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Spie Press Book

Materials for Infrared Windows and Domes: Properties and Performance
Author(s): Daniel C. Harris
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Book Description

This text provides a comprehensive introduction to infrared-transparent materials for windows and domes that must withstand harsh environmental conditions, such as high-speed flight or high-temperature process monitoring. Each section contains introductory material that makes the book readable by anyone with a background in science or engineering.

Book Details

Date Published: 16 August 1999
Pages: 428
ISBN: 9780819459787
Volume: PM70

Table of Contents
SHOW Table of Contents | HIDE Table of Contents
Preface. . . . . . . . . . . . . . . . . . . . . . . . .xi
0. The Heat of the Night and the Dust of the Battlefield. . . . . .1
0.1 Electromagnetic spectrum and atmospheric transmission. . . . . . .2
0.2 Blackbody radiation. . . . . . . . . . . . . . . . 3
0.3 Transmission through rain, snow, fog and dust. . . . . . . . . . .6
References . . . . . . . . . . . . . . . . . . . .11
1. Optical Properties of Infrared Windows . . . . .12
1.1 A day in the life of a photon. . . . . . . . . . .12
1.2 Refraction and refractive index. . . . . . . . . .15
1.2.1 Birefringence. . . . . 19
1.2.2 Preference for cubic materials . . . . . . 20
1.2.3 Reproducibility of the refractive index. . . . . . . 22
1.3 Reflection and transmission. . . . . . . . . . . .22
1.3.1 Transmission of an absorbing window. . . . . . .24
1.3.2 Etalon effect. . . . . . . . . . . . . . . . . .25
1.4 Optical constants: n and k. . . . . . . . . . . .27
1.5 General behavior of absorption coefficient and refractive index. . . . . . 28
1.6 Transmission spectra of infrared materials . . . . . . . . . . . . . . . . 30
1.7 Measuring the absorption coefficient . . . . . . .36
1.7.1 Direct transmittance measurements. . . . . . . .36
1.7.2 Laser calorimetry. . . . . . . . . . . . . . . .37
1.8 Emissivity . . . . . . . . . . . . . . . . . . . .39
1.9 Effect of temperature on absorption and emission . . . . . .42
1.10 Free carrier absorption in semiconductors. . . . .46
1.11 What makes a window midwave or long wave?. . . . .47
1.12 "Two-color" materials. . . . . . . . . . . . . . .58
References . . . . . . . . . . . . . . . . . . . .60
2. Optical Performance of Infrared Windows. . . . . . . 63
2.1 Resolution . . . . . . . . . . . . . . . . . . . .63
2.2 Scatter. . . . . . . . . . . . . . . . . . . . . .64
2.3 Modulation transfer function: a measure of imaging quality. . . . . .68
2.4 Degradation of infrared sensing by a hot window. . . . . . . . . . . .71
2.4.1 Emittance from a hot window. . . . . .71
2.4.2 Temperature gradients in windows . . . . . 74
2.5 Frequency doubling . . . . . . . . . . . . . . . .76
2.6 Microwave transmission properties of infrared materials. . . . . 77
References . . . . . . . . . . . . . . . . . . . .81
3. Mechanical Properties. . . . . . 84
3.1 Elastic constants. . . . . . . . . . . . . . . . .84
3.2 Measuring the strength of brittle materials. . . . . . 88
3.2.1 3-point and 4-point flexure tests. . . . . 89
3.2.2 Equibiaxial disk flexure test. . . . . . . 92
3.3 Ceramics fracture at pre-existing flaws. . . . . .94
3.3.1 Stress concentration by cracks . . . . . . 96
3.3.2 Strain rate dependence of strength . . . . 98
3.4 Weibull statistics . . . . . . . . . . . . . . . .98
3.4.1 The Weibull distribution . . . . . . . . . 99
3.4.2 Safety factors . . . . . . . . . . . . . .101
3.5 Strength scales with area (or volume) under stress . . . . . . . . . . . . . . . . .103
3.6 Strengths of optical ceramics. . . . . . . . . . 105
3.6.1 Strength is not an intrinsic property of a material. . . . . .106
3.6.2 Temperature dependence of strength . . . .107
3.7 Window and dome design . . . . . . . . . . . . . 109
3.7.1 Designing a circular window. . . . . . . .109
3.7.2 Designing a dome . . . . . . . . . . . . .111
3.8 Hardness and fracture toughness. . . . . . . . . 114
3.8.1 Relation of strength to fracture toughness and grain size. . . . . 119
3.8.2 Temperature dependence of hardness and fracture toughness. . . . . 121
References . . . . . . . . . . . . . . . . . . . 122
4. Thermal Properties . . . . . . .126
4.1 Thermal expansion and heat capacity. . . . . . . 126
4.2 Thermal conductivity . . . . . . . . . . . . . . 128
4.3 Thermal shock. . . . . . . . . . . . . . . . . . 132
4.3.1 Hasselman figures of merit . . . . . 134
4.3.2 Klein figure of merit for minimum thickness dome . . . . . . .140
4.3.3 Mach-altitude limits for a dome. . . . . . . . . . . . . . . .141
4.4 Aerodynamic domes. . . . . . . . . . . . . . . . 144
4.5 Thermal stability of window materials. . . . . . 145
References . . . . . . . . . . . . . . . . . . . 148
5.Fabrication of Infrared Materials 150
5.1 Classes of infrared materials. . . . . . . . . . 150
5.1.1 Glass-ceramics . . . . . . 153
5.2 Fabrication of polycrystalline materials by powder processing. . . . . . .155
5.2.1 Yttria: an example of dome fabrication from a powder. . . . . . . 155
5.2.2 Methods of densifying ceramics: sintering, hot pressing and hot isostatic pressing. . . . . . . . . . . 159
5.2.3 Annealing. . . . . . . . . . . . . . . . . . . 162
5.3 Chemical vapor deposition. . . . . . . . . . . . 163
5.3.1 Zinc sulfide and zinc selenide . . . . . . . . 163
5.3.2 Silicon carbide and silicon nitride. . . . . . 167
5.4 Single-crystal materials . . . . . . . . . . . . 170
5.4.1 Gallium arsenide, gallium phosphide, germanium and silicon . . . . . . .170
5.4.2 Sapphire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .173
5.4.3 Hot forging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177
5.5 Optical finishing. . . . . . . . . . . . . . . . 177
5.5.1 Scratch/dig specifications . . . . . . . . . . . . . . . . . . . . . . .180
5.5.2 Optical polishing. . . . . . . . . . . . . . . . . . . . . . . . . . . .181
5.6 The effect of surface finish on mechanical strength. . . . . . . . . . .183
5.7 Polymer infrared windows . . . . . . . . . . . . 189
References . . . . . . . . . . . . . . . . . . . 191
6. Optical Coatings . . . . . 195
6.1 Antireflection coatings. . . . . . . . . . . . . 195
6.1.1 Moth eye surfaces. . . . . 199
6.1.2 Interference fringes for measuring coating thickness . . . . .201
6.1.3 Adherence of coatings. . . . . . . . . . . . . . . . . . . . .202
6.1.4 Emittance from coatings. . . . . . . . . . . . . . . . . . . .202
6.1.5 Rugate filters . . . . . . 203
6.2 Stress in coatings . . . . . . . . . . . . . . . 205
6.3 Conductive coatings for electromagnetic shielding. . . . . . . . . . 207
References . . . . . . . . . . . . . . . . . . . 213
7. Erosion and Erosion Protection . . . . . .215
7.1 Rainfall characteristics . . . . . . . . . . . . 218
7.2 The raindrop impact event. . . . . . . . . . . . 220
7.3 Raindrop damage threshold velocity . . . . . . . 224
7.3.1 Threshold velocity for fracture or loss of mechanical strength . . . . .224
7.3.2 Threshold velocity for loss of optical transmission or contrast. . . . .228
7.3.3 Threshold velocity for loss of mass. . . . . . . . . . . . . . . . . . .231
7.4 Rain erosion test facilities . . . . . . . . . . 233
7.4.1 Whirling arm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .233
7.4.2 Single-impact waterjet . . . . . . . . . . . . . . . . . . . . . . . . .235
7.4.3 Multiple-impact jet apparatus (MIJA) . . . . . . . . . . . . . . . . . .237
7.4.4 Single-drop impact testing . . . . . . . . . . . . . . . . . . . . . . .240
7.5 Aerodynamic effects in rain erosion. . . . . . . 241
7.6 Erosion by solid particles . . . . . . . . . . . 243
7.6.1 Combined effects of sand and rain. . . . . . . . . . . . . . . . . . . .248
7.7 Effect of angle of incidence on erosion. . . . . 249
7.7.1 Waterdrop impact at inclined angles. . . . . . . . . . . . . . . . . . .249
7.7.2 Sand impact at inclined angles . . . . . . . . . . . . . . . . . . . . .250
7.7.3 Comparative erosion testing of materials . . . . . . . . . . . . . . . .250
7.8 Protective coatings for erosion. . . . . . . . . 252
7.8.1 Mechanisms of protection by coatings . . . . . . . . . . . . . . . . . .252
7.8.2 Diamond-like carbon and germanium-carbon coatings. . . . . . . . . . . .258
7.8.3 "Boron phosphide" and other phosphorus-based coatings. . . . . . . . . .259
7.8.4 "REP" coating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .264
7.8.5 Claddings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .266
7.8.6 Diamond coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . .270
References . . . . . . . . . . . . . . . . . . . 273
8. Proof Testing. . . . . . . 280
8.1 Case study: proof testing of zinc selenide. . . . . .281
8.1.1 An example of an unsuccessful proof test . . . . . .283
8.2 What is the stress intensity factor? . . . . . . 284
8.3 Slow crack growth. . . . . . . . . . . . . . . . 285
8.4 The theory of proof testing. . . . . . . . . . . 290
8.4.1 How strength changes during a proof test . . . . . .292
8.4.2 A theoretical example: proof testing of sapphire. . . . . . .293
8.5 Designing a proof test for the space shuttle window. . .. . .295
8.5.1 Minimum time to failure after a proof test . . . . .295
8.5.2 Crack growth parameters for space shuttle window material. . . . . 296
8.5.3 Proof test design. . . . . . . . . . . . . . . . . .297
8.6. Fatigue. . . . . . . . . . . . . . . . . . . . . 299
References . . . . . . . . . . . . . . . . . . . 300
9. Optical-Quality CVD Diamond. . . . . 303
9.1 What is diamond and how is it made?. . . . . . . 304
9.1.1 Chemical vapor deposition of diamond . . . . . 306
9.1.2 The two surfaces of CVD diamond. . . . . . . . 309
9.2 Mechanical and thermal properties of diamond . . . . . .311
9.2.1 Hardness, toughness and elastic properties . . . . .312
9.2.2 Mechanical strength. . . . . . . . . . . . . . 313
9.2.3 Thermal expansion. . . . . . . . . . . . . . . 317
9.2.4 Thermal conductivity and heat capacity . . . . 318
9.2.5 Commercial grades of CVD diamond . . . . . . . 321
9.3 Optical properties of diamond. . . . . . . . . . 321
9.3.1 Absorption and scatter . . . . . . . . . . . . 322
9.3.2 Refractive index . . . . . . . . . . . . . . . 327
9.3.3 Microwave properties of diamond. . . . . . . . 328
9.4 Diamond windows and domes. . . . . . . . . . . . 329
9.4.1 Polishing diamond. . . . . . . . . . . . . . . 330
9.4.2 Mechanical and erosion performance . . . . . . 332
9.4.3 Oxidation of diamond . . . . . . . . . . . . . 334
9.4.4 Prospects. . . . . . . . . . . . . . . . . . . 336
References . . . . . . . . . . . . . . . . . . . 336
Appendix A: Physical Constants and Conversion Factors. . . . . .344
Appendix B: Suppliers of Infrared Materials and Sources of Information . .346
Appendix C: Optical Properties of Infrared Materials . . . . . .351
C.1 Refractive index, absorption coefficient, and dn/dT. . . . . .352
C.2 Dispersion equations for refractive index. . . . . . . . . . .356
C.3 Absorption coefficients of selected materials calculated by OPTIMATR . . . . . . . . . . . 362
C.4 Change of refractive index with isotropic pressure . . . . . .365
Appendix D: Definitions from Radiometry. . . . . 371
Appendix E: Elastic Constants. . . . . . . . . . 374
Appendix F: The Weibull Distribution . . . . . . 382
F.1 Weibull probability distribution. . . . . 382
F.2 Effective volume or area. . . . . . . . . 384
F.3 Weibull equations for different kinds of test specimens . . . . . .384
F.4 Relative strengths of different kinds of test specimens . . . . . .387
F.5 Weibull scaling by area instead of volume . . . . . . . . . . . . .389
Appendix G: Thermal Properties of Selected Materials . . . . . .391
Index . . . . . . . . . . . . . . . . . . . . . . . . 403

PREFACE

This text is intended to provide a comprehensive introduction to infrared-transparent materials for windows and domes that must withstand harsh environmental conditions, such as high-speed flight or high-temperature process monitoring. Each section contains sufficient introductory explanation so that the book should be readable by anyone with a background in science or engineering. The current volume builds on its predecessor, Infrared Window and Dome Materials, published in 1992 as part of the SPIE Tutorial Text series. The book you are holding incorporates seven years of new developments in the field of infrared windows and includes additional reference information and some more theory to make it more useful.

My wife, Sally, prepared many of the illustrations and contributed in many ways to the production of this volume. The manuscript benefited from critical reviews by Mike Thomas of the Applied Physics Lab and Lee Goldman of Raytheon and many helpful discussions with Claude Klein. Mel Nadler, Mike Seltzer and Andy Wright recorded some of the spectra that appear in this book. I also wish to express my appreciation to the management at China Lake, which values creative and scholarly activities, and to the Office of Naval Research for ongoing support of window material research and development.

I welcome your comments, corrections and suggestions. I can be reached at dan_harris@alum.mit.edu.

Dan Harris
June 1999
China Lake, California


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