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IAC-08-C2.6.07

Low CTE Waveguide for Extreme Thermal Environment

Sandro Mileti sandro.mileti@uniroma1.it;

Davide Micheli, Plinio Coluzzi, Piersante Miccichè, Mario Marchetti

Dep. of Aerospace and Astronautic Engineering (DIAA), “Sapienza” University Rome, Italy plinio.coluzzi@uniroma1.it; davide.micheli@uniroma1.it; piersante.micciche@uniroma1.it;

mario.marchetti@uniroma1.it Via Eudossiana 18, 00184 Rome ; Phone: 0039-06-44585800; Fax: 0039-06-44585670 Keywords: waveguide, carbon-carbon, high temperature

ABSTRACT

Probes are being sent towards extreme environments of our solar system. The delivery of the science in these environments is a challenge for waveguide systems. The crucial aspects of the design are associated with, and constrained by, the high-temperature, high-radiation exposure and suitable conductivity, requiring advanced materials having a very low Coefficient of Thermal Expansion (CTE). The combination of these requirements led to the selection of carbon-carbon as an ideal material due to its low CTE, good strength, and excellent high temperature characteristics. The purpose of this research was to investigate the feasibility of using carbon-carbon composite for waveguide applications. The characterizations of the mechanical properties are determined through several room temperature tests. A prototype rectangular tube-type assembly was fabricated and copper was plated on the surface of the composite material as conducting surfaces. Specific experiments aimed to study the electromagnetic and physical properties of the composite material were performed. Particularly, the in plane and transverse CTE were examined to determine the proper milling procedure and exploit material qualities. In addition, the machining direction was essential to satisfy the dimensional stability specifications. Outgassing qualification of the composite was analyzed and some coating solutions were also studied. Prototype waveguide frequency range measurements and efficiency were tested to evaluate the compatibility of C/C for waveguide applications.

FULL TEXT 1 Introduction

The materials used in waveguide applications, for future deep-space missions [1], will need to survive and operate over a wide temperature range (-200 °C to 300 °C), be radiation tolerant, exhibit low outgassing and be lightweight. For high-efficiency communications, the materials must also exploit a low coefficient of thermal expansion (CTE).

The combination of these requirements led to the selection of carbon-carbon (C/C) as a promising candidate for such application due to its low density, high strength, and excellent high-temperature characteristics [2]. Once selected the material an investigation of its feasibility in waveguide system applications was performed. Specific experiments were conducted to characterize its mechanical and electromagnetic properties and two prototypes of rectangular waveguide

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were built in order to evaluate the scattering parameters. The composite material used in this study was supplied by NTT Aerospace. It is an 8mm thick C/C laminate having 1.48 gr/cm3 density. The material reportedly has acceptable thermal and electrical conductivity (Table 4). In addition, there is a substantial weight saving of 40% compared to its metallic counterpart. A manufacturing process had also to be developed in order to mill a waveguide section directly from the 2D C/C laminate. The selected waveguide configuration is detailed in Fig. 1. It is a standard WR28 rectangular waveguide having aluminium flanges at its ends. The transmitting performance capability of the C/C and copper-plated C/C composite waveguide were also investigated.

Fig. 1 Waveguide prototype configuration

2 Mechanical Testing C/C was selected mainly for its low sensitivity to length changes due to thermal variations and for its adequate mechanical properties [3]. For proper design, material characterization must be fully performed. The selected C/C composite underwent a mechanical test campaign to evaluate its intrinsic material properties. At this purpose tensile, flexural and interlaminar shear tests have been performed, following the ASTM International Standards. All tests have been performed using a Trebel Schenck testing

machine implementing a HBM U2b force transducer. 2.1 Tensile test Tensile strength and modulus of the composite were estimated under the ASTM C1275 through dumbbell specimens having nominal size shown in Fig. 2. Five samples have been cut from different areas of a 1 m2 slab of material. Fig. 3 shows a dumbbell specimen milled using graphite dedicated cutting tools. All samples underwent visual inspection in order to be sure that no delaminations occurred during machining.

Fig. 2 Dumbbell dimensions

Fig. 3 Milled sample for tensile test The samples were mounted on special fixtures made for CMC material samples. On each sample, two strain gages were glued for modulus and Poisson ratio measurements (Fig. 4). The samples were tested up to rupture to measure also the ultimate tensile strength. A representative stress/strain graph acquired on one of the tests is given in Fig. 5.

Fig. 4 Mounted sample

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Tensile strength, elastic modulus and Poisson ratio are reported in summary Table 1.

Fig. 5 Tensile Stress/Strain graph 2.2 Interlaminar shear test A main material characteristic which had to be determined, typical of composite materials, was the interlaminar shear strength. The presence of shear loads during launch, flight and operative conditions may heavily damage the waveguide compromising its functionality. Moreover, shear stress is a threat for CMC materials since their resistance to these loads is relatively low. For proper design needs, the interlaminar shear strength was determined through the ASTM C1292 double-notch test method (Fig. 6). The average shear strength calculated is reported in Table 1.

Fig. 6 Double-notch shear test 2.3 Three-point bending test Three-point bending tests were conducted to determine the flexural strength and flexural modulus by following ASTM standard

C1341-00. A span to depth ratio of 20 and crosshead speed of 144 mm/min were applied to three-point bending test specimen. The average flexural strength and modulus are listed in Table 1 and a force/elongation representative graph is given in Fig. 8.

Fig. 7 Bending test

2.4 Mechanical properties Experimental results are listed in Table 1. The mean values of tensile strength, ultimate elongation and elongation at break are indicated in Table 1 along with their standard deviation. Below (Figs.8, 9 and 10) are included three graphs, each displaying a representative force/elongation rate graph for each test.

Property Density(g/cm3)

Tensile Strength(MPa)

Elastic Modulus (GPa)

Flexural Strength (MPa)

Flexural Modulus (GPa)

Poisson Ratio υ (in plane)

Shear Strength(MPa)

Value 1.48 125 100 152 50 0.36 4.7

Stand. Dev.% - 4.4 4.5 4.0 4.0 - 1.9

Table 1 C/C Mechanical Properties

Fig. 8 Tensile force/displacement graph

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Fig. 9 Flexural force/elongation graph

Fig. 10 Shear force/displacement graph

3 Thermal, Electrical and Dielectric Properties

The coefficient of thermal expansion measurement was carried out using a Linseis L75 push-rod dilatometer (ASTM E 228). Both in-plane and crosswise measurements have been performed up to 400°C and results are given in Table 2. Thermal conductivity was calculated through the experimental data of thermal diffusivity and specific heat provided by Linseis using XFA 500 Xenon Flash analyzer. C.T.E (in-plane at 400°C) (×10-6/K)

C.T.E (crosswise at 400°C) (×10-6/K)

Electrical Conductivity(Siemens/m)

Thermal Conductivity (W/mK)

0.8 6 3,8 E4 5

Table 2 CTE and Electrical properties A comparison with copper electrical conductivity, which is around 106-107 S/m, it is evident (Table 4) that the C/C conductivity value of is about two times an order of magnitude order lower.

As far as microwave measurements are concerned, a Vector Network Analyzer (Agilent PNA-L), has been used in order to determine the Scattering parameter and the dielectric constant in the X-Band. A measurement using the Transmission/Reflection line technique involves placing a sample in a section of waveguide or coaxial line and measuring the two ports complex scattering parameters with a vector network analyzer (PNA). The technique involves measurement of the reflected (S11) and transmitted signal (S21). The relevant scattering parameters relate closely to the complex permittivity and permeability of the material by equations. The conversion of s-parameters to complex dielectric parameter is computed by solving the equations using a program. In many cases, the technique requires sample preparation such as machining so that the sample fit tightly into the waveguide or coaxial line. The method adopted for the measurement of the electromagnetic properties of the C/C material (7mm thickness) is the standard waveguide measurement system, where the C/C is mounted in a sample holder located between two waveguides, as in Figs.11a and 12.

Fig. 11 Waveguide measurement method

a

b

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Fig. 12 Sample mounting There are various approaches for obtaining the permittivity and permeability from s-parameters. In our research we adopted the Nicholson-Ross-Weir (NRW) technique [5] which provides a direct calculation of both the permittivity and permeability from the s-parameters. It is the most commonly used technique for performing such conversion. The measurement results are given here after. In particular linear S11 and S21 scattering parameters (Fig. 13), are respectively close to the unity and to 10-5. These values mean that the C/C material tends to reflect almost all of the electromagnetic incident wave when the incidence direction is perpendicular to the sample surface. This behaviour is typical of a conductor (where the average S11→1 and S21→0 i.e. meaning that almost all of the incident microwave power is reflected back to the emitting sources).

Fig. 13 S11 & S21 linear scattering parameters Regarding the electrical losses of the C/C, it is possible to observe (Fig. 14), which the sum of squares of S11 and S21 parameters are always lower than one:

Circuit Losslessfor 12111

Circuit Lossesfor 1211122

22

=+

≤+

SS

SS

Fig. 14 Sum of squares of parameters This evidences that a fraction of the incident microwave power is absorbed by C/C so some losses had to be expected [6]. These losses are also observable by the values exhibited by the complex part of the dielectric constant (Fig. 16) and Loss Tangent (Fig. 17) as well [7]. Real part of the dielectric constant is also given in Fig. 15.

Fig. 15 Real part of dielectric constant

Sample Holder

Wave Guide Flanges

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Fig. 16 Complex part of dielectric constant

Fig. 17 Electric loss factor

4 Waveguide Manufacturing and Assembly

Once characterized the C/C material, the manufacturing phase of two WR28 waveguides followed. For low thermal variations in operative environment, the in-plane CTE was to be exploited in the waveguide attenuators axial direction. Each waveguide was divided into two U shaped cross-section segments in order to satisfy the manufacturing requisites of geometry and

edge corners needed for its proper functioning. To achieve this, high precision milling of an 8mm thick C/C was carried out through the laminate thickness (Fig. 18) using dedicated graphite milling heads. A close view of the milled surface and a finished U shaped segment are given in Figs.19 and 20.

Fig. 18 Milling Fig. 19 Milled C/C surface

Fig. 20 U Shaped segment Two segments are then joined together in order to shape the rectangular hollow. The alignment tolerances are satisfied using a metallic frame appositely built to maintain the two halves in correct position during bonding procedure. Bonding was provided by the use of graphite adhesive applied only on the external joining surfaces once the two halves have been matched. The configuration is completed adding aluminum end flanges for the scattering parameter measurements. The first of the two prototypes was assembled without copper plating the internal surfaces (Fig. 21). This was intentionally done to evaluate the attenuating performance simply using C/C as conductive surfaces.

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Fig. 21 C/C Waveguide The second prototype instead had copper plating as conductive surfaces. The C/C one attenuates high frequencies more than so it was plated in order to achieve such requirement. Copper was chosen as plating material for the conducting surfaces of one prototype to improve the conductivity. The plating development effort investigated and determined a plating process for copper on C/C composite materials. Plating was carried out through electrolysis method using less than 1µm copper chemical deposition on C/C as adhesion promoter. A light surface cleaning using IPA was done before the deposition process. The plating thickness was foreseen to be around 50µm but due to non adequate electrode fixtures, clamped externally on the end tips, the deposition was much lower on the conductive surfaces (5-10µm). The plating process was performed in collaboration with ELITAL S.r.l. experts in plating electronic boards and other devices. Fig. 22 shows the assembled copper plated prototype.

Fig. 22 Plated waveguide

5 Experimental Results and Scattering Parameters

To determine the reflective coefficient of the C/C composite, a sample material which completely fills the cross-section of a WR28 sample holder (Fig. 23), was mounted on an Agilent PNA X analyzer. The frequency range tested was from 23 to 30GHz. Results evidenced that the material has a high reflective behaviour. In fact, the S22 parameter (Fig. 24) is close to the zero value indicating that the C/C material effectively well reflects other than transmit the electromagnetic incident wave. This result suggested that C/C can replace metal as base material for waveguide applications.

Fig. 23 C/C sample The most significant test for a waveguide, and also for other transmitting devices, is the signal attenuation performance at certain frequencies [4].

Fig. 24 C/C Sample Reflection Scattering Parameter

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Moreover, if the values exhibited are not acceptable then all other test become useless. For this reason this electromagnetic characteristic of the waveguides was first studied. The tests were carried out using the Agilent PNA X analyzer in the suitable range going from 23 to 30 GHz. A standard reference copper waveguide has been used to compare all results. The waveguide lengths are indicated in Table 3. Waveguide Length

(mm) Reference WR28 C/C Copper plated C/C

120 150 60

Table 3 Waveguide lengths

However, even though an optimal comparison should be performed with all waveguides having of the same length, a valid indication of the behavior and suitability of the materials can still be evaluated. The transmission coefficient (scattering parameter S21) and the reflection coefficient (scattering parameter S11) are the two characteristics investigated on the two waveguides. The following graphs (Figs.25, 26 and 27) are the reflection and transmission scattering parameters measured for all three waveguides. In relation to the C/C waveguide (Fig. 14), it can be noticed that the attenuation (transmission S21 parameter), for some frequencies, is higher than 10dB.

Fig. 25 Carbon-Carbon Wave Guide

Fig. 26 Plated Carbon-Carbon

Fig. 27 Reference Wave Guide WR28 This behaviour is typical of an attenuator and not of a wave-guide transmission line. Furthermore, an interesting aspect to be highlighted is that the attenuation behaviour decreases at higher frequencies. This fact is of interest for future investigations and at the moment it leads to the conclusion that the electric conductivity of C/C increases at higher frequencies. The following Fig. 28 is a comparison graph which evidences the S21 parameter of all three waveguides.

Fig. 28 Comparison between Reference

Calibration Kit Wave Guide, C/C Wave

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Guide, C/C-Cu Wave Guide, Transmission Scattering Parameter S21 (dB)

The black line in the graph is referred to the reference waveguide while the blue and red ones are respectively those of the C/C and plated C/C. The values exhibited indicate that the average attenuation of the copper plated C/C is around <0.1dB which is a promising result for the type of application. The slight fluctuation is probably due to the non uniform thickness of the copper deposition along the waveguide and also to a not continuous electrical contact between the two U shaped segments. The relative quality of this result, from the first fabricated plated C/C prototype, indicates not only the feasibility but also the accuracy of the fabrication and material properties suitability.

Fig. 29 C/C Waveguides

6 Conclusions This study assessed the feasibility of using C/C technology for space based waveguide instruments. This technology potentially allows low mass, rapid development, thermal stability and manufacture of integrated waveguide components. It allows simple machining of discrete components and provides feasible assembling. The technology will also reduce mass of systems allowing instruments to provide results comparable with conventional ones, and the use in a broader range of environment scenarios. The advantages and restrictions of using this technology in space will be traded off against conventional

technologies where extreme temperatures are foreseen for the mission. Results evidence that C/C waveguides (Fig. 29) can satisfy the specifications imposed in waveguide applications. Several improvements can be performed to optimize the attenuation behavior. A plating process using dedicated clamping electrodes for more uniform thickness deposition, thicker plating and silver deposition instead of copper. An interesting aspect was evidenced on the C/C material sample. High reflection S22 resulted in the scattering parameter measurements. This feature suggests that C/C composite acts as an almost perfect mirror for these frequencies and can also be suitable for electromagnetic shielding applications.

Acknowledgements The authors would like to thank the Electromagnetism and BioEngineering Department of the “Politecnica delle Marche” University for their contribution on scattering parameter measurements and ELITAL S.r.l. for the plating process development.

References [1] Alberto Anselmia, George E.N. Scoon, BepiColombo, ESA’s Mercury Cornerstone mission Planetary and Space Science 49 (2001) 1409–1420. [2] John D. Buckley, Carbon-Carbon Materials and Composites 1993 ISBN: 0-8155-1324-0. [3] E. Fitzer and L.M. Manocha, Mechanical properties of carbon/carbon composites, Carbon fibre reinforcement and carbon/carbon composites, Springer, Berlin (1998), pp. 190–236. [4] R. E. Collin, Field Theory of Guided Waves, The Institute of Electrical and Electronics Engineers, Inc., New York, 1991.

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[5] Microwave Electronics Measurement and Materials Characterization; L. F. Chen, C. K. Ong and C. P. Neo National University of Singapore V. V. Varadan and V. K. Varadan Pennsylvania State University, USA [6] Microwave Engineering, Third Edition by David Pozar [7] J. Abdulnour, L. Marchildon,“ Scattering by dielectric obstacle in rectangular waveguide”, IEEE Trans. Microwave Theory Tech., vol. MTT-41, No. 11, pp. 1988-1994, Nov. 1993.