MAS Spinner Materials

Doty spinner assemblies utilize super-precision, wear-resistant ceramic stators of silicon nitride or zirconia. The stator is supported in a housing of plastic or ceramic. The importance of the spinner assembly in the MAS probe requires that the materials be chosen carefully based on background signals and temperature ranges. Ceramic rotors and plastic caps of various materials are available to provide fast spinning with limited background problems.

Material

Major

Constituents

Minor
Elements

Dielectric
constant / tanδ

RT,
400 MHz

Maximum
Material

Temp

Silicon
Nitride

98%
Si3N4

2%
Y2O3 , .005 Al

8
/ 0.0012

1400°C
*

Zirconia

94
ZrO2 , 4 Y2O3

Hf,
0.3% Si, .02 Al

26
/ 0.004

650°C
*

Macor

Al,
Si, O, B, K

2%
F, Mg

5.5
/ 0.003

650°C
*

Vespel

H,
C, O

N

3.5
/ 0.004

300°C
*

Torlon
H, C, O
Ti, N
4.2/0.05
260°C
GF Torlon
H, C, O, Si
Ti, N, F
260°C

Aurum

H,
C, O

N

3.4
/ 0.0015

240°C
*

Kel-F

F,
C, Cl

 

2.5
/ 0.002

160°C
*

This chart represents only material characteristics. Check the Probe Specifications for its operating limits.

* Refer to the following sections for temperature ranges for turbine caps.

Ceramics

Silicon Nitride: With a density of about 3.18 g/cm3 and a (working) tensile strength of 700 MPa, silicon nitride has the highest strength-to-weight ratio of any ceramic available today. Its hardness and toughness make it very difficult to grind, but it has the lowest dielectric loss and lowest permittivity of any engineering ceramic, making it the best choice for high frequencies. A HIPed (Hot Isostatic Pressed) variety is available with improved strength and dielectric properties. Its superior electrical properties make it the preferred material for most applications (often, even for silicon and nitrogen), because of silicon’s long T1 and nitrogen’s low natural abundance. Silicon nitride stators, housings, and rotors are available in all sizes.

Zirconia: The yttia-stabilized, high-purity material is glossy white and has a density of about 6 g/cm3 and a (working) tensile strength of 700 MPa. Alumina content can be kept below 100 ppm.

Macor: This white, machineable, boro-silicate glass-ceramic is loaded with synthetic mica to inhibit crack propagation. It is easily machined with conventional tooling. Macor housings may be provided for carbon and proton studies at temperatures over 160°C in our standard speed MAS probes. Macor is usable up to 650°C, but it has very poor thermal shock tolerance. Macor rotors are only available for the high speed and standard spinners. Macor rotors are made with a thicker wall and are machined with an integral turbine at one end. Standard speed rotors require a plug-cap at the open end, while high-speed rotors require a rear turbine. Macor rotors are recommended for wet samples, air sensitive samples, and temperatures from -150°C to 250°C. Major constituents: Al, Si, O, B, K, F.

Boron Nitride: The hexagonal hot-pressed variety, with 6% calcium borate binder, is easily machineable and is used for disposable inserts in the high-temperature ceramic rotors. The material is soft enough to scratch easily and may absorb up to 1% moisture.

Plastics

MAS Turbine Caps Glass-fiber-reinforced torlon grade 5030 is now used on most MAS turbine caps (except for low proton Kel-F caps) for greatly improved VT performance and all-round better reliability and performance. This new material stands out from the rest with respect to isotropic thermal expansion (only 16E-6/°C), tensile strength at 200°C (120 MPa), and heat distortion temperature (282°C). It also has rather low moisture absorption, high wear resistance, and high impact strength.

Kel-F: This is our standard housing material for Standard and High Speed probes. A translucent plastic, Kel-F is background free for all nuclei except F, Cl, and C. Kel-F is also excellent for carbon studies since the strong fluorine coupling effectively broadens the Kel-F carbon signal, and there are no protons to cross polarize. Kel-F turbine cap pairs are used with silicon nitride and zirconia rotors. The turbine caps can be used at temperatures from -20°C to 70°C. (In parts of the spinner assembly that do not spin, the usuable temperature range is -100°C to 150°C.)

Aurum: This dark brown to black thermoplastic polyimide has excellent dielectric properties. Aurum can be used for low silicon applications when fast spinning is desired. Aurum is supplied for most turbine caps and spinner parts on probes designed for fluorine studies and other applications where carbon was not a problem. Turbine caps may be used from -30°C to 80°C repeatedly. (In non-spinning parts of the spinner assembly, the upper temperature limit is 240°C.)

Aurum is no longer available in appropriate forms for our purposes, so Aurum is being replaced by Torlon

Torlon: Torlon is used for fastest spinning of DI3 turbines. This green thermoplastic polyamide-imide, has exceptional chemical resistance. Torlon is now the standard material for fastest spinning turbine caps and can be used as a substitute for Aurum as the NMR characteristics are similar. Torlon is not recommended for proton studies or for some carbon studies. Caps may be used from -30°C to 80°C repeatedly. The caps may be used once to higher temperatures but they will be too loose after that. (In non-spinning parts of the spinner assembly, the upper temperature limit is 260°C.)

Vespel: This brown plastic is used for the extended temperature caps described below and for nonspinning spinner assembly parts that will reach temperatures over 200°C. Vespel is not recommended for carbon or proton studies.

•  Glass-fiber-reinforced Torlon: ………… Temperature range:   -120°C to 160°C.

• Aurum Turbine Caps:   ………… Temperature range: -30°C to 80°C.

• Kel-F Turbine Caps:    ………….. Temperature range: -20°C to 70°C.

• Torlon Turbine Caps:     …………. Temperature range: -30°C to 80°C.

O-Ring Caps (Only for 5mm and 7mm probes)

For wet samples, air sensitive samples, and variable temperatures

O-ring Caps: Turbines and plug caps with dual Viton o-ring seals are available for Macor, silicon nitride, and zirconia rotors. Macor rotors, (available only for standard and high speed probes) are recommended for wet samples, air sensitive samples, and temperatures from -60°C to 250°C. A single (rear) cap with o-rings is used with a Macor rotor. However, if faster spinning is critical, VT and air-sensitive experiments can be done in silicon nitride or zirconia rotors using o-ring-sealed cap pairs.

O-ring turbine caps are normally inserted and removed by hand. Turbines with threaded holes can be ordered (with a threaded insertion tool) for use when loading samples in a glove box. (A threaded insertion tool is always required to insert and remove the plug cap of standard speed cap pairs.) Caps can be ordered with axial holes for out-gassing during higher temperature work or to remove air bubbles from wet samples.

•  Kel-F O-ring Caps:   …………….. Temperature range with o-rings: -45°C to 80°C.

•  Torlon or Aurum O-ring Caps: …Temperature range with o-rings: -45°C to 120°C.

•  Note: Although the supersonic o-ring caps can be used for air sensitive samples in XC5 and XC7 probes, XC sealing cells are usually prefered.

Extended Temperature Caps

•  Glass-fiber-reinforced Torlon glued in ……. Temperature range:  -170°C to 200°C.

SuperSonic, High Speed, and Standard extended VT caps include a small screw along the axis of the rotor to retain the caps. A tamp with a hole in the middle is required for sample packing.

•  Kel-F Extended VT Caps:   ……. Temperature range: -270° to 80°C.

•  Vespel Extended VT Caps:   ….. Temperature range: -270° to 240°C.

 Notes:

• O-ring caps are easier to pack than VT axial screw caps.

Extended VT caps with axial screws and o-ring caps are made only for thin wall rotors.

• XC, SuperSonic, and high-speed rotors require a front turbine and a rear turbine.

• Standard speed rotors require a front turbine and a plug.

wp ecommerce - e-commerce wordpress