps can be used compared to air, allowing for more concise geometry on the vacuum side without compromising safety. This principle is crucial where size constraints are critical, and space is at a premium. Designing around customers’ requirements is paramount in ensuring efficient use of materials for the performance required.

For electrical feedthroughs – where high current flow is desired – the capacity is determined by the size and choice of conductive materials, such as copper or copper alloys, known for their low electrical resistance and high thermal conductivity. In high-power applications, like power supply units for medical imaging equipment, ensuring the correct conductor size and material is vital in handling the current-induced thermal load without significant resistive losses.

Power feedthroughs integrate design elements of high voltage and current, focusing on materials and geometries that can withstand high electrical throughput and dissipate heat efficiently. This is crucial in high-power electronics where overheating can lead to failure or reduced lifespan.

2. Joint Construction

The joint construction used between ceramic and metal components is critical for achieving mechanical strength and hermeticity. The choice of joint – compression, butt/face, or internal – depends on the application's specific requirements, such as mechanical or thermal loading (thermal-cycling), or simply space constraints within the given system.

Compression seals, which apply external pressure to maintain the seal, are suitable for applications requiring robustness, such as in pressure sensors used in deep-sea oil and gas. This type of joint construction can induce stress on ceramic, necessitating precise tolerancing and materials selection to avoid unnecessary stress concentrations.

Butt or face seals offer a direct material interface and require precise control over thermal expansion coefficients to prevent cracking due to thermal stress gradients. This is particularly relevant in larger assemblies, where the relative displacement of adjoining parts is much greater at elevated temperatures.

Internal seals, protected from external environmental factors, are complex to manufacture but essential in devices like pacemakers, where internal stress management and protection from bodily fluids are paramount.

3. Design Loads

Designing ceramic-to-metal seal assemblies requires an intricate understanding of the mechanical properties of ceramics, as well as the stresses these materials may encounter in their final application. Ceramics are renowned for their exceptional hardness, often displaying values on the Vickers scale significantly higher than many metals. In addition, ceramics exhibit excellent properties when placed under compressive loads, but less favourable properties when stressed in tension. These factors make ceramics both desirable for their durability and challenging due to their brittleness and susceptibility to fracture under certain conditions. The key to optimising ceramic design lies in managing how loads are applied to these materials, ensuring that stresses are oriented in a manner that the ceramic can withstand.

Optimising Joint Construction

The design of the joint between ceramic and metal components is critical in managing the inherent weaknesses within the ceramic substrate. Different joint constructions (whether that be compression, butt/face, or internal) provide various approaches to stress management:

Managing Pressure

Design loads, particularly those involving internal and external pressurisation of a ceramic seal assembly, require careful consideration. Excessive pressure can induce stress concentrations in ceramics, leading to premature failure. Designs must account for the maximum expected pressure differential and ensure that the ceramic components are supported or reinforced accordingly to ensure even stress distribution. Empirical testing in our production facility allows for real-world validation of design concepts, ensuring that assemblies can withstand their intended operational pressures without compromising integrity.

Lateral and Torsional Loading

Lateral and torsional loads are particularly detrimental to ceramic seal assemblies as they induce bending and axial tensile stresses, respectively. Ceramics' poor tolerance for tensile stress makes it critical to minimise these load types. Brittle fracture of ceramics under tensile stress can be understood through the Griffith criterion, which account for the presence of microscopic flaws in a brittle material and how these can lead to catastrophic failure under relatively low tensile stresses. Reducing this eventuality can be achieved by controlling the maximum permittable stress in the ceramic when subject to tensile forces.

Complex Loading and Residual Stresses Post-Brazing

The brazing process, essential for creating ceramic-to-metal seals, introduces another layer of complexity in stress management. The heating and cooling cycles can induce residual stresses due to the differential thermal expansion between the ceramic and metal components. These residual stresses must be carefully managed to prevent the initiation and propagation of cracks in the ceramic or braze joint. This is particularly important in the context of the previously discussed loading conditions, as residual stresses can exacerbate the effects of external loads.

Design strategies to manage these challenges include the selection of braze alloys with compatible thermal expansion characteristics, the use of gradual heating and cooling cycles to minimise thermal shock, and the incorporation of stress-relieving features in component parts.

4. Termination

The termination method must ensure mechanical robustness and, where applicable, hermeticity. Typical choices include welded, flanged, and threaded terminations.

Welding provides a strong, permanent connection but can introduce thermal stresses, requiring careful selection of alloys. Welded terminations have the advantage of smaller footprint, making them the preferred solution where space is limited and high hermeticity is required.

Flange connections offer a balance between strength and assembly flexibility, suitable for high vacuum systems in various applications, and allowing for easy integration with standard components. Typical flange choices include ConFlat® and Klein.

Threaded connections that utilise O-rings are preferred for their simplicity and effectiveness in sealing, though they do not offer the same level of hermeticity as flanged connections. As such, these are commonly used in low vacuum applications.

5. Working Environment

The choice of seal assembly must also account for the operating temperature, working atmosphere, and potential for magnetic interference. Each require careful consideration as these determine the most appropriate material and design choices for the respective application.

The operating temperature range influences metal and braze alloy selection, ensuring thermal compatibility across the assembly's lifespan. The operating atmosphere, particularly in vacuum applications, demands materials that minimise outgassing and other sources of contaminants such as braze migration. Moreover, correct choice of braze alloy much be ensured to prevent loss of braze volume during repeat thermal-cycling. Materials' magnetic permeability is also a consideration in environments where magnetic interference could be detrimental. In high magnetic fields, magnetic components can displace considerably, resulting in additional mechanical loading on the assembly.

6. Tolerancing

Brazing ceramic-to-metal assemblies is a complex process that balances precision with the inherent variability of high-temperature bonding techniques. Achieving the desired positional accuracy of components during the brazing cycle presents unique challenges, necessitating specialised approaches to ensure assembly integrity and dimensional accuracy.

Materials

1. Alumina & Sapphire

Understanding the materials used in ceramic-to-metal seal assemblies is pivotal for achieving the desired performance and reliability. These assemblies rely heavily on the unique properties of ceramics like alumina and sapphire, each offering distinct advantages for specific applications. Alumina substrates form the backbone of ceramic-to-metal seal assemblies, serving multiple roles from electrical insulation and thermal management to withstanding mechanical loads induced by pressure, vacuum, or thermal changes. Alumina's inherent strength and durability are crucial, yet it requires careful handling to prevent failure.

High-Purity Alumina

Aluminium oxide (Al2O3) is available in various purity grades, typically ranging from about 95% to 99.7%. These grades offer a spectrum of properties tailored for different applications.

Sapphire

While also a form of aluminium oxide (Al2O3), sapphire represents the higher end of the purity spectrum, typically exceeding 99.997%. It is distinguished by its single-crystal structure, often produced using the Kyropoulos method, which contributes to its remarkable insulating, thermal, mechanical, and optical properties:

The choice between alumina and sapphire in seal assemblies is driven by the application's specific needs – whether it demands the versatile and robust performance of alumina or the exceptional purity and optical qualities of sapphire. Both materials, through their unique properties and the manufacturing techniques used to integrate them into metal assemblies, facilitate the development of solutions capable of withstanding the most challenging environments. Our expertise in selecting and processing these materials ensures that each assembly is not only functional but also exemplifies the pinnacle of durability and performance.

2. Metals & Metal Alloys

Metals and metal alloys are integral to the construction of ceramic-to-metal seal assemblies, playing a pivotal role in ensuring robust interfacing with existing equipment, forming hermetic seals with ceramics, and facilitating the insulation properties critical to the assembly's function. In applications requiring some form of electrical transmission, metals act as essential conductors. Typically, high-purity coppers with a low oxygen content are favoured for their ability to maximise current capacity while minimising the need for extensive thermal management; however, alternative alloys may be employed, albeit with adjusted expectations regarding current capacity.

Metals We Use

Our expertise encompasses a wide range of metals and alloys, each selected for its unique properties and suitability for specific applications:

Considerations in Alloy Selection and Processing

It is crucial to recognise that different alloys necessitate specific processing and braze alloy combinations to forge durable joints. Inappropriate selection can lead to undesirable changes in material properties, such as reduced ductility or poor adhesion, compromising the assembly's integrity.

The optimal manufacturing approach often involves the use of controlled expansion alloys, particularly Fe-Ni alloys like Alloy 42 (Nilo® 42) or Alloy K (Kovar®). Their coefficient of thermal expansion (CTE) closely aligns with that of alumina across a broad temperature range, minimising relative displacement and residual stresses resulting from the brazing cycle. In situations where iron's presence is undesirable due to requirements for low magnetic permeability, alternative materials such as Cu-Ni alloys or stainless steel may be considered; however, the application of stainless steel may be restricted by significant mismatches in CTE.

Through careful selection and processing of metals and metal alloys, we ensure the creation of ceramic-to-metal seal assemblies that are not only mechanically robust and electrically efficient, but also tailored to withstand the demanding conditions of their intended applications. Our comprehensive understanding of material properties and compatibility allows us to navigate the complexities of material selection, ensuring optimal performance and longevity of the assemblies we produce.

3. Braze Alloys

In the design of ceramic-to-metal seal assemblies, the choice of braze alloy is critical for forming a robust mechanical bond between the metallized ceramic and the metal substrate. This is equally important when metal-to-metal brazing is required by the design. Braze alloys, which may consist of high-temperature eutectic mixtures or sometimes pure metals like silver, copper, or gold, are essential for their ability to create durable and reliable joints. The meticulous control over the alloy's composition enables the precise determination of liquidus temperatures, facilitating controlled brazing cycles that are crucial for achieving successful brazing outcomes.

Typical Braze Alloys Employed

Avoiding Material Issues

Successful brazing hinges on a thorough understanding of the materials involved, and how to process them through a high-temperature environment.

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The key to avoiding material issues lies in:

• Careful Alloy Selection

Aligning the braze alloy's properties with the intended use-case and working environment of the assembly ensures compatibility and longevity.

• Understanding Interfacial Reactions

Recognising how braze alloys interact with both ceramic and metal substrates at high temperatures can prevent adverse reactions that compromise joint integrity.

• Temperature and Environment Control

Precise control over the brazing temperature and atmosphere is vital to ensure that the materials undergo the intended reactions without inducing detrimental side effects.

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By integrating these considerations into the brazing process, we navigate the complexities of material compatibility to produce ceramic-to-metal seal assemblies of the highest reliability and performance. Our expertise in selecting and applying the appropriate braze alloys, coupled with our thorough process control, ensures the successful formation of bonds that meet the demanding requirements of our customers’ applications.

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Material Properties

1. Alumina & Sapphire

High-Purity Alumina (97.6%)

This alumina ceramic boasts a 97.6% Al2O3 purity level and is extensively utilised in critical components where its superior electrical and thermal characteristics are vital for maintaining operational stability and dependability.

Sapphire (Optical Grade)

Synthetic, optical grade sapphire is a premium material known for its exceptional clarity and durability, making it an ideal choice for precision optical components and applications where high strength and optical purity are crucial. This material, comprised of crystalline aluminium oxide (Al2O3), is engineered to achieve superior optical properties.

2. Metals & Metal Alloys

Alloy 42 (Nilo® 42)

Alloy 42, also known as Nilo® 42, is a controlled expansion alloy consisting primarily of iron and 42% nickel, with trace amounts of other elements. This alloy is renowned for its predictable and uniform coefficient of thermal expansion (CTE), making it an ideal choice for applications requiring tight dimensional tolerances over a range of temperatures, or when bonding to alumina substrates. Alloy 42's exceptional properties make it particularly suitable for ceramic-to-metal seals, where its expansion characteristics can be matched to those of high-purity alumina or synthetic sapphire.

Alloy K (Kovar®)

Alloy K, commonly referred to as Kovar®, is a nickel-cobalt ferrous alloy that is designed to exhibit a coefficient of thermal expansion (CTE) closely matched to that of borosilicate glass and ceramics over a certain temperature range. This unique property makes Alloy K suitable for creating hermetic seals between metal and glass/ceramic materials. The alloy typically consists of approximately 29% nickel, 17% cobalt, and the balance iron, along with trace amounts of other elements to enhance its properties.

3. Coefficients of Thermal Expansion

The following section presents a comprehensive graph illustrating the Coefficients of Thermal Expansion (CTE) of regularly used materials. The Coefficient of Thermal Expansion is a fundamental material property that quantifies how a material's dimension changes with temperature. Materials expand or contract when exposed to temperature fluctuations, and the CTE value is critical for applications where dimensional stability of bonded surfaces is required across a range of temperatures. Materials with greater CTE differences pose challenges insofar as the stresses imposed on the braze joint and metallizing bond. Consequently, certain material combinations and joint sizes may not be advisable.

Manufacturing

2. Alumina & Sapphire

The manufacturing processes of alumina and sapphire are defined by precision engineering and careful consideration of material properties, each tailored to meet the specific demands of their applications. This section explores the intricate steps involved in producing these materials, and how to achieve the desired tolerances, surface finishes, and structural integrity.

Manufacturing Alumina: Process and Precision

In the production of alumina components, the initial design phase is crucial, with a specific focus being on tolerancing. This is because the final geometry of alumina parts is largely determined post-firing, making it essential to design with an 'as-fired' finish in mind to minimise costs. Standard 'as-fired' tolerances are typically +/-1% for length, width, or diameter, with a minimum tolerance window of +/-0.15mm. Designs should also avoid sharp external corners to reduce the chance of chipping, and sharp internal corners to avoid unnecessary stress raisers. Caution should be taken with rapid changes in section thickness, especially in applications prone to thermal cycling or where high mechanical loads are present.

Pressing Process: Isostatic vs. Dry Pressing

Alumina is manufactured using different pressing methods, with each method offering being more suitable for different production demands:

Firing and Post-Processing

After pressing, alumina parts are fired at temperatures up to 1750°C, necessitating precise control over the kiln's temperature and atmosphere. During this stage, the pressed alumina powder forms a non-porous crystalline structure, and the loss of the pressing binder results in shrinkage of ~25%. After firing, achieving tight tolerances and improved surface finishes involves processes using diamond abrasives, such as grinding, lapping, honing, and polishing. Precision-machined features can achieve tolerances of sub 10 microns, though they can add significantly cost to the purchase price.

Crafting Sapphire: Growing and Finishing

3. Metalwork

We pride ourselves on our comprehensive in-house manufacturing capabilities, which extends far beyond ceramic. All of our metalwork is manufactured in-house by our parent company, Normec (Manchester) Limited, ensuring absolute control from inception to final manufacture. Component parts are produced using various methods that are usually combined to produce the finished parts, including:

• CNC Turning and Milling

CNC machines provide high precision and flexibility, allowing us to produce complex shapes and geometries with exceptional accuracy.

• Pressing

Pressing techniques are used to shape metal parts efficiently and with high repeatability.

• Spinning

Spinning offers versatile shaping options for both standard and complex parts.

• EDM Wire and Drilling

For intricate details and precision cuts, EDM capabilities are unparalleled, ensuring exactness down to the minutest specifications.

• Grinding and Polishing

Smooth finishes and precise dimensions can be achieved through surface grinding.

• Lapping

For parts requiring ultra-smooth surfaces or tight dimensional tolerances, our surface lapping processes provide the perfect solution.

These manufacturing methods allow us to create just about anything and can fit most requirements, whether than be off-the-shelf or bespoke components. Normec are skilled in machining exotic materials, such as titanium, Inconels or controlled expansion metals such as Alloy 42 (Nilo 42).

4. Metallization

5. Brazing

Facilities are available for brazing components, in vacuum, inert, or reducing atmosphere, up to 250mm diameter × 300mm long. The choice of atmosphere and braze alloy is critical in achieving a successful homogeneous joint. The most commonly employed alloy will be Ag/Cu eutectic, however, the eventual choice of alloy(s) will depend on assembly geometry, material substrate, working environment, or manufacturing method. Ag/Cu is preferred material as it produces a high seal strength at low cost. For applications where higher operating temperatures or other design constraints are involved, Au/Cu, Au/Ni or Cu braze materials are employed, the latter braze material having a liquidus point of approximately 1100°C.

6. Welding

In-House

All in-house welds are made using the Tungsten Inert Gas (TIG) method with high purity argon shielding to exclude atmospheric gasses. This helps to prevent the formation of oxides and nitrides in the weld, thereby allowing the molten metals to run together without employing any flux, and achieving a clean, homogeneous weld. After welding, all welds are carefully checked for pits, cracks, and vacuum leaks.

7. Inspection & Quality