Iridium

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Small crucibles made of iridium have been used for studying high-temperature reactions. A major use for iridium is crucibles for producing large, pure, defect-free man-made crystals for electronic and industrial applications. Single crystals so formed are used as substrates in magnetic bubble memory devices, solid-state lasers, insulating substrates for semiconductors, monoclinic filters, and substitutes for natural gemstones in jewelry. Iridium also is used as an alloying element to harden platinum; as electrodes in spark plugs for severe operating conditions, such as those experienced by jet engine igniters; as thermo-couple elements; and as radioactive isotopes for industrial applications and cancer therapy.

General Characteristics:

Atomic Weight: 192.9

Density: 22.65 g/cm3 at 20 oC

Melting Point: 2447 oC

Boiling Point: 4500 oC

Coefficient of Linear Thermal Expansion: 6.8 um/m ⋅ K at 20 oC

Specific Heat: 0.130 kJ/kg ⋅ K

Thermal Conductivity: 147 W/m ⋅ K at 0 to 100 oC

Electrical Resistivity: 47.1 nΩ ⋅ m at 0 oC; 53 nΩ ⋅ m at 20 oC

Temperature Coefficient: 0.00427 nΩ ⋅ per oC at 0 to 100 oC

Thermal Electromotive Force: Pt 67 (reference junction at 0 oC):

                                                 +3.626 mV at 400 oC

                                                 +6.271 mV at 600 oC

                                                 +12.741 mV at 1000 oC

Tensile Properties of Iridium Annealed at 1500 oC:

Temperature    Tensile Strength     0.2% Yield Strength    Reduction in Area

       oC                    MPa                         MPa                                %

        24                    623                          234                                 6.8

       500                   530                          234                                 12.7

       750                   450                          142                                 51.0

      1000                  331                          43.4                                80.6

Magnetic Susceptibility Mass: 0.19 x 10-8 mks at 18 oC

Reflectivity:    64% at λ = 0.45 um

                       70% at λ = 0.55 um

                       78% at λ = 0.75 um

Emissivity: 0.30 at 0.65 micron for solid unoxidized metal.

General Corrosion Behavior:

Iridium is the most corrosion-resistant element. It is not affected by common acids, including hot sulfuric acid. It is slightly attacked by sodium hypochlorite solutions but not by aqua regia at ordinary temperatures. However, at elevated temperatures and pressures, aqua regia does attack iridium, and it may be used under these conditions for dissolving iridium and its refractory alloys for analysis. Iridium is virtually insoluble in lead even at high temperatures, and use is often made of this fact in preliminary steps in chemical analysis.

Fabrication Characteristics:

Iridium can be arc melted (inert-gas cover), electron beam melted, or consolidated by powder metallurgy techniques. It is hot worked using procedures similar to those used for tungsten. Final working is done at warm temperatures, which produce a fibrous structure. Iridium has limited malleability at room temperature.

Tensile Properties (of 0.5 mm wire):

              Tensile Strength:    Annealed at 1000 oC: 1100 to 1240 MPa

                                                             Hot Drawn:  2070 to 2480 MPa

               Elongation:                             Annealed:     20 to 22%

                                                             Hot Drawn:     13 to 18%

Hardness:

Annealed at 1000 o:  200 to 240 HV

                              As-Cast:   210 to 240 HV

                          Hot Drawn:   600 to 700 HV

Modulus of Elasticity:

              Tension:  517 GPa (Static); 527 GPa (Dynamic) 

      Compression:  210 GPa

  Poisson's Ratio:  .26