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Improved densification of SiCf/SiC composites by Microwave-assisted Chemical Vapor Infiltration process based on multifrequency solid-state sources excitation
... The CVI process can also be combined with other processes such as photopolymerization [44] and PIP [45]. In addition, additional processes during the CVI process can also be carried out, such as the addition of microwaves [46]. The products of the CVI process can be applied to various mechanisms, including a locking mechanism at high temperatures [47]. ...
Ceramic Matrix Composites (CMCs) are transformational materials with outstanding thermal stability, mechanical strength, and resilience to severe conditions, making them important in aerospace, energy, and defence applications. This paper examines the novel approaches and problems of creating CMCs for ultra-high-temperature settings, emphasizing material selection, reinforcing strategies, and advanced production techniques. Recent improvements include using silicon carbide (SiC) and zirconium oxide (ZrO?) as matrix materials and reinforcements, such as continuous fibers and whiskers, to improve performance. Advanced processing methods, such as Polymer Infiltration Pyrolysis (PIP) and Chemical Vapor Infiltration (CVI), provide precise microstructure customization for high-demand applications. Despite these gains, challenges such as oxidation resistance, surface degradation, and cost-effective scaling persist. Integrating non-destructive evaluation techniques and adhering to high-quality standards is essential for boosting reliability. This review emphasizes the promise of CMCs in satisfying critical technological objectives while underlining the need for continuous research into processing advancements and environmental durability to enhance their application in ultra-high-temperature sectors.
In a classical chemical vapor infiltration (CVI) process, the competing effects of chemical kinetics and reagent gas transport lead to non‐uniform depositions such that outer layers of a preform densify faster leaving the core highly porous. Currently, CVI must be performed at a sufficiently low temperature to achieve good densification quality which leads to high processing time and cost. Volumetric heating of the preform, especially through microwaves, can create temperature inversion such that the core is hotter than the outer surface and potentially, overcome the challenges associated with isothermal CVI. Direct numerical simulations (DNS) of densification under various such temperature distributions indicate that microwave heating in CVI processing can lead to better (uniform) densification of porous preforms. The role of key parameters describing the temperature distributions on the densification behavior is investigated. Strategic temporal control of the temperature distribution shows that processing times can be reduced by almost half while maintaining a good densification quality similar to that of low‐temperature isothermal processing. Inside‐out densification due to the inverted temperature profile is a key distinguishing characteristic of microwave assisted CVI.
Handling plastic waste through recycling allows extending the life of polymeric materials, avoiding recurrence to incineration or landfilling. In contrast with traditional mechanical recycling technologies, chemical recycling enables the obtention of the virgin monomers by means of depolymerisation to create new polymers with the same mechanical and thermal properties as the originals. Research presented in this paper is part of the polynSPIRE project (Horizon 2020 European funding programme) and develops and scales-up a heated reactor to carry out the depolymerisation of polyamide-6 (PA6), polyamide-6,6 (PA66) and polyurethane (PU) using microwave (MW) technology as the heating source. The purpose is to design and optimize a MW reactor using up to eight ports emitting electromagnetic waves. Finite element method (FEM) simulation and optimisation are used to design the reactor, considering as parameters the data obtained from experimental dielectric testing and lab-scale characterisation of the processes and materials studied. Two different COMSOL Multiphysics modules are involved in this work: Radio Frequency (RF) and Chemical Reaction Engineering (RE), to simulate the reactor cavity using two frequency levels (915 MHz and 2.45 GHz) with a power level of 46 kW, and the chemical depolymerisation process, respectively. A sensitivity study has been performed on key parameters such as the frequency, the number of ports, and position inside the reactor to consolidate the final design. It is expected that these results assist in the design and scale-up of microwave technology for the chemical recycling of plastics, and for the large-scale deployment of this sustainable recovery alternative.
During the recent years, considerable attention has been given to the development of efficient and environmentally friendly technologies for the production of ceramic matrix composites. However, little research has been carried out on use of methylsilane (MS) as a halogen‐free precursor in the CVI process. The goal of this work was therefore to study the production of SiC‐matrix composites by pulse chemical vapor infiltration (PCVI) method with MS as a precursor and recently developed organomorphic frameworks as preforms. Experimental research was combined with numerical modeling to get deeper insight into the physical mechanisms underlying the technological process. Results of the study demonstrated that the PCVI allows performing the process with higher densification rate compared to continuous‐flow modifications of CVI. It is shown that optimization of PCVI providing a compromise between the utilization efficiency of the precursor, the densification uniformity, and the process time requires a proper choice of duration of the deposition stage and the process temperature. Numerical modeling proved to be a helpful tool in research and development of PCVI technology.
The SiCf/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t0, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t0, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiCf/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiCf/SiC composites for industrial applications.
The use of high-quality resonant cavities in microwave-assisted reactors is highly desirable. Thanks to the low losses in the cavity walls and to the possibility to reach critical coupling conditions, high energy efficiency can indeed be achieved.
This paper describes the design, building, and test of a microwave-assisted Chemical Vapor Infiltration reactor at a pilot scale, based on an overmoded resonant cavity operating at 2.45 GHz.
First, a general design method is proposed, in which the dimensions of the cavity are chosen in order to obtain a defined mode density and a high fraction of microwave power dissipated in the sample. Among the modes of this cavity, the most efficient one for the microwave heating of the sample is then determined by means of rigorous numerical analysis.
In the graphite reactor cavity built using this approach, the condition of critical coupling is met in a large variety of samples and operating conditions. Moreover, a good agreement between experimental and simulated heating dynamics is obtained, which confirms the validity of the proposed method.
As a result, disc-shaped samples with a diameter of the order of 10 cm and a thickness of the order of 1 cm can be brought to an infiltration temperature of about 900 °C within 5 minutes, using a microwave power of 1000 W, with a well controllable and reproducible temperature variation. In the infiltration of Nicalon preforms, more than 60% of the microwave power is dissipated in the sample.
In a SiC/SiC ceramic matrix composite, oxygen penetrates through unprotected ends of boron nitride coated silicon carbide fibers by ends-on oxidation in ambient air at 1200–1285 °C. For oxidation times and temperatures that cause only a few microns of oxide to form on the matrix, the boron nitride coatings can be oxidized to a depth of several hundred microns. Sub-surface oxide penetration is deeper for thicker coatings.
Whispering-gallery mode spectroscopy is employed to study the dielectric permittivity and losses of Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3, and Ba(Co1/3Nb2/3)O3 ceramics in the frequency range of 40-90 GHz and temperature interval of 5-295 K. Below 50 GHz, the room-temperature extrinsic dielectric losses in Ba(Mg1/3Ta2/3)O3 and Ba(Mg1/3Nb2/3)O3 manifest themselves as a sublinear loss factor tan delta(omega) dependence until the intrinsic losses start to dominate the spectra around 50-70 GHz. Based on these data we obtain the room-temperature intrinsic limit of dielectric loss of Ba(Mg1/3Ta2/3)O3 (Q×f=430+/-20 THz) and of Ba(Mg1/3Nb2/3)O3 (Q×f=230+/-20 THz). In the case of Ba(Co1/3Nb2/3)O3, the extrinsic losses were predominant throughout the studied frequency range. The possible sources of this loss are discussed.
(Zr0.8, Sn0.2)TiO4 ternary compounds (ZST) have been prepared by conventional solid-state reaction from raw materials. The effects of such sintering parameters as sintering temperature, sintering time, and NiO addition on structural and dielectric properties were investigated. The material exhibits a dielectric constant ɛr ∼36.0 and high values of the product Qf of the intrinsic quality factor Q and the frequency f from 32,170 to 50,000 at microwave frequencies. The dielectric loss tanδ values of ZST ceramics are decreased by low-level doping of NiO, while the temperature coefficient of the resonance frequency τf takes values in the range −2 to +4ppm/°C. Investigations on whispering gallery modes revealed low dielectric loss in millimetre-wave domain. An intrinsic quality factor of 480 was measured at 115.6GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter-wave applications, such as dielectric resonators, filters, planar antennas, hybrid microwave integrated circuits, etc.
Introduction and Background Detailed Description of the CVI Method Applications in Processing/Fabrication of Ceramics and Composites General Discussion Concluding Remarks and Future Directions References
Overview of frequency domain measurement techniques of the complex permittivity at microwave frequencies is presented. The methods are divided into two categories: resonant and non-resonant ones. In the first category several methods are discussed such as cavity resonator techniques, dielectric resonator techniques, open resonator techniques and resonators for non-destructive testing. The general theory of measurements of different materials in resonant structures is presented showing mathematical background, sources of uncertainties and theoretical and experimental limits. Methods of measurement of anisotropic materials are presented. In the second category, transmission–reflection techniques are overviewed including transmission line cells as well as free-space techniques.
The introduction of ceramic matrix composites (CMCs) into the hot sections of aircraft engines offers significant weight and fuel efficiency gains. The result of decades of research and development, CMCs are based on silicon carbide. They consist of a matrix reinforced with the latest generation of continuous fibres. The non-brittle behaviour of these materials is achieved by the ''interphase'', a thin coating of fibres. Boron nitride is the interphase material chosen to act as a mechanical fuse and to withstand the conditions of matrix manufacture and composite application. This paper considers the influence of the interphase on the mechanical properties of these CMCs and their behaviour in the different environments for which they are intended. Finally, the different preparation methods studied in the laboratory and the production methods used in industry are discussed, focusing on the gas phase routes.
The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 ◦) and off-axis (±45 ◦) direction are summarized. The tensile properties are determined at room temperature and 1300 ◦C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 ◦C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 ◦C.
Hexagonal BN fiber coatings and BN powders were prepared by pyrolysis of the raw materials boric acid and urea in an atmosphere consisting of hydrogen and nitrogen. The powders were used to determine the appropriate mixing ratio of the raw materials to produce a BN with the desired composition and crystal structure. The pyrolysis of boric acid and urea in a molar mixing ratio of 1:2 resulted in a BN that was hexagonal and had a near-stoichiometric composition.
To prepare a solution for the coating of fibers, boric acid and urea were dissolved in an ethanol-water mixture. The coating was then applied to SiC filaments using a continuous roll-to-roll dip-coating process. It could be shown by SEM/EDS that BN layers were applied to the fibers. No significant bridging in the fiber bundle was found. Furthermore, it could be demonstrated by grazing incidence x-ray diffraction that the layers were crystalline.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC CMCs) are promising candidates for components in the hot gas section of jet engines, as they exhibit high temperature resistance and low density compared to their metal alloy counterparts. Three common manufacturing routes are chemical vapor infiltration, reactive melt infiltration (RMI) and polymer infiltration and pyrolysis (PIP). This work describes a new approach of a combined PIP and RMI process. A combination of the processes seems attractive: the remaining porosity after PIP process can be closed by subsequent siliconization, resulting in a dense material. SiC/SiC CMCs were manufactured by PIP process using Hi-Nicalon Type S fibers. Generally, the processing of SiC/SiC, produced solely by PIP route, is rather time-consuming and the composites show a certain residual porosity. In order to obtain a dense matrix and to reduce the processing time, an additional RMI with silicon alloy is carried out after a reduced number of PIP cycles. To protect the fibers during the siliconization, a CVD fiber coating was applied. Microstructure was examined via microCT, SEM and EDS. Bending strength was determined to 433 MPa; strain to failure was 0.60%. The overall processing time was reduced by 55% compared to standard PIP route. The hybrid material contained 70% less unreacted carbon than material produced by LSI process alone.
Fiber coatings based on BN, BN/SiC and BN/Si3N4 were deposited on Hi Nicalon type S SiC fibers. The coating parameters were optimized using a design of experiments study. With optimized parameter sets, the coatings exhibited a high degree of coverage on the fibers and almost no fiber bridging could be observed. The coated fiber bundles are flexible and can be processed further by techniques such as filament winding. In comparison to a non-processed reference sample, the maximum tensile load of the fiber bundles with BN, BN/SiC and BN/Si3N4 coatings was reduced by only 5 %, 13 % and 10 %, respectively. The coated fiber bundles retained their tensile strength after thermal annealing up to 1650 °C in a nitrogen atmosphere for 0.5 h. SiCf/SiC samples with BN/SiC fiber coatings exhibited higher values of bending strength and strain-to-failure as a reference sample without fiber coating indicating the functionality of the fiber coatings.
A hybrid Microwave-assisted Chemical Vapor Infiltration (MW-CVI) pilot plant to produce silicon carbide-based Ceramic Matrix Composites was designed, built and setup, as a part of the European project HELM. Being different from the existing lab-scale MW-CVI equipment, this pilot plant was designed with the idea of a further industrial scale-up.
In order to enable the infiltration of the large samples of interest in industrial applications, the reactor was designed with the internal microwave cavity acting as an overmoded resonator at the frequencies of interest. The designed pilot plant allowed proper microwave heating of cylindrical samples of diameter doubled with respect to typical lab-scale preforms, with reproducible operating conditions in terms of transmitted/reflected power. First infiltration trials resulted in an average reaction efficiency of 25% with the desired inside-out silicon carbide infiltration. The main steps of the design and the results of the first infiltration tests are discussed in this paper.
SiC/SiC composites reinforced with 3rd generation SiC fibers (Hi-Nicalon S and Tyranno SA3 fibers) are promising candidates for thermomechanical applications in high technology industries. Both composites exhibited a pseudo-ductile mechanical behavior but the HNS/PyC/SiC composite reaches higher failure strains than TSA3/PyC/SiC ones. The mechanical behavior of SiC/SiC composites is linked to the way PyC is bonded to the fiber surface. Analyses have shown that these interactions and the Fiber/Matrix debonding behavior depend strongly on the nature of the carbon on the SiC fiber surface, which is different according to the SiC fiber. In order to understand the mechanism governing the chemical adhesion at the PyC/SiC fiber interface, the formation, the chemistry and the structure of the surface carbon layer were studied. Understanding the origin of this carbon has allowed elucidating the local interaction mechanisms of the studied SiC/SiC composites.
Ceramic materials have a very attractive package of properties: high strength and high stiffness at high temperatures, chemical inertness, low density, etc. What they lack is toughness. The ceramic matrix composites have the main objective of enhancing toughness. In this chapter, we first describe some of the processing techniques for CMCs, followed by a description of some salient characteristics of CMCs regarding interface and mechanical properties and, in particular, the various possible toughness mechanisms, and finally a description of some applications of CMCs, including the SiC/SiC composites introduced in aircraft engines.
High-power solid-state sources are expected to be the core elements in efficient controlling characteristics of microwave thermal processing. In this paper, we analyze the effect made by frequency alteration within the 2.4–2.5 GHz ISM band on the patterns of dissipated power density in the processed material. A dual-source rectangular cavity loaded by a rectangular block characterized by different dielectric properties is considered. Two regimes of alteration involving equidistant and resonant frequencies are examined using an finite-difference time-domain model validated against theoretical data on eigen frequencies and corresponding eigen fields. The resulting heating patterns generated after a full cycle of alteration are qualitatively analyzed as superpositions of contributions by the distributions at different frequencies. It is shown that both regimes are capable of effectively mixing up the diverse heating patterns and thus produce more uniform distributions of dissipated power. Alteration over resonant frequencies appears to have the potential for leading to more energy-efficient processes.
Carbon/carbon and ceramic-matrix composites are frequently processed by chemical vapor infiltration, a technique that ensures a very good quality of the interphase and matrix, while preserving the fibers. We review in this chapter the principle and several implementations and applications of this process; in a second part, a more precise description of the physico-chemistry and the bases of process modeling are treated.
A system has been developed, based on the cavity frequency shift technique, for doing rapid scoping measurements of the complex permittivity and scalar permeability from 50 to 2450 MHz andfrom-160°C to +1400°C. The measurement system, including multimode rf cavities, is described, and some measurements on a selection of materials are presented to demonstrate the use of the system.
Solid state amplifiers are being increasingly used instead of electronic vacuum tubes to feed accelerating cavities with radio frequency power in the 100 kW range. Power is obtained from the combination of hundreds of transistor amplifier modules. This paper summarizes a one hour lecture on solid state amplifiers for accelerator applications.
Ceramic matrix composites (CMCs) generally consist of ceramic fibers or whiskers in a ceramic matrix. CMCs are designed to overcome the main drawback of monolithic ceramics, namely their brittleness. They are referred to as inverse composites, which is to say that the failure strain of the matrix is lower than the failure strain of the fibers, whereas it is the reverse in most polymer or metal matrix composites. Hence, under load it is the matrix which fails first. In order to prevent an early failure of the brittle fibers when the matrix starts to microcrack, the fiber/matrix (FM) bonding should be controlled during processing. CMCs are tough materials and display a high failure stress when the FM bonding is not too strong or too weak, which is usually achieved through the use of a fiber coating referred to as the interphase. The fabrication of CMCs requires specific processing techniques. Gas-or liquid-phase routes (or a combination of both), in which the interphase and the matrix are formed around the fibers from gaseous or liquid precursors, are usually preferred. CMCs are used as thermostructural materials under severe service conditions, for example, high temperatures under load and in corrosive atmospheres, such as combustion gases. The most commonly used CMCs are nonoxide CMCs, namely carbon/carbon (C/C), carbon/silicon carbide (C/SiC), and silicon carbide/silicon carbide (SiC/SiC), the fibers being specified first.
Ceramic matrix composites reinforced with long fibers are commonly fabricated by infiltration methods, in which the ceramic matrix is formed from a fluid infiltrating into the fiber structure. Infiltration techniques differ from each other in the types of fluids and the processes for converting the fluid into a ceramic: polymer infiltration and pyrolysis (PIP), chemical vapor infiltration (CVI), reactive melt infiltration (RMI), slurry infiltration, sol-gel infiltration. This chapter discusses the formation of the ceramic matrix microstructure, properties of the interface and the benefits and drawbacks of the composites prepared by the different techniques. Fabrication routes including the stages of preform preparation, interphase deposition, preceramic fluid infiltration and thermal processing are described.
The chemical vapor infiltration process is used to fabricate the interphases and matrices of ceramic-matrix composites. This process involves complex physicochemical phenomena such as the transport of precursor, carrier, and by-product gases in the reactor and inside a fibros preform, chemical reactions (pyrolysis and deposition), and the structural evolution of the preform. It is able to provide high-quality materials; however, it is expensive and sometimes difficult to optimize. Many variations of the basic concept have been proposed in the past decades, introducing thermal and pressure gradients, in order to increase its efficiency.
(Zr0.8Sn0.2)TiO4 ceramic (ZST) has been prepared and characterized. The effects of sintering parameters such as sintering temperature, sintering time and NiO addition on structural and dielectric properties were investigated. The material has a dielectric constant εr ∼ 36.0 and high values of the Q·f product from 32,170 to 50,000 at microwave frequencies. The tan δ values are decreased by low level doping of NiO, while the temperature coefficient of the resonance frequency τf takes values in the range (-2÷+4) ppm/°C. Investigations on whispering gallery modes revealed low dielectric losses in millimeter wave domain. An intrinsic quality factor of 480 was measured at 136. 3 GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter wave applications such as filters, hybrid microwave integrated circuits, etc.
Carbon/carbon and ceramic-matrix composites are frequently processed by chemical vapor infiltration, a technique that ensures a very good quality of the interphase and matrix, while preserving the fibers. We review in this chapter the principle and several implementations and applications of this process; in a second part, a more precise description of the physico-chemistry and the bases of process modeling are treated.
Ceramic materials in general have a very attractive package of properties: high strength and high stiffness at very high temperatures, chemical inertness, low density, and so on. This attractive package is marred by one deadly flaw, namely, an utter lack of toughness. They are prone to catastrophic failures in the presence of flaws (surface or internal). They are extremely susceptible to thermal shock and are easily damaged during fabrication and/or service. It is therefore understandable that an overriding consideration in ceramic matrix composites (CMCs) is to toughen the ceramics by incorporating fibers in them and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure. It is worth pointing out at the very outset that there are certain basic differences between CMCs and other composites. The general philosophy in nonceramic matrix composites is to have the fiber bear a greater proportion of the applied load. This load partitioning depends on the ratio of fiber and matrix elastic moduli, E
f/E
m. In nonceramic matrix composites, this ratio can be very high, while in CMCs, it is rather low and can be as low as unity; think of alumina fiber reinforced alumina matrix composite. Another distinctive point regarding CMCs is that because of limited matrix ductility and generally high fabrication temperature, thermal mismatch between components has a very important bearing on CMC performance. The problem of chemical compatibility between components in CMCs has ramifications similar to those in, say, MMCs. We first describe some of the processing techniques for CMCs, followed by a description of some salient characteristics of CMCs regarding interface and mechanical properties and, in particular, the various possible toughness mechanisms, and finally a description of some applications of CMCs.
A microwave heated methyl trichlorosilane based chemical vapour infiltration technique has been used to form SiCf/SiC composites from SiC fibre preforms preimpregnated with SiC powder using two different fabrication techniques. While infiltration rates obtained from samples loaded with powder were generally higher than for preforms without powder, preferential infiltration occurred in regions where the SiC powder was most concentrated as a result of the initial non-uniform distribution of the powder across the preforms. The consequence was density gradients in the final composites. Nevertheless, average densities as high as 75% could be achieved in a 10 h process.
Techniques based on vacuum bagging (VB) and electrophoretic impregnation (EPI) have been investigated for the impregnation of SiC powder into layered Nicalon SiC fabric preforms. The aim was to produce preimpregnated samples for subsequent chemical vapor infiltration (CVI) with reduced intertow porosity that arises from the construction of the fabric layers while leaving unblocked the intratow porosity that is so indispensable for a successful infiltration. Because the goal was simply to learn about the ability to impregnate the samples, no interphase coating was applied to the fibers as would normally be used when producing SiCf/SiC composites. While the VB process generally yielded much stronger preforms, depending on the pressure used and the powder particle size, it resulted in powder becoming located in the intratow rather than the intertow porosity. In contrast, provided an appropriate electrode arrangement was used, EPI offered the potential for a more controlled impregnation process with the powder primarily found in the intertow porosity; however, the preforms were very weak and delaminated easily. The combination of the two processes resulted in a very successful approach, with greater uniformity of particle infiltration and higher green strengths, while largely avoiding impregnating the intratow porosity.
Ba(Mg1/3Ta2/3)O3 ceramic possessing extremely high Q×f value of more than 300THz at microwave frequency was developed in our previous study. It is of great interest to understand the mechanism of microwave absorption in such a practical material. In the present study we report on the temperature dependence of the dielectric loss in the Ba(Mg1/3Ta2/3)O3. The mechanism of the microwave absorption is discussed using two phonons difference process. The samples were prepared by conventional solid state reaction and sintered at 1893K in oxygen atmosphere. Dielectric properties in the microwave range were measured by Hakki & Colemann and resonant cavity methods in the temperature range of 20–300K. Whispering gallery mode technique was used for the measurement of the dielectric properties at the millimeter wave frequency. Dielectric loss of the Ba(Mg1/3Ta2/3)O3 at the microwave frequency increases with temperature between 200 and 300K in general agreement with the theory of intrinsic dielectric loss derived from the two phonon difference process. However below 200K, the dielectric loss has shown a distinctive behavior with a loss peak at 40K. It was inferred that the loss peak of the Ba(Mg1/3Ta2/3)O3 was caused by the local orientation polarization having dispersion at the microwave frequency.
Carbon and SiC fiber-reinforced ceramic matrix composites were prepared via infiltration of fiber preforms using the polymer infiltration technique and polymer pyrolysis. Suitable silazane (SiCN) precursors with appropriate thermosetting behavior, viscosity and ceramic yield were synthesized, starting from functionalized chlorosilanes. Microstructural development and fracture behavior was studied after various infiltration and pyrolysis cycles. Residual stresses induced during processing were evaluated. Mechanical and thermo-physical properties of the composites with polymer-derived matrix, i.e. 3-pt bending strength and thermal expansion coefficients (CTE), were measured dependent on reinfiltration cycles and fiber orientation. The oxidation resistance was investigated. Specific pyrolyzed samples were infiltrated via silicon melts in order to enhance corrosion and wear resistance.
The effect of both A-site and B-site non-stoichiometry on the microstructure and microwave dielectric properties of the perovskite niobates Ba(A 2 + 1/3 Nb 2/3 )O 3 (A 2 + = Co, Zn) has been examined. Both the chemical composition of the ceramics and their sintering regimes have been shown to significantly influence the type and the amount of impurity phases which consecutively demonstrate a prevailing effect on the microwave dielectric loss. The highest magnitudes of the Qxf product have been obtained in the Co-deficient BCN (Qxf = 100 000 GHz) and Ba-deficient BZN (Qxf = 90 000 GHz) which both demonstrate noticeable amount of secondary Ba-rich phases.
A microwave assisted process for production of continuous fiber reinforced ceramic matrix composites is described. A simple apparatus combining a chemical vapor infiltration reactor with a conventional 700 W multimode oven is described. Microwave induced inverted thermal gradients are exploited with the ultimate goal of reducing processing times on complex shapes. Thermal gradients in stacks of SiC (Nicalon) cloths have been measured using optical thermometry. Initial results on the “inside out” deposition of SiC via decomposition of methyltrichlorosilane in hydrogen are presented. Several key processing issues are identified and discussed.
The use of microwave energy to heat substrates during chemical vapor infiltration (CVI) is described. Both numerical predictions and experimentally observed thermal profiles indicate that steep inverted thermal gradients can be established in fibrous preforms subjected to microwave radiation. When combined with the CVI technique, the inverted gradients can be exploited to promote inside-our densification. Recent microwave assisted CVI results, which focus on silicon based matrix materials, are described. Emphasis is placed on the role played by the relative ability of fiber and matrix to dissipate microwave energy. Issues central to further process development, such as the effect of interfacial coating on subsequent microwave processing, are discussed.
A model describing the reactive melt infiltration (RMI) of liquid silicon into a carbonaceous preform is solved and the simulation is compared with experimental results. The finite-element method is used to solve the equations in arbitrary geometries. The model is consistent with experimental results in that it predicts the infiltration rate in a reacting and nonreacting system and the magnitude and functional form of the temperature rise in the sample caused by the exothermic reaction of silicon with carbon to form silicon carbide. The model is particularly sensitive to two parameters, the pore neck diameter (d1) as defined by an ideal repeating pore unit, and the SiC reaction barrier thickness (δ0) which forms when silicon reacts with carbon to form a SiC skin around the particles. The sensitivity of the solution to these two parameters is demonstrated. It is suggested that be chosen based on experimental measurements of infiltration rate and temperature excursion during silicon infiltration.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiCf/SiC) have been produced using microwave heated chemical vapor infiltration. Preferential densification of the composite from the inside out was clearly observed. Although an average relative density of only 55% was achieved in 24 h, representative of an ∼26% increase over the initial fiber vol%, the center of the preform densified to 73% of the theoretical. The densification mechanisms were investigated using X-ray absorptiometry and scanning electron microscopy. The initial inverse temperature profile obtained, which was found to result in the efficient filling of the intratow porosity, although not the intertow porosity, flattened out after approximately 6 h as the densification front moved outward toward the edges. Although not investigated directly, the evidence suggested that this was caused by changes in both the thermal conductivity and microwave absorption characteristics as the samples densified.
Pyrocarbon (PyC), the common interphase for SiC/SiC, is not stable under severe environmental conditions. It could be replaced by boron nitride more resistant to oxidation but poorly compatible with nuclear applications. Other materials, such as ternary carbides seem promising but their use in SiC/SiC has not been demonstrated. The most efficient way to improve the behavior of PyC interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself. Issues related to the design and behavior of layered interphases are reviewed with a view to demonstrate their interest in high-temperature nuclear reactors.
Al2O3–TiO2 composite dielectric ceramics have been prepared by different sintering techniques including high-temperature, high-pressure sintering (2.5 GPa, T = 1000 °C) and conventional pressureless sintering (T = 1350 °C). Formation of the Al2TiO5 secondary phase has been completely suppressed by optimization of the sintering and annealing temperatures. Dielectric properties were measured in the 10–11 GHz range using the cylindrical resonant cavity technique and in the 40–92 GHz range using the open resonator whispering gallery mode technique. At 10 GHz, the optimized composite material exhibited and at room temperature. As an evidence of an additional, low-frequency (possibly Debye-type), extrinsic contribution to the dielectric loss, the Q × f value gradually increased with frequency and reached a plateau of Q × f at approximately 80 GHz. It is demonstrated that Al2O3–TiO2 composites have considerable potential as dielectric resonators in the output multiplexers of communication satellites.
Chemical vapor infiltration (CVI) is simply chemical vapor deposition (CVD) on the internal surfaces of a porous preform and has been used to produce a variety of developmental and application materials. The greatest use of CVI is to infiltrate continuous-filament preforms taking advantage of the relatively low-stress CVD process. In CVI, reactants are introduced in the porous preform via either diffusion or forced convection and the CVD precursors deposit the appropriate phase(s). As infiltration proceeds, the deposit on the internal surfaces becomes thicker. Thus, after some length of time, the growing surfaces meet bonding the preform and fill much of the free volume with deposited matrix. The forced-flow/thermal-gradient technique (FCVI) developed at Oak Ridge National Laboratory overcomes the problems of slow diffusion and restricted permeability, and has demonstrated a capability to produce thick-walled, simple-shaped, SiC-matrix components in times of the order of hours. A model has been developed for the process that predicts flow, thermal and density profiles as a function of time. The results have been compared with an initial set of experiments and indicate qualitative agreement. It is expected that improved property relationships, such as permeability and thermal conductivity as a function of density, will allow the model to closely represent the FCVI process and be useful in fabrication and product optimization.
The status of vapor-phase routes for the rapid densification of high-temperature composite materials, primarily ceramic-matrix composites, is reviewed. Conventional densification of composites such as carbon-carbon and SiC-SiC is accomplished by isothermal, isobaric chemical vapor infiltration (CVI), either alone or in combination with liquid resin impregnation and thermal annealing. These are multi-step processes which take from several hundred to thousands of hours at high temperature. In this paper we review approaches designed to significantly reduce the processing time and the number of steps required for densification, while producing materials with the desired properties. We describe techniques such as inductively-heated thermal-gradient isobaric CVI, radiantly-heated isothermal and thermal-gradient forced-flow CVI, liquid-immersion thermal-gradient CVI and plasma-enhanced CVI. Different heating methods, such as radiative and inductive, and both hot-wall and cold-wall reactors are compared. Available material properties of composites produced by these techniques are given.
Boron Nitride (BN) coatings deposited by chemical vapor deposition (CVD) have been increasingly used as an interface material for SiC/SiC composites. In this work, the CVD of BN was investigated using a statistical design of experiments (DOE) approach. In order to determine the most significant parameters for the process a two-level screening design (Plackett–Burman) was employed. The deposition pressure, gas mixture dilution factor, deposition time, and the reaction gas flow ratios were found to be the most significant factors that influenced coating thickness. To optimize the deposition process, a three-level surface response design (Box–Behnken) was used with the aim of producing a predictive mathematical model of the process. The generated response surface modeling (RSM) showed that deposition time had the greatest effect on coating thickness while, temperature–time and temperature–NH3/BCl3 interactions may be large at low/high NH3/BCl3 ratios and high deposition time, respectively. Tensile strength was strongly influenced by the deposition temperature and deposition time. The response model showed the dependence of tensile strength on coating thickness, NH3/BCl3 gas flow ratios and time. The model interaction plots suggested a dependence of temperature–gas flow ratio on tensile strengths of BN coated SiC fibers.
The film-boiling densification process is an alternative of chemical vapor infiltration involving a strong thermal gradient. It allows to fabricate composite materials starting from a fibrous preform lying in a boiling hydrocarbon precursor, the cracking of which results in a solid deposit constituting the matrix of the carbon/carbon composite. A modelling approach is presented and validated with respect to experimental data. Then, the sensitivity of the process is studied with respect to various parameters. Optimization guidelines are proposed, in conjunction with a discussion on the densification front that characterizes the process. It is thus possible to evaluate the minimal amount of power required, while maintaining the quality of the produced material, i.e., its bulk density and homogeneity.