ELECTRON TUBES AND VACUUM EQUIPMENT · 2015. 12. 21. · INTRODUCTION 578 CHAPTER 33—ELECTRON TUBES AND VACUUM EQUPMENT AWS BRAZING HANDBOOK The brazing of structures for vacuum

Prepared by:

C. WalkerSandia National Laboratories

T. HirtheTru-Mar Manufacturing Services, Incorporated

Contents

Introduction 578

Base Materials 578

Brazing Filler Metals 581

Inspection 584

Bibliography 586

SuggestedReading List 586

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ELECTRON TUBES AND VACUUM EQUIPMENT

Photograph courtesy of Wesgo Metals Division, Morgan Advanced Ceramics

AWS BRAZING HANDBOOK 577

INTRODUCTION

578 CHAPTER 33—ELECTRON TUBES AND VACUUM EQUPMENT AWS BRAZING HANDBOOK

The brazing of structures for vacuum tubesand other high-vacuum devices has required thedevelopment of highly refined cleaning and handlingprocedures, furnace equipment, and high-purity,low-vapor-pressure brazing filler metals. Vacuumtubes, of necessity, are operated at very low pressures[10–6 torr to 10–8 torr (10–4 Pa to 10–6 Pa)] and mustmaintain this very low pressure for the thousands ofhours of their useful life.

High-vacuum devices and specialized equipmentcannot tolerate conditions that inhibit the ability tosecure and maintain extremely low pressures. As afurther complication, electron tubes and other vac-uum devices are heated to temperatures beyond932°F (500°C) for extended periods during their gasevacuation cycle (bake out) to drive out the gasesentrapped in their metal structures.

The operating conditions of vacuum tubes includeelevated temperatures, high voltages, and/or highelectrical currents. To meet the rigorous operatingconditions, widely dissimilar materials must be usedin construction. Thus, prior to the brazing of compo-nents for electron tubes and vacuum devices, theproblems of differential thermal expansion, particu-late control, and outgassing must be addressed dur-ing the design stage after a thorough analysis of therequired operating environment and conditions hasbeen completed.

BASE MATERIALS

An entire class of metal alloys has been developedto address the problem of thermal expansion mis-match, which arises when brazing ceramics tometals, a common occurrence in vacuum tube fabri-

cation or vacuum equipment electrical feedthroughs.The majority of controlled-expansion alloys can beseparated into two systems, iron- and nickel-basedalloys or iron, nickel, and cobalt (fenico). Althoughmany compositions of fenico are in use, the mostcommon composition is Fe-29Ni-17Co.

COPPER AND NICKEL ALLOYSCopper-based alloys containing high-vapor-pressure

elements should be avoided when fabricating electrontubes and other vacuum devices. Oxygen-free high-conductivity copper (OFHC) is the preferred materialin most cases when the annealed strength of purecopper is not an issue.

Although most nickel-based alloys and super-alloys are vacuum compatible at elevated tempera-tures, high coefficients of thermal expansion oftenprevent their use as structural materials in electrontubes. Commercially pure nickel (UNS N02200),however, being more ductile than nickel alloys, isoften used in electron tubes.1

STAINLESS STEELSStainless steels are used frequently in the manufac-

ture of vacuum equipment and hardware. For struc-tural assemblies and vacuum chambers, lowoutgassing rates, high strengths, low costs, highavailabilities and ease of joining make stainless steelsthe material of choice.

1. ASTM International, Standard Specification for Nickel Rodand Bar, ASTM B160, West Conshohocken, Pennsylvania: ASTMInternational.

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