Multifunctional syntactic foams

A multifunctional syntactic foam including a matrix material and hollow particles, where a first and second material property of the syntactic foam are tailored on the basis of the wall thickness and volume fraction of the hollow particles.

BACKGROUND OF THE INVENTION

The present invention is generally related to materials that are used in the field of packaging or structural applications, such as syntactic foams, and is more specifically related to materials that have properties, such as dielectric constants, that may be tailored so as to be useful in such applications.

The dielectric constant is an important property in electronic substrate or packaging applications. Materials that have a possibility of tailoring their dielectric constant may be very useful in such applications. In addition, an increase in temperature during the usage of electronic components may be a challenge that may lead to thermal stresses and failure. Syntactic foam composite materials have been employed in such applications.

Very large scale integration of electronic circuits has drastically reduced the size of circuit boards used in electronic devices. This has created a desire to develop materials with low dielectric constant, high specific strength, low density, low moisture absorption and high durability. Integrated circuit boards, which form the heart of computers, require electrical insulators with low and preferably tunable dielectric properties. Polymers and polymeric composites have found applications in such fields due to their low dielectric properties.

Epoxy resins, which are often used as matrix materials for composites, may also be used in electrical and electronic fields as insulators, dielectrics and as underfills in circuit boards. One of method of decreasing the dielectric constant of the polymer is to introduce porosity in the polymer. Since air has a low dielectric constant of 1, the dielectric constant of polymer foams is low and may also accompany low strength and stiffness, which are undesirable. In addition, irregular size and distribution of gas voids in polymer foams may lead to mechanical property variation within the material.

The instant inventors are not aware of an instance where the dielectric constant and the coefficient of thermal expansion of a syntactic foam have been tailored simultaneously. Accordingly, there is a need for a syntactic foam composite material that may be simultaneously tailored for multiple properties, such as dielectric constant, coefficient of thermal expansion (CTE), and density,

SUMMARY OF THE INVENTION

A method is provided including: calculating, utilizing at least one computer, a range of potential values for a first material property of a syntactic foam, the syntactic foam including a matrix material and hollow particles, on the basis of a wall thickness and a volume fraction of the hollow particles; selecting a first material property value from the range of potential values for the first material property; calculating, utilizing at least one computer, a range of potential values for a second material property of the syntactic foam on the basis of the wall thickness and the volume fraction of the hollow particles while maintaining the selected first material property value; selecting a second material property value from the range of potential values for the second material property; and calculating, utilizing at least one computer, the wall thickness and the volume fraction of the hollow particles that will produce a syntactic foam with the selected first material property value and the selected second material property value. The first material property and the second material property may be selected from the dielectric constant, coefficient of thermal expansion (CTE), damping capacity, elastic modulus, storage modulus, loss modulus, and density, and the first material property is different than the second material property.

A method is provided including: calculating, utilizing at least one computer, a range of potential values for a first material property of a syntactic foam, the syntactic foam including a polymer matrix material and hollow glass particles, on the basis of a wall thickness and a volume fraction of the hollow glass particles; selecting a first material property value from the range of potential values for the first material property; calculating, utilizing at least one computer, a range of potential values for a second material property of the syntactic foam on the basis of the wall thickness and the volume fraction of the hollow glass particles while maintaining the selected first material property value; selecting a second material property value from the range of potential values for the second material property; and calculating, utilizing at least one computer, the wall thickness and the volume fraction of the hollow glass particles that will produce a syntactic foam with the selected first material property value and the selected second material property value. The first material property and the second material property may be selected from the dielectric constant and the coefficient of thermal expansion (CTE), and the first material property is different than the second material property.

A syntactic foam is provided. The syntactic foam may have a coefficient of thermal expansion of about 30×10−6° C. to about 70×10−6° C., and a dielectric constant of about 2.6 to about 4.9. The syntactic foam may include a matrix material and hollow particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Following below are more detailed descriptions of various concepts related to, and embodiments of, inventive processes for the selection of a volume fraction and wall thickness of hollow particles in a syntactic foam to produce a syntactic foam with two independently tailored material properties. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

The present invention includes a method that may be applied to any combination of hollow particles and a matrix material to produce syntactic foams with two individually tailored properties. For example, a syntactic foam composite material may be simultaneously tailored for a desired dielectric constant and coefficient of thermal expansion (CTE). A range of compositions may be developed for syntactic foams that have the same dielectric constant but have different CTE values, and vice versa. Selection of an appropriate material composition allows the production of syntactic foams with both desired properties, such as dielectric constant and CTE.

As utilized herein, a syntactic foam includes hollow particles distributed in a matrix material. Syntactic foams may be low density composite materials with a significant volume fraction of porosity. The mechanical and thermal properties, such as elastic modulus, loss modulus, density, coefficient of thermal expansion (CTE) and thermal conductivity, of a syntactic foam may be tailored. The variables in designing syntactic foams include: the particle and the matrix materials, the volume fraction of the particles, and the wall thickness of the particles. In many applications, weight is an important consideration in using syntactic foams. A combination of hollow particle wall thickness and volume fraction may be used to independently tailor the CTE and the density of syntactic foams to achieve weight savings in structural applications.

The matrix material may be any suitable material, such as a polymer, metal or ceramic. According to one embodiment, the matrix material may be a vinyl ester resin. According to another embodiment, the matrix material may include aluminum.

The hollow particles may be any suitable particles. The particles may be spherical, cubic, cuboidal, cylindrical or any other appropriate geometry. The particles may include a ceramic, glass, polymer or carbon material—such as alumina, boron carbide, silicon carbide, carbon, phenolic polymers, or epoxy resin. According to one embodiment, the hollow particles may be spherical glass microballoons. According to another embodiment, the hollow particles may be hollow spherical silicon carbide particles. The hollow particles may be any suitable size. According to one embodiment, the hollow particles may be microparticles.

The properties of the syntactic foams may be tailored based on the volume fraction and wall thickness of the hollow particles. According to one embodiment, at least two properties of the syntactic foam may be independently tailored. Theoretical calculations allow the prediction of the volume fraction and wall thickness of the hollow particles that will produce a desired property. The theoretical calculations may be performed on at least one computer. A desired material property may be produced for a range of volume fraction and wall thickness values. Thus, a first desired material property may be selected, and a range of potential values for a second material property may be calculated that may be achieved by altering the volume fraction and wall thickness while also maintaining the desired first material property. The effect of the volume fraction and wall thickness on various material properties of the syntactic foam are described below.

The appropriate volume fraction and wall thickness for achieving a desired first and second material property may be determined by a material property tailoring process. First, a potential range of first material property values may be calculated based on the alteration of the volume fraction and the wall thickness of the hollow particles, and then a desired first material property value may be selected. A potential range of a second material property may then be calculated based on the range of volume fraction and wall thickness of the hollow particles that may produce the desired first material property. The selection of a desired second material property value from the calculated second material property potential range then allows the calculation of the volume fraction and wall thickness of the hollow particles that may produce a syntactic foam that exhibits both the first and the second selected material properties. The first and the second material properties may be any material property of the syntactic foam that exhibits a relationship to the volume fraction and wall thickness of the hollow particles, such as those properties described below. The first and second properties be selected from the dielectric constant, coefficient of thermal expansion, density, stiffness, thermal conductivity, glass transition temperature, loss modulus, storage modulus, damping parameter, and dynamic properties. The first and second material properties may be different material properties.

In addition to the volume fraction and wall thickness of the hollow particles, other factors may influence the calculated potential material property ranges. These additional factors may include the matrix material selected, the hollow particle material selected, and the type of processing utilized to produce the syntactic foam. For example, a syntactic foam produced by a shear mixing process may include an increased amount of broken hollow particles, adversely impacting the properties of the produced syntactic foam. Additionally, the properties of the raw materials employed in the production of the syntactic foams may impact the material properties of the produced syntactic foams.

The syntactic foams described herein may be especially suited for a variety of applications. The syntactic foams may be utilized in marine applications, including submarine components, ship components, deep sea vehicle components, undersea pipeline components, buoy components, and equipment platform components. Alternatively, the syntactic foams may be utilized in aerospace components, including spacecraft components, aircraft components, and antenna units. The syntactic foams may also be utilized in electronic device components, such as radio equipment and printed circuit board components. Other applications for which the syntactic foams may be suited include sports equipment, furniture, food and beverage containers, composite tooling components, vacuum forming plug assists, blast protection components, fire protection components, furniture, and medical devices.

Viscoelastic Properties

The understanding of the viscoelastic properties of syntactic foams at a wide range of temperatures and loading frequencies may be helpful for developing composites with tailored vibration response, high temperature mechanical properties, and energy absorption capabilities.

Dynamic mechanical analysis (DMA) is a technique for the characterization of viscoelastic material parameters. In this technique a sinusoidal force is applied to a specimen and the phase difference in the stress and strain response due to viscoelasticity is recorded. The in-phase and out-of-phase components of stress are used to calculate the storage modulus (E′) and loss modulus (E″), respectively. The storage modulus provides a measure of energy stored in the material, and the loss modulus refers to the amount of energy dissipated in each cycle of the sinusoidal deformation. The ratio of the loss modulus to the storage modulus is the damping parameter, Tan δ. Loss modulus-temperature data may be used to measure the glass transition temperature (Tg). Correlations of these properties with the microballoon wall thickness and microballoon volume fraction (Φ) of syntactic foams may allow the production of syntactic foams with selected properties. Such correlations between the fundamental microstructural features and dynamic properties of syntactic foams may allow the engineering of syntactic foams to various requirements despite the wide variety of matrix and microballoon materials employed.

Twelve compositions of syntactic foams were fabricated with a systematic variation in microballoon wall thickness and volume fraction to enable the establishment of a correlation between these parameters with the measured properties of the composites. The combined effect of frequency and temperature on the dynamic mechanical properties was analyzed. The William-Landel-Ferry (WLF) equation may be employed for the time-temperature superposition (TTS) to obtain the variation of viscoelastic properties such as storage modulus, loss modulus, and damping parameter over a wide frequency range.

Vinyl ester resin was used as the matrix material for preparing the syntactic foams. Methyl ethyl ketone peroxide was used as the catalyst to polymerize the vinyl ester resin. Hollow glass microballoons of three nominal true particle densities, namely 220 kg/m3, 320 kg/m3and 460 kg/m3, were used in volume fractions of 0.3 to 0.6. Since the density of the microballoons is less than half the density of the resin system, flotation of the microballoons may occur and the quality of the composite may not be uniform at microballoon volume fractions lower than 0.3. The properties of the glass microballoons employed are presented in Table 1.

Since the particles have nearly the same outer diameters, the main varying parameter is the wall thickness (w), as illustrated inFIG. 1. The radius ratio, η, defines the geometric properties of hollow particles as
η=ri/ro(1)
where riand roare the internal and the outer radii of the microballoon. The particle wall thickness may be defined in terms of radius ratio as w=r0(1−η). Thus, η is lower for a thicker walled hollow particle of the same radius. The outer diameter of the particles and the wall thickness may be varied independently.

FIGS. 2A and 2Bshow the microstructures of syntactic foams containing Φ of 0.3 and 0.6, respectively. The measured and the theoretical densities of the composites are shown in Table 2. The theoretical density may be obtained using the rule of mixtures. Some undesired voids, termed matrix porosity, may be present in the matrix. The volume fraction of the matrix porosity Φpmay be given by

Φp=(ρth-ρexpρth)×100(2)
where ρthand ρem, are the theoretical and the experimental densities, respectively, of syntactic foams. The calculated matrix porosity content is given in Table 2, which is of similar level and below 5 vol. % for all the produced syntactic foams, except for VE220-60. The specimens in Table 2 are named according to the following nomenclature: VE for vinyl ester, followed by the true particle density and the particle vol. %.

Specimens of nominal dimensions 30×11×1.75 mm (length×width×thickness) were prepared for all experiments. The specimen width and thickness were controlled within ±1% accuracy. The specimens were cut using a low speed precision diamond blade saw and then dried in a convection oven for 3 h at 70° C. prior to testing. A Dynamic Mechanical Analyzer was utilized to conduct the experiments. The experiments were performed in single cantilever mode with a span length to thickness ratio of 10. A frequency of 1 Hz was employed for studying the effect of temperature on dynamic properties. The temperature range used in the study was from −75° C. to 195° C., at a heating rate of 5° C./min and an isothermal soak time of 1 minute. The specimens were tightened in the cantilever clamp using a torque of 0.8 Nm.

A TTS study was also performed under the single cantilever mode on specimens of the same geometry. The experiments were conducted in a frequency range of 1-100 Hz, divided linearly into 20 divisions. The testing temperature range was 30° C. to 140° C., with an isothermal soak time of 1 minute and the frequency sweep at every 5° C.

FIGS. 3A, 3B, and 3Cdepict representative graphs for storage modulus (E′) variation with respect to temperature in the range −75° C. to 195° C. for VE220, VE320, and VE460 syntactic foams, respectively. Neat resin results are also plotted in each figure for the purposes of comparison. As a general characteristic, the graphs have three distinct regions as illustrated inFIG. 4. In Region I, E′ slowly decreases with an increase in temperature. In Region II, the rate of E′ reduction increases drastically. This region appears around Tg. In Region III, E′ stabilizes to a very low value compared to that in Region I. The Region III defines the flow region where E′ variation is negligible. The E′ values at three representative temperatures, −50° C., 30° C. and 175° C. (marked inFIG. 4), are presented in Table 3 to demonstrate the extent of change with respect to temperature.

The E′ for neat resin was higher than any measured composition of VE220 and VE320 syntactic foams at low temperatures (in Region I). However, VE460 syntactic foams exhibit E′ comparable to that of neat resin. These trends are similar to the room temperature tensile and compressive modulus values previously reported for the corresponding syntactic foam compositions. An increase in Φ produced a decrease in E′ for all types of microballoons.FIGS. 3A, 3B, and 3Cshow that the curves for different Φ of the same microballoon type cross over. The storage modulus of neat resin becomes the least after the cross over point. The cross over points shift to lower temperature as η increases. In Region III, E′ was lowest for the neat resin, which was measured to be 76-96% lower than any produced syntactic foam.

The maximum use temperature is the temperature at which the storage modulus starts to decrease drastically. The intersection point of the tangents drawn to the Region I and Region II curves is defined as the maximum use temperature (Tmax), as shown inFIG. 4. The values of the maximum use temperature are presented in Table 4. The Tmaxwas higher for the neat resin compared to any of the produced syntactic foams. Compared to the Tmaxof the neat resin (122° C.), many syntactic foams have a Tmaxbelow 96° C. The difference between E′ calculated at room temperature and at maximum use temperature was found to be higher for neat resin (38%) than that of any of the produced syntactic foams, with a maximum reduction of 27% for VE460-30.

FIGS. 5A, 5B, and 5Cdepict representative sets of graphs for loss modulus (E″) variation with respect to temperature in the range −75° C. to 195° C. for VE220, VE320, and VE460 syntactic foams, respectively. Neat resin results are also plotted in each figure for comparison. The glass transition temperature (Tg) may be obtained as the temperature corresponding to the maximum of the loss modulus curve and is presented in Table 4. Tgfor the neat resin was higher than many of the syntactic foams. Tgof the syntactic foams increased with Φ and became nearly equal to that of the neat resin at Φ=0.6. The values of the maximum loss modulus for the neat resin and syntactic foams are presented in Table 5.

At room temperature E″ was lower for the produced syntactic foams than the neat resin. The lowest E″ was observed for VE220-60 syntactic foams, which is 73% lower than the neat resin. An increase in Φ produced a decrease of E″ for all types of microballoons. The E″maxwas higher for the neat resin when compared to various types of syntactic foams. An increase in Φ produced an increasing trend in Tg.

FIGS. 6A, 6B, and 6Cdepict representative sets of graphs for Tan δ variation for various syntactic foams and neat resin. Tan δ is defined as the ratio of the loss modulus to the storage modulus (E″/E′) and is a measure of the damping capability of the material. An increase in Φ produced a decrease of the maximum and the room temperature values of Tan δ, as shown in Table 6. It may be observed fromFIG. 6that with an increase in Φ, the area under the Tan δ curve decreased. This indicates that the mechanical properties, and thus the stability, of the syntactic foams increase with increasing Φ at higher temperatures.

The combined effect of temperature and frequency on the properties of syntactic foams was characterized. The combined effect was studied for the neat resin and four compositions of syntactic foams: VE220-30, VE220-60, VE460-30 and VE460-60. These compositions were selected to account for the particles of the minimum and maximum density in the two extremities of volume fractions. The experiments were conducted in the frequency range of 1-100 Hz and in the temperature range of 30° C. to 140° C. TTS schemes may be used to develop master curves for the material over a wide range of temperatures and frequencies from a limited set of data.

The E′ values for the neat resin at different temperatures and frequencies are plotted inFIG. 7A. Two trends may be observed in this figure: (i) with increasing temperature the E′ values decrease, which is similar to the trend observed inFIG. 3A, and (ii) the dependence of E′ on the test frequency increases with temperature. The TTS principle may be used to construct master curves in frequency regions that are outside of the experimental frequency range (1-100 Hz). To construct a master curve, one of the curves inFIG. 7Ais selected as the pivot and the remaining curves are shifted on the frequency axis towards the pivot curve until a continuous plot is obtained, which is the master curve. The reference temperature was selected as 120° C. for the pivot curve and the shift factor aTfor each temperature was determined.FIG. 7Bdepicts the master curve for the neat resin at 120° C. The master curve is shown in the frequency range 10−2to 106Hz, which is useful for many applications. In general, the master curve may be constructed for a wide range of frequencies using the available data. The corresponding shift factor values are presented inFIG. 7C. The shift factor values for all three specimens matched closely, as shown by the small error bars inFIG. 7C. Similar master curves may be constructed for any desired temperature by selecting an appropriate reference temperature for the pivot curve.

The same procedure was employed to develop master curves for VE220-30, VE220-60, VE460-30 and VE460-60 syntactic foams. Three specimens for each syntactic foam type were tested and the results were consistent. For simplicity, the curves for only one specimen of each type are presented here. The master curves are plotted inFIGS. 8A, 8B, 8C, and 8Dfor VE220-30, VE220-60, VE460-30 and VE460-60 syntactic foams, respectively, at reference temperatures of 60° C., 90° C. and 120° C. The curves demonstrate that as the frequency is increased the sensitivity of E′ for the test frequency reduces. The sensitivity of E′ to frequency becomes negligible after 1 MHz frequency for all materials tested. The calculated shift factors for syntactic foams VE220 and VE 460 are plotted inFIGS. 9A and 9B, respectively. The shift factors are independent of the reference temperature of the pivot curve. The shift factors are of the same order for vinyl ester and the produced syntactic foams, as shown inFIGS. 9A and 9B.

The syntactic foam density may decrease with (i) decreasing wall thickness of the microballoons, (ii) increasing microballoon volume fraction, and (iii) increasing matrix porosity. Either the individual effects of these parameters or their combined effects on density may produce an identifiable trend with the viscoelastic properties of syntactic foams. The observed experimental trends are further discussed below.

The E′ variation with respect to temperature for various syntactic foams was investigated, as reported in Table 3. E′ may increase with the wall thickness of the microballoon for a given Φ. The variation with respect to the Φ is not the same for all the microballoon types. Thin walled microballoons may produce a decrease in E′ with an increase in Φ while thicker walled microballoons may produce an increase in E′ with an increase in Φ. The E′ values at room temperature for the syntactic foams in Table 3 demonstrate a linear trend with respect to the syntactic foam density. Identifying this trend allows a combination of microballoon wall thickness and Φ to be used to obtain a range of syntactic foam compositions with the same density. This provides a flexible approach to tailor the viscoelastic properties of syntactic foams within the available material parameters.

Syntactic foams may be stiffened with the addition of thicker walled particles, with the opposite trend observed in the case of thin walled particle reinforcement. These trends have also been observed for the tensile and the flexural modulus of the syntactic foams tested at room temperature. The stiffening or softening effects with increasing Φ may depend on the ratio of microballoon modulus to matrix modulus. Hollow particles include an air void inside the glass shell. Therefore, the effective modulus of a hollow particle (Ē) is lower than the modulus of the glass material. For hollow particles, Ē can be estimated by

E_=Eg⁡(1-2⁢v)⁢(1-η3)(1-2⁢v)+(1+v2)⁢η3(3)
where Egis the modulus of the glass material (60 GPa) and v is the Poisson's ratio of the glass material (0.21). In addition, density of the glass is estimated as 2540 kg/m3. The average η values calculated from Equation (1), based on the nominal true density of particles, are presented in Table 1. The estimates of Ē for S22, S32, and K46 microballoons are 2.7 GPa, 4.0 GPa, and 5.9 GPa, respectively. These theoretical values assume that the particles are defect free. The presence of defects within the particles may further reduce the modulus values. The elastic modulus of the neat vinyl ester is 3.4 GPa. Therefore, it may be predicted that incorporation of the S22 particles will produce a softening effect in the vinyl ester matrix, incorporation of the S32 particles will produce a mild stiffening or no effect in the vinyl ester matrix, and incorporation of the K46 particles will produce a stiffening effect in the vinyl ester resin. These predicted results are confirmed by the experimental results for E′.

The storage modulus variation with respect to temperature has been studied for many polymers including amorphous and crystalline thermoplastics and thermosets. The stiffness variation has been modeled with respect to temperature. This model accounts for the relaxation of the polymer by quantifying the breakage of the secondary bonds. The model is governed by a Weibull distribution which estimates the bond rupture based on the parameters (mi) referred to as Weibull moduli corresponding to the statistics of bond breakage. These parameters are dependent on various factors such as degree of cross-linking, molecular weight and crystallinity of the polymer. Based on the material stiffness transitions with respect to temperature, the general equation for the Weibull distribution may be expressed as

A similar calculation was conducted to estimate the m2values for syntactic foams. Table 7 lists the E′ values at the three reference temperatures that are used as inputs in equation (4), with E′1selected at a constant temperature of 27° C., E′2selected at Tg as provided in Table 4, and E′3selected at a constant temperature of 170° C. A parametric study was performed to estimate the Weibull coefficient m2, which is plotted inFIG. 11. It may be observed that m2decreases with increase in Φ. This result corroborates the observed effect of variation of carbon black content on polybutadiene and shows that with an increasing reinforcement content (0-37.5 wt. % range), the Weibull parameter decreased up to 50%. It was observed that Φ has a stronger influence on E′ than the microballoon wall thickness. Additionally, if the m2values are plotted with respect to the syntactic foam density they fit along a straight line. These trends illustrate that with the availability of only a small set of parameters, the behavior of a wide variety of syntactic foam compositions may be determined.

The damping parameter Tan δ exhibits a decrease in a maximum with increasing Φ. This effect may be related to the brittle nature of the microballoons and the viscoelastic nature of the polymeric matrix material. These effects are similar to those the previously reported on the dynamic mechanical behavior of syntactic foams at room temperature. Since Tan δ presents a relationship between E′ and E″, it is also analyzed with respect to the syntactic foam density inFIG. 12. The maximum Tan δ values fit along a straight line in this figure for many syntactic foams. This linear relationship may be useful in analyzing the weight saving potential through the use of syntactic foams in applications that are based on the damping characteristics, which contribute to increased Tg.

Tgof the neat resin was higher than the syntactic foams analyzed herein. This may be related to the lower specific heat capacity of glass compared to the neat resin. An increase in Tgmay be observed with increasing Φ, as reported in Table 4. A similar trend has been observed in the Tgof epoxy matrix syntactic foams. The increase in Tgwith increasing Φ may be attributed to the reduction in the mobility of polymer chains due to the interfacial bonding between microballoons and resin. The microballoon-matrix interfacial area increases with increasing Φ. A similar trend has been observed in other particle reinforced composites. For example, a silica filled polymeric matrix composite exhibited a stiffening effect adjacent to the filler particle-matrix interface, which may be responsible for an increased Tg.

A Cole-Cole plot may be used to interpret trends observed in TTS curves. This plot between log E″ and log E′ may show a semi-circular curve for homogeneous materials.FIG. 13depicts the Cole-Cole plot for the neat vinyl ester resin and syntactic foams. The curves for the neat resin and syntactic foams exhibit similar trends, and demonstrate the applicability of TTS for syntactic foams. The imperfect semi-circle observed inFIG. 13indicates a good interfacial bonding between particles and the matrix. Strong interfacial bonding may be attributed to similar deviations from the semi-circular shape of Cole-Cole plots in composites.

Further analysis of TTS results may be conducted by the William-Landel-Ferry (WLF) equation

log⁢⁢aT=-c1⁡(T-T0)c2+(T-T0)(5)
where T0is the reference temperature for which the master curve is constructed (60° C.) and c1and c2are constants calculated from the slope and the y-intercept of the (T−T0)/log aTversus (T−T0) plot. The values of c1and c2are calculated as 96.8 and 357.7, respectively, for the neat vinyl ester resin. The values of c1and c2for various syntactic foams are plotted inFIG. 14. Although microballoons of different wall thickness and Φ are used in the four syntactic foams, both constants c1and c2exhibit a linearly increasing trend with the increasing syntactic foam density. The linear trend may allow the engineering of properties of syntactic foams to produce desired properties.

The master curves obtained for the syntactic foams were compared with previously reported data. The room temperature dynamic mechanical properties of syntactic foams were previously investigated by using the vibration response of cantilever beam specimens subjected to impulse excitation using an instrumented hammer. The storage modulus values were calculated from these results at certain frequencies, as plotted inFIG. 15. The figure also contains lines for the E′ values obtained herein at 30° C. temperature. The testing herein was conducted within the frequency range of 1-100 Hz, and the values were extrapolated in the 100-150 Hz range to cover the last data point. A maximum variation of ±14% was observed in both sets of results for all four types of syntactic foams and the neat resin.

The experimental results support the following general conclusions. The room temperature (30° C.) storage modulus of syntactic foams may increase with an increase in the wall thickness of the microballoons. A stiffening effect may be observed for an increase in the volume fraction of thick walled microballoons and a decrease in the volume fraction of thin walled microballoons. The room temperature storage modulus and Tan δ may vary linearly with the density of syntactic foams. The presence of glass microballoons may increase the retention of mechanical properties of syntactic foams at temperatures beyond Tg. The Tgmay increase with an increasing microballoon volume fraction. The addition of microballoons may produce a 14-66% decrease in Tan δ compared to the Tan δ of the neat resin, as well as an increase in the storage modulus of the syntactic foams post Tg. Time temperature superposition (TTS) may be employed to develop master curves over wide temperature and frequency ranges from the tests conducted in the frequency range 1-100 Hz. A Cole-Cole plot and the WLF equation may be employed to analyze the data obtained from the temperature and frequency dependent testing.

Electrical Properties

Vinyl ester-glass microballoon (GMB) syntactic foams were characterized for dielectric properties with a specific focus on understanding the relation of the volume fraction of the microballoons, Φmb, and hollow particle wall thickness to the dielectric constant of the syntactic foams. In addition, theoretical models were developed to predict the dielectric constant of syntactic foams. The Maxwell-Garnett and Jayasundere-Smith (J-S) equations, applicable to solid particle filled composites, were modified to include the hollow particle wall thickness. These theoretical predictions were validated with experimental results. The models are used to conduct parametric studies to understand the weight saving potential of syntactic foams in applications where the dielectric constant is the primary consideration.

Glass microballoons and a vinyl ester resin were employed to fabricate syntactic foam slabs for the purpose of studying electrical properties. According to one embodiment, neat vinyl ester resin and GMBs were measured in appropriate proportions and mixed in a beaker. To the uniform mixture, a hardener was added and continuously stirred. The resulting slurry was poured into aluminum molds coated with a lubricant and allowed to cure at room temperature for at least 24 h.

The electrical impedance was measured using a CH Instruments 700D potentiostat by the AC impedance method, as schematically represented inFIG. 16. The experiments were conducted in a frequency range of 10−2-106Hz, with an applied AC wave amplitude of 500 mV. According to one embodiment, a specimen size of 18×14×1 mm was used in performing the experiments. The specimens were cut using a low speed precision diamond blade saw to ensure that the surfaces were parallel to each other. Five specimens were tested for each composition type of syntactic foams and the average values along with the standard deviations are reported.

GMBs of three different nominal true particle densities (220 kg/m3, 320 kg/m3and 460 kg/m3) were used in four different volume fractions (30%, 40%, 50% and 60%) to fabricate twelve types of syntactic foams. The scanning electron micrographs of 460 type microballoon reinforced vinyl ester matrix syntactic foams containing 30% and 60% are shown inFIGS. 17A and 17B. The radius ratio of the microballoons may be calculated using previously described equation (1). Further, the radius ratio may be related to the GMB wall thickness (w) as
w=ro(1−η)  (6)
An increasing value of η refers to a decreasing hollow particle wall thickness. The GMB properties, including radius ratio, are provided in Table 8. These GMBs have been extensively characterized in previous studies and information on the measured average diameter, size distribution, and density are widely available. These experimental values are used in herein for determining the GMB parameters.

The specimen nomenclature starts with VE representing vinyl ester resin, followed by three digit true particle density and two digits of microballoon volume fraction. The entrapped air porosity in the matrix may be calculated by using previously described equation (2). The theoretical density calculated using the rule of mixtures and the experimentally measured density of syntactic foams are reported in Table 8. The estimated matrix porosity is low and is between 1 and 4.5 vol. % for most syntactic foam slabs.

The impedance obtained from the experiment is a complex quantity, containing the real (resistance, R) and the imaginary (reactance, Xc) parts and may be defined
|Z|=√{square root over (R2+Xc2)}  (7)

The potentiostat provides measurements of these quantities. The phase angle may be given as

The phase angle is found to be around −90°, which indicates the capacitive nature of the neat resin. The capacitance may be obtained as

The dielectric constant may be obtained from the calculated capacitance as

The values of t the thickness of the sample, A the contact area, and the dielectric constant of a vacuum ∈0are taken as 10−3m, 25.2×10−5m−2and 8.854×10−12F/m, respectively.

First, the impedance of the neat vinyl ester resin employed as the matrix in the syntactic foams was measured. The variation of impedance with respect to frequency for the vinyl ester resin is depicted inFIG. 18. The impedance was found to decrease with increasing frequency across the selected range. The impedance values were used to calculate the dielectric constant of the neat vinyl ester resin at various frequencies.

Next, syntactic foams were tested in a similar manner and their dielectric constants were calculated from the experimental results. The impedance-frequency plots of the syntactic foams exhibit characteristics similar to the neat resin. These plots may be used to determine the dielectric constant at various frequencies of interest.

The dielectric constants of the neat resin and the syntactic foams at a representative frequency of 100 kHz are presented inFIG. 19. As a general trend, the dielectric constant of the syntactic foams is lower than that of the neat resin. It is also observed that GMB volume fraction has a prominent effect on the dielectric constant of the syntactic foams. The dielectric constant of the syntactic foams decreases with increasing GMB volume fraction. This relationship may be produced because the air porosity volume fraction increases with GMB volume fraction, and air has a lower dielectric constant than that of the matrix resin and GMB material (glass). The plot of the dielectric constant with respect to syntactic foam density depicted inFIG. 20demonstrates that both η and GMB volume fraction affect the density of the syntactic foams. The neat resin had the highest density observed in the experimental data set. The dielectric constant varied almost linearly with respect to the syntactic foam density.FIG. 20includes the dielectric constant at frequencies of 100 Hz and 100 kHz. In both cases the slope of the line is the same.

The syntactic foam density is dependent on both the GMB volume fraction and η. The individual effect of these parameters may be better understood by theoretical analysis. Although the interfacial bonding between GMB and vinyl ester is expected to play a role in the measured dielectric constant, this parameter is not included in the study. The interfacial bonding characteristics may be assumed to be the same for all syntactic foams for the sake of simplicity. In addition, the mean radius may be the same for all types of GMBs used in syntactic foams, indicating that the syntactic foams having the same volume fraction of different types of GMBs will have the same interfacial area and the effect of bonding characteristics will also be the same. Hence, the effect of GMB volume fraction and η is evaluated through theoretical models. In addition, including interfacial bonding as a parameter in mathematical models would require quantitative information, but quantitative information on interfacial bonding is not presently available. The interfacial bonding may include the combined effects of chemical bonding, mechanical interlocking and interfacial friction.

Estimation of the dielectric constant of hollow particles, which comprise a thin shell filled with air, may be the first step in theoretically obtaining the effective dielectric constant of syntactic foams. In order to find a closed form expression for the dielectric constant of GMBs, Maxwell's theory may be utilized. The polarization of GMBs may be equated to the polarization of an equivalent sphere and the dielectric constant of GMB may be obtained as follows. The potential ψ of the GMB may be obtained by solving the Laplace equation in polar coordinates, as depicted inFIG. 21, given by the equation
∇2ψ=0.  (11)

The solution of the Laplace equation may be obtained in the form

ψ⁡(r,θ)=∑n=o∞⁢⁢(an⁢r-(n+1)+bn⁢rn)⁢Pn⁡(cos⁢⁢θ).(12)
where Pn(cos θ) is the nthorder Legendre polynomial and anand bnare constants. Taking into account the axial symmetry of the sphere, the potential may be given by

The constants Dn, En, Gn, Hn, In, and Jnmay be found by applying the following boundary conditions and utilizing the orthogonality property, given by

ψ1⁡(ri,θ)=ψ2⁡(ri,θ)(17⁢a)ψ2⁡(ro,θ)=ψ3⁡(ro,θ)(17⁢b)ψ3⁡(re,θ)=ψ4⁡(re,θ)(17⁢c)ɛa⁡(∂r⁢ψ1)r=ri=ɛg⁡(∂r⁢ψ2)r=ri(18⁢a)ɛg⁡(∂r⁢ψ2)r=ro=ɛe⁡(∂r⁢ψ3)r=ro(18⁢b)ɛe⁡(∂r⁢ψ3)r=re=(∂r⁢ψ4)r=re(18⁢c)∫θ=0π⁢Pn⁡(cos⁢⁢θ)⁢Pm⁡(cos⁢⁢θ)⁢sin⁢⁢θ⁢ⅆθ=2(2⁢m+1)⁢δm,n(19)
where δm,nis the Kronecker delta function. Applying the above boundary conditions the value of the constant J1may be

J1=[(ɛe-1)(ɛe+2)⁢re3⁢K-(2⁢ɛe+1)⁢ro3⁢L(ɛe+2)⁢(2⁢ɛe+ɛg)]⁢Se0(20)
where the constants K, L and S are represented by

The dipole moment induced in the sphere due to the applied external field e0is 4π∈0J1. In addition, the polarization of the system, α, is the ratio of the induced dipole moment to the external applied field e0. Hence the polarization may be written as

The effective dielectric constant of GMB may be found by equating the polarization of the system consisting of a hollow glass shell containing the air void to the polarization of an equivalent solid sphere. The dielectric constant of GMBs may be written as

Although the syntactic foams are assumed to be filled with GMBs that are identical in all respects, a distribution may exist in their size and radius ratio. Experimental data are available on the size and radius ratio distribution of GMBs used in the present work, and may be used in the models to account for size and radius ratio variability range. The radius ratio values calculated from the experimentally measured density values of GMBs are 0.958, 0.948 and 0.928 for the 220, 320 and 460 type microballoons, respectively.

The effective dielectric constant for GMBs may be used in the Maxwell-Garnett and the J-S equations to predict the dielectric properties of particulate reinforced composites. The Maxwell-Garnett equation may be given by

The J-S equation is based on the Kerner's equation, and takes into account the particle-to-particle interaction between GMBs and may be given by

Previously reported data validated the results produced by these theoretical models.FIG. 23shows that both the J-S and the Maxwell-Garnett model predictions closely match the experimental results, with only ±6% and ±5% deviation, respectively. This experimental data was obtained on syntactic foams containing GMB of 600 kg/m3density tested at 1 MHz frequency and using ∈g=5.6. The value of ∈gwas not experimentally measured in this study and was assumed based on the general range for the soldalime borosilicate glass.

The dielectric constants predicted by the modified Maxwell-Garnett and J-S equations for the syntactic foams characterized are provided in Table 9. These predictions employ the experimentally measured dielectric constant of the neat resin as an input parameter, and ∈gwas taken as 5.6, which is consistent with previously reported values. Since the dielectric constant is dependent on the frequency, selection of ∈mat appropriate frequency helps in obtaining predictions for syntactic foams at that frequency. The modified Maxwell-Garnett model predictions demonstrates a larger deviation from experimental values (±22%) than those obtained from the J-S model (±12%), as listed in Table 9. Although the modified J-S model predictions are closer to the experimental results, both models consistently under-predict the dielectric constant of the syntactic foams. To better understand this trend, the J-S model may be employed for conducting reverse calculations using the experimentally measured dielectric constant of syntactic foams for calculating the dielectric constant of glass ∈g, while taking ∈mas 3.87. Experimentally measured values of η from previously reported studies were used in the calculation. The calculated ∈gvalue is obtained as 12.1±1.8. This value is high compared to the expected range for the sodalime borosilicate glass. Therefore a sensitivity analysis may be conducted on the model with respect to the two important input parameters: ∈gand ∈m. The results are depicted inFIGS. 24A and 24B, where it is observed that the model is much more sensitive to variation in ∈mthan in ∈g.FIG. 24Autilizes values of ∈gof 5.6, 8, 10, and 12.15 with ∈mof 3.87.FIG. 24Butilizes values of ∈mof 3.87, 4, 4.1, and 4.2 with ∈gof 5.6. Changing the value of ∈gfrom 3.87 to 4.2 shifted the theoretical curves to match with the experimental data. Small variations in a number of parameters such as cure conditions and resin to hardener ratio may affect the measured value of ∈g. This analysis demonstrates that the modified J-S model is capable of capturing the trend observed in the dielectric constant of syntactic foams and provides predictions with a reasonable degree of accuracy. The model also has a strong sensitivity to the dielectric constant of the resin, which is the continuous phase of the experimentally produced syntactic foams.

The modified J-S model was then employed for conducting a parametric study to understand the effect of GMB volume fraction and η on the dielectric constant of syntactic foams. The dielectric constant of the neat resin was taken as 3.87 at 100 kHz frequency in the parametric study.

FIGS. 25A and 25Bdepict the variation of the dielectric constant of syntactic foams with respect to the GMB volume fraction and wall thickness, respectively. It may be observed that with the appropriate selection of GMB volume fraction and η, the dielectric constant of syntactic foams may be tailored over a wide range of 2.6-4.9. Thin walled GMBs, where η>0.7, may produce syntactic foams with a dielectric constant lower than that of the neat resin. This opposite trend is observed for GMBs where η<0.7.

The specific dielectric constant normalized with respect to the syntactic foam density is presented as a function of volume fraction and radius ratio of the GMBs inFIGS. 26A and 26B, respectively. The density is observed to decrease more rapidly than the dielectric constant of the syntactic foams. Therefore, GMBs of lower wall thickness may provide a higher specific dielectric constant.FIG. 26Balso shows that the effect of wall thickness variation on the specific dielectric constant may become negligible where η>0.7 for all GMB volume fractions. This observation may be very important in evaluating the weight savings obtained by using certain combinations of GMB volume fraction, Φmb, and η. In syntactic foams, Φmband η may be independently varied. Several combinations of Φmband η may provide the same syntactic foam density. Therefore, the relationship between the dielectric constant, Φmband η is further evaluated as depicted inFIGS. 27A and 27B.

FIG. 27Adepicts a 3D contour plot of dielectric constant variation along the vertical axis, whereas the floor of the graph contains a 2D contour plot of syntactic foam density variation with respect to GMB volume fraction and η marked on two different axes. The 2D contour plot is reproduced inFIG. 27Bfor the sake of clarity, where the axes are marked in terms of Φmband η to illustrate the contribution of these parameters to syntactic foam density. The scale bar inFIG. 27Arepresents both dielectric constant and density of syntactic foams in their appropriate units.

Two representative values of the dielectric constant, namely ∈=2.75 and 3, are denoted by a solid and a dashed line, respectively, inFIG. 27A. All the syntactic foam compositions along a given line have the same dielectric constant. These two lines are also projected on the density contour plot. It is noted that syntactic foams having densities in the range of 630-740 kg/m3may be selected to have the same dielectric constant of 2.75. The syntactic foams having this range of densities have a GMB volume fraction in the range of 0.5-0.6 and η in the range 0.987-0.936. Since the mechanical properties of syntactic foams are also related to Φmband η; the flexibility of selecting these parameters in a range, instead of a fixed value, provides the possibility of independently tailoring the dielectric constant and modulus within the ranges given by such charts. Similar observations can also be made for the second representative value of the dielectric constant of 3, where syntactic foam density may vary in the range of 790-930 kg/m3, which can be obtained by GMB volume fractions of 0.35-0.5 and η of 0.987-0.898. Many widely used GMBs in syntactic foams are within these ranges of Φmband η. Appropriate combinations of Φmband η may be selected to have the same syntactic foam density and dielectric constant, while having the desired mechanical properties.

The dielectric properties of vinyl ester/glass microballoon syntactic foams were found to decrease with an increase in the volume fraction of the microballoons and increase with an increase in the wall thickness of the microballoons. The dependence of the syntactic foam dielectric constant on microballoon volume fraction may be more pronounced in comparison to the wall thickness.

A linear relation was observed between the dielectric constant and the density of syntactic foams at all testing frequencies. An increase in the testing frequency produced a reduced dielectric constant of the syntactic foams. An equivalent sphere approach may be employed to obtain the dielectric constant of the hollow glass microballoons. This closed form expression was employed in coherence with the Maxwell-Garnett and the Jayasundere-Smith (J-S) models to obtain predictions of the dielectric constant of syntactic foams. The J-S model predictions were closer to the experimental values than the Maxwell-Garnett predictions. The J-S model was also validated with previously reported experimental data. The J-S model was then employed to perform a parametric study to further understand the relationship between a syntactic foam dielectric constant and microballoon volume fraction and wall thickness. The investigation shows that several compositions of syntactic foams may be developed to obtain the same dielectric constant. This approach allows the tailoring of the dielectric constant independent of other properties and the density of syntactic foams over a wide range.

Thermal Expansion

Experimental and theoretical studies were conducted to relate the coefficient of thermal expansion (CTE) of syntactic foams with the volume fraction and wall thickness of the GMBs and gain insight into the possibility of designing lightweight materials for thermal applications. GMBs of three different nominal densities (220 kg/m3, 320 kg/m3, and 460 kg/m3) were used in four different volume fractions (30%, 40%, 50%, and 60%) to fabricate twelve types of syntactic foams.

The mean particle size and wall thickness of the syntactic foams are presented in Table 10. A graphical representation of the relationship of the inner radius and the outer radius of the microballoon is shown inFIG. 28. The calculated values of η for the selected particles are also provided in Table 10. The η values were in a narrow range for the selected particles. With an increasing volume fraction of lighter particles the composite modulus may decrease, whereas the composite modulus may increase with an increasing volume fraction of higher density particles. Therefore, in syntactic foams these particles are defined as thin and thick walled, respectively.

Table 10 provides the theoretical and the experimental densities of the syntactic foam specimens along with the matrix porosity content. The matrix porosity values may be considered an indicator of the quality of the fabricated composites. These values are not related to the CTE of syntactic foam because in the small size specimens used for CTE measurement, the specimens containing voids may generally be detected and avoided by careful surface inspection. In addition, the weight and dimensions of each CTE specimen were measured to calculate their density. The presence of a large matrix porosity produces a low density specimen, and may be easily detected and excluded from the experimental scheme. The representative microstructures of syntactic foams containing 30 vol. % and 60 vol. % GMB of the 220 type are shown inFIGS. 29A and 29B, respectively. A uniform distribution of GMBs may be observed in these figures. A similar microstructure was observed in foams containing the 320 and the 460 type particles.

Specimens of nominal dimensions of 11×5.5×3.5 mm (length×width×height) were prepared for CTE measurement. The specimens were heated in a convection oven for 3 hours at 70° C. to remove any adhered moisture.

The CTE characterization of the specimens was conducted using a Q400 Thermomechanical Analyzer. The schematic of the experimental setup is depicted inFIG. 30. An expansion type probe was used for measuring the temperature dependent dimensional changes. A preload of 0.02 N was applied for all tests. The measurements were conducted from room temperature to 75° C. Time, temperature, and change in specimen height were recorded during the test and used to estimate the CTE. In initial experiments 1° C./min, 3° C./min, and 5° C./min heating rates were employed. Based on the results, a 3° C./min heating rate was found to produce thermal equilibrium in the specimens this heating rate and the results were consistent.

FIGS. 31A and 31Bdepict representative plots of thermal strains with respect to temperature for syntactic foams having 30 vol. % and 60 vol. % GMB of different η, respectively. Similar trends were observed for other volume fractions as well. It was observed that the dimensional change was the steepest for thin walled particles, indicating that the dimensional stability of the composite increases with an increase in the particle wall thickness.FIGS. 31C and 31Dcompare a representative set of results for composites containing GMB densities of 220 kg/m3and 460 kg/m3in different volume fractions, respectively. The trends observed for other particles are similar.FIGS. 31C and 31Ddemonstrate that increasing the GMB volume fraction in syntactic foams produces an increase in dimensional stability. The slope of such graphs may be used to calculate the CTE of syntactic foams (α) as given by

α=1l×ⅆlⅆT(25)
where l is the initial specimen length and dl/dT is the slope of the dimension change-temperature plot.

The experimental measured values of the CTE are plotted inFIG. 32. A trend was observed that an increase in the GMB volume fraction provides improved dimensional stability to the composite. For comparison, the CTE of the neat resin was also plotted inFIG. 32. It was observed that the value of the CTE of the syntactic foams is up to 60.4% less than the CTE of the neat resin. The lowest CTE value was observed for the VE460-60 syntactic foam, which contains GMB of the highest wall thickness in the maximum volume fraction investigated.

The experimental results were analyzed to understand the effect of several parameters on the CTE of the syntactic foams.FIG. 32shows that increasing the volume fraction and wall thickness of GMBs decreases the CTE of the syntactic foams. It was observed that increasing the volume fraction of the 220 type particles from 30 vol. % to 60 vol. % decreases the CTE of composites by 27-50% compared to the neat resin. Similarly, the 320 and the 460 type GMBs produce a 30-53% and 23-60% decrease in the CTE of the syntactic foams compared to neat vinyl ester resin, respectively. In comparison to the volume fraction, microballoon wall thickness exhibits a milder effect on the CTE of the syntactic foams within the investigated range. At the same GMB volume fractions, the syntactic foam CTE values demonstrate a difference within 10% for most particle types. Only VE220-50 exhibited a deviation from this trend due to the higher CTE value than would be expected from the experimental trends observed for the entire set of compositions. The density of the syntactic foams also follows a qualitatively similar trend with respect to the microballoon volume fraction and wall thickness, as seen in Table 10. Among the present batch of composite specimens, VE460-60 has the highest glass content and the lowest measured CTE value.

Developing a correlation between composite density and the CTE through theoretical models may be useful because common applications of syntactic foams are based on their low density and the ability to tailor the mechanical and thermal properties. Two different models applicable to solid particle reinforced composites were analyzed for the possibility of modifying them for hollow particle reinforced composites and the predictions are validated with experimental results.

The rule of mixtures (ROM) is widely employed to obtain an upper bound of various properties of composite materials. The ROM for the CTE estimation may be written as
α=αmφm+αbφb(26)
where αmand αbare the CTE of the matrix resin and the GMB, respectively, and φmand φbare the volume fraction of the matrix and the GMB, respectively. ROM is the basis for several theoretical models. Other theoretical models for predicting the CTE values of solid particle reinforced composites are also available. These models are mainly applicable to composites containing solid fillers and do not include particle wall thickness, which is an additional parameter available for variation in syntactic foams. Turner's and Kerner's models have been modified herein to include the effect of wall thickness of hollow particles on the CTE of the composite material. Turner's model may be given by

α=αm⁢ϕm⁢Km+αb⁢ϕb⁢Kbϕm⁢Km+ϕb⁢Kb(27)
where Kmand Kbare the bulk moduli of the matrix and the microballoon, respectively. The bulk modulus may be estimated from the Young's modulus of the constituents of the composite as
K=E/(3(1−2v))  (28)
where E is the Young's modulus and v the Poisson's ratio of the respective constituents.

The effective modulus of the GMB differs from the modulus of the glass and depends on the wall thickness. To determine the effective GMB modulus, an equivalent sphere with the same properties as the hollow GMB was assumed. The radial displacement at the outer surface of the two systems were compared and the effective modulus (E*) as a function of the microballoon radius ratio may be given by

E*=Eg⁡(1-2⁢v)⁢(1-η3)(1-2⁢v)+(1+v2)⁢η3(29)
where Egis the modulus of the microballoon glass material and was assumed to be 60 GPa. The modified Turner's model for the CTE of the hollow particle filled composites may be given by substituting Eq. (28) and (29) in Eq. (27) producing

α=αm⁢ϕm⁢Em⁡[(1-2⁢vg)+(1+vg2)⁢η3]+⁢⁢αb⁢ϕb⁢Eg⁡(1-η3)⁢(1-2⁢vm)ϕm⁢Em⁡[(1-2⁢vg)+(1+vg2)⁢η3]+ϕb⁢Eg⁡(1-η3)⁢(1-2⁢vm)(30)
where vgis the Poisson's ratio of the microballoon material and was assumed to be 0.21, En, is the modulus of the matrix and was assumed to be 2.82 GPa, and vmis the Poisson's ratio of the matrix material and was assumed to be 0.35.

The Kerner's model may be given by

α=αm⁢ϕm+αb⁢ϕb+ϕb⁢ϕm⁡(αb-αm)×(Kb-Kmϕm⁢Km+ϕb⁢Kb+3⁢Kb⁢Km4⁢Gm)(31)
where Gmis the shear modulus of the matrix material. The Kerner's model (Eq. 31) may be modified to obtain a new version (Eq. 32) for hollow particle filled composites given by

Equations (30) and (32) were employed to determine the theoretical estimates of CTEs of various syntactic foams and the results are presented inFIG. 33A. The CTE of the neat resin was determined to be 76.5 μm/m° C. based on the experimental results presented inFIG. 32, and the CTE of the microballoon glass was assumed to be 4 μm/m° C. A comparison demonstrates that the theoretical values presented inFIG. 33Aclosely match with the experimental values as shown inFIG. 32, and the observed difference was in the range of ±15% for the modified Turner's model and ±18% for the modified Kerner's model. There are several factors such as matrix porosity and use of literature values for Poisson's ratio that may produce some difference in the theoretical and experimental results. In addition, polydispersion in particle size and volume fraction may also produce some variation in the measured properties. Accounting for these parameters may provide more accurate theoretical estimates.

Since both the modified Turner's and Kerner's models demonstrated a close match with the experimental results, they were each also validated with previously reported experimental values of CTE. Syntactic foams with microballoon densities of 600 kg/m3and 150 kg/m3were used to analyze the CTE, and the comparison of theoretical and experimental values for these studies is shown inFIGS. 33B and 33C, respectively. Close matching between the theoretical and the experimental results provides further validation of the models utilized to predict the CTE of syntactic foams. Experimental data on syntactic foams containing φb<0.3 is not widely available as such high density syntactic foams may be unsuitable for applications in which weight savings through the use of lightweight materials is desired.

The modified Turner's model was selected to perform a parametric study on the variation of CTE with respect to radius ratio and volume fraction of the GMBs.FIG. 34Aplots the normalized CTE as a function of η for various values of φb. It was observed that in the case of (1−η)<0.4 (i.e. thin walled particles), which may be important for weight saving materials, there is a steep change in CTE. Many previously reported syntactic foams have employed particles with a (1−η) value in the range of 0 to 0.1, where CTE decreases sharply. However, the rate of CTE change reduces substantially in the region (1−η)>0.4 (i.e. thick walled particles). For (1−η)>0.5 the rate of CTE change was almost negligible. This trend indicates that for a fixed volume fraction of GMB, the use of microballoons of (1−η)>0.5 produces an increase in the composite density without further improvement of the dimensional stability of the composite. A comparison of trends inFIG. 34Bdemonstrates that increasing the volume fraction of GMB with a constant wall thickness may produce an over 80% reduction in the CTE of the composite compared to the matrix material. Within the range 0.8<η<1, the CTE behavior of syntactic foams changes rapidly, but the rate of change decreases at η<0.8. The line for η=0 inFIG. 34Bpresents the case of solid particle reinforced composite and is the lower bound of the CTE.

The weight saving potential through the selection of material parameters was explored for GMB/vinyl ester syntactic foams. The 3D contour plot inFIG. 35Ashows the CTE variation with respect to the composite density. The composite's density variation as a function of GMB volume fraction and wall thickness (or density) is plotted on the bottom ofFIG. 35Aas a 2D contour plot. The scale bar inFIG. 35Ademonstrates both the CTE variation and the composite density variation in their appropriate units. The relationship between the composite density and the microballoon volume fraction and density is further clarified inFIG. 35Bas a 2D contour plot. The syntactic foam density ranges depicted inFIGS. 35A and 35Bare the same. Reduction in the composite's CTE was evident with an increase in the GMB volume fraction and wall thickness. A solid and a dotted line are shown inFIGS. 35A and 35B, for two representative constant values of α=300×10−7° C.−1and α=500×10−7° C.−1, respectively. It may be observed inFIG. 35Athat a variety of GMB/vinyl ester syntactic foam compositions having densities over a large range of 572-1050 kg/m3may be employed to obtain a CTE of 500×10−7° C.−1. Correspondingly,FIG. 35Bdemonstrates that a combination of GMB volume fractions in the range of 24-60% with the GMB densities in the range of 180-700 kg/m3may provide the same CTE value of 500×10−7° C.−1. Similar observations may be made for α=300×10−7° C.−1, where numerous syntactic foam compositions having densities in the range of 784-935 kg/m3may be used to obtain the CTE value. Such flexibility may allow the tailoring of the composite properties for thermal applications. The parametric study results for the CTE observed inFIGS. 35A and 35Bmay be combined with the existing understanding of mechanical properties of syntactic foams. Such combination of mechanical and thermal properties may allow the selection of the best possible parameters in designing syntactic foams for a given application.

The CTE of polymer matrix syntactic foams was studied with respect to the hollow particle wall thickness and volume fraction. The CTE of the syntactic foams was found to decrease with an increasing microballoon wall thickness and volume fraction. The CTE of the composites was found to be up to 60.4% lower than the CTE of the neat resin for the composites characterized herein. Within the range of parameters studied herein, the microballoon volume fraction was found to be effective in modulating the CTE of syntactic foams. Turner's and Kerner's models were modified to include the effect of particle wall thickness and develop predictive models for syntactic foams. The modified Turner's model provided predictions within ±15% of the experimental results. The parametric study demonstrates that a combination of the microballoon wall thickness and the volume fraction may be effectively employed to produce composites with a desired CTE. In addition, the wide range of choices available for the microballoon volume fraction and wall thickness for obtaining syntactic foams with the same CTE value may facilitate the tailoring of the density and mechanical properties at the same time.

As shown inFIG. 36A, various compositions of syntactic foams may be produced that have a CTE of 40×10−6/° C., indicated by the dashed line. Similarly, the dashed-dotted line inFIG. 36Amarks the compositions that have a CTE of 60×10−6/° C. A theoretical model was used to obtain the syntactic foam modulus in the same range of volume fraction and wall thickness, as shown inFIG. 36B. Further, a density map of the syntactic foams having the same range of volume fraction and wall thickness was developed using rule of mixtures and is plotted inFIG. 36C. Since the variation of the CTE, modulus and the density 2D-maps are based on the same rage of η and Φmb, these 2D plots may be superimposed on each other to find the range of modulus and densities for the same values of CTE and the corresponding η and Φmbmay be determined. Tables 11 and 12 provide the compositions, densities, and modulus of syntactic foams corresponding to the CTE values of 40×10−6/° C. and 60×10−6/° C., respectively.

Table 11 shows that the syntactic foams having densities in the range 644-990 kg/m3may provide a CTE of 40×10−6/° C. In this composition range, the modulus of syntactic foams may vary in the range 2.8-4.1 GPa by changing the Φmbin the range 0.37-0.6 and correspondingly selecting appropriate η that ranges between 0.898 and 0.959. The second representative example for syntactic foams having a CTE of 60×10−6/° C. is presented in Table 12, where the density may vary in the range 556-1091 kg/m3and the modulus may vary in the range 1.226-3.231 GPa by tailoring the combination of η and Φmb. Depending on the requirements of an application, either syntactic foams of lowest density or desired modulus may be selected from this available composition range. This possibility of obtaining multifunctionality in the syntactic foam properties provides greater flexibility than solid reinforcement filled composites.

The advantage of utilizing syntactic foams in comparison to the neat resin, closed-cell air filled foams and solid particulate reinforced composites may be observed inFIG. 37. This figure includes results obtained from the same theoretical model and input parameters that are used forFIG. 36C. The addition of solid particles may produce higher modulus values in comparison to the matrix resin, at the cost of increasing the composite density. The addition of air pores, which creates closed-cell foams containing gas porosity, reduces the density while also decreasing both modulus and specific modulus. In contrast, the addition of hollow particle fillers may decrease the composite density while tuning the modulus over a wide range based on the wall thickness and the amount of microballoon added to the composite. It may be observed inFIG. 37that the modulus of syntactic foams may be significantly higher than closed-cell foams at the same density level, and the use of syntactic foams may produce significant weight savings in structural applications. The combination of increased specific mechanical properties and decreased density may allow for a reduction of the weight of a component while retaining the desired behavior. As shown, the modulus of closed-cell foam is significantly lower than that of the matrix material, which is why their structural applications are scarce. A large number of compositions have a higher modulus than that of the matrix resin. Such compositions may be useful in load bearing structural applications.

Syntactic Foams with SiC Particles

Hollow silicon carbide sphere (SiCHS) reinforced vinyl ester matrix resin syntactic foams were characterized for thermal and mechanical properties. The porous nature of the walls of these hollow particles may pose a challenge in the estimation of the effective properties of the SiCHS. Such analysis may allow the exploration of the potential for these particles for use in syntactic foam applications.

SiCHSmay be prepared by mixing template spheres with silicon powder and heating to 1300° C. The hollow core may be formed by calcining the particles, thereby removing excess carbon. A molten salt synthesis comprising a salt bath, a carbon black template and silicon may also be utilized to obtain SiCHS. Herein, a polyethylene core material was employed as a template, on which SiC was deposited by a chemical vapor deposition technique. SiCHSand a vinyl ester resin was employed to fabricate syntactic foam slabs. Methyl ethyl ketone peroxide was used as the catalyst for the resin. Syntactic foams were prepared with 60% of SiCHSby volume by a mechanical mixing method. The cast syntactic foam slabs were cured at room temperature for 24 hours, and then post cured at 70° C. for 3 hours in a convection oven.

Two types of SiCHSwere employed herein. The density of the tapped bed of these particles was measured as 440 kg/m3and 790 kg/m3. Due to the porosity open to the surface in the walls of these particles, the direct measurement of a true particle density using equipment such as a pycnometer is not possible.FIG. 38Ashows exemplary SiCHSemployed in fabricating syntactic foams.FIG. 38Bshows the wall thickness of a broken SiCHS. The average wall thickness and the outer diameter of the SiCHSwere evaluated as an average of 25 measurements taken on micrographs of several particles, and the results are shown in Table 13. The average radii of the two types of particles, designated as S1 and S2, were measured as 400 μm and 510 μm, respectively. The average wall thickness was measured as 36.1 μm and 81.6 μm, respectively, for these particles. The density of SiC was assumed to be 3200 kg/m3to evaluate the ideal true particle density, which is the density of hollow particles considering these measured diameters and wall thickness and fully dense walls, using
ρHS=ρSiC(1−η3)  (33)

The ideal true particle density was evaluated using equation (33) and was found to be 787 kg/m3and 1293 kg/m3for the S1 and S2 particles, respectively, as shown in Table 13. However, the true particle density of the particles may be lower due to the porosity present in the particle walls.FIG. 39shows the microporous voids present in the wall of the SiCHS. The experimental density of the fabricated syntactic foams containing 60 vol. % SiCHSwas measured to be 858±33 kg/m3and 1048±44 kg/m3for SF1 and SF2 type syntactic foams containing the S1 and S2 particles, respectively.

The quasi-static compression testing was performed on cylindrical specimens with nominal dimensions of 6.5 mm diameter and 8.5 mm thickness at a constant loading rate of 1 mm/min using an electromechanical universal test system. The CTE study was performed on a thermomechanical analyzer using cylindrical specimens with nominal dimensions of 7 mm diameter and 5 mm thickness from room temperature to 80° C. at a constant heating rate of 3° C./min.

A representative compressive stress-strain curve of the SF1 syntactic foam is presented inFIG. 40. Similar behavior has been observed for GMB/vinyl ester and GMB/epoxy syntactic foams in previously reported experiments. The deformation features of the specimen corresponding to points a, b and c marked inFIG. 40are shown inFIGS. 41A, 41B, and 41C, respectively. The SiCHSare largely intact inFIG. 41A, which corresponds to the end of the elastic deformation region. Point a inFIG. 40indicates the onset of particle failure, where some weak particles begin to exhibit cracking. The weak particles may include thin walled or defective particles. The particle fracture corresponds to the drop in stress shown inFIG. 40. Upon further compression, crushing and compaction of the particles corresponding to the plateau region marked by point b inFIG. 40occurs. Fracture of the particles transfers the load to the surrounding matrix material. The failure of the matrix is evidenced by the shear bands in the specimen inFIG. 41B, and crushed and compacted particles along the shear band are also observed. At specimen failure, the cracks propagate through the entire thickness of the specimen and constitute final failure, as observed inFIG. 41C. The trends of the stress-strain graph and the failure features are found to be similar for the SF1 and the SF2 syntactic foams.

The stress-strain graphs may be used to calculate the modulus, peak strength, and plateau strength of the syntactic foams, as presented in Table 14. The higher density particles provided higher peak strength but lower plateau strength and modulus. Studies on GMB filled syntactic foams have consistently shown higher strength and modulus for syntactic foams containing higher density particles. However, a similar trend is not present in this case because the particle walls are porous and some of the pores present in the walls may be larger than the critical size to initiate failure at a low stress level. The standard deviation in the plateau stress and compressive modulus is within ±11%. The specific compressive strength (normalized with respect to the corresponding syntactic foam density) of the SF1 and the SF2 syntactic foams were 33.4 kPa/kg/m3and 38.8 kPa/kg/m3, respectively. The specific compressive modulus of the SF1 and the SF2 syntactic foams were 0.8 MPa/kg/m3and 0.6 MPa/kg/m3, respectively. These values of the SiCHS/VE syntactic foams are lower in comparison to previously reported GMB/VE syntactic foams. These lower compressive properties may be ascribed to the microporous voids in the walls of the particles, as shown inFIG. 39, which may produce undesirably low effective mechanical properties and early failure of some particles.

Thermal strain-temperature graphs were used to calculate the CTE of the produced syntactic foams. The measured CTE of the SF1 and the SF2 syntactic foams were found to be 26.7±2.7 and 15.9±1.8×10−6/° C., respectively. The thicker walled SiCHSproduced syntactic foams with a lower CTE than the thinner walled SiCHS. The CTE values of SiCHS/VE were compared with those previously reported for GMB/VE syntactic foamsFIG. 42. The GMB/VE syntactic foams contain 60 vol. % of three different types of GMBs having true particle densities of 220 kg/m3, 320 kg/m3and 460 kg/m3. The CTE values are plotted with respect to the syntactic foam density inFIG. 42. It was observed that the CTE of SiCHS/VE is lower than the CTE values obtained for GMB/VE syntactic foams. In the case of GMB/VE syntactic foam a maximum decrease of 60.4% in CTE was observed in comparison to the CTE of the neat resin. By contrast, for the SiCHS/VE syntactic foams a CTE decrease of 65.1% and 79.3% was observed for the SF1 and the SF2 syntactic foams in comparison to the CTE of the neat resin, respectively. It is expected that syntactic foams including SiC hollow spheres with non-porous walls will exhibit property trends based on the volume fraction and wall thickness similar to those observed for glass microballoon containing syntactic foams.

To evaluate the porosity in the wall of the S1 and the S2 SiCHS, the density of the hollow particles was calculated from the experimentally measured density of the syntactic foams using the rule of mixture. It was assumed that there was no additional porosity present in the matrix. The true particle densities evaluated by this method are 656 kg/m3and 973 kg/m3, for S1 and S2 SiCHS, respectively. Using these true particle density values in Equation (33), the density of the porous SiC material was obtained as 2668 kg/m3and 2408 kg/m3for S1 and S2 particles, respectively, which is lower than the density of a solid SiC material (3200 kg/m3). The density difference demonstrates that the S1 and the S2 SiCHScontain 16.6% and 24.8% porosity, respectively.

Theoretical models have been previously reported for the purposes of estimating the CTE of syntactic foams with respect to the volume fraction (Φ) and radius ratio (η) of the hollow particles. The CTE experimentally measured herein was used in these models to estimate the properties of the porous SiC material of SiCHS. The variation of CTE (α) of the syntactic foams based on Φ and η of the filler material is given by the Turner's model modified for application to syntactic foams

SiCHS/VE syntactic foams were studied for compressive properties and coefficient of thermal expansion (CTE). Two types of SiCHSwere used in 60 vol. % quantities to fabricate two types of syntactic foams. Direct measurement of the properties of SiCHSmay be difficult due to their porous walls. Hence, the measured CTE and density values of syntactic foams were used to estimate the properties of SiCHS. The ratios of the inner to the outer radius of the two types of particles, S1 and S2, were measured to be 0.91 and 0.84, respectively. The compressive modulus of the two syntactic foams, SF1 and SF2, was measured to be 725 MPa and 692 MPa, respectively. The higher density syntactic foams exhibited a lower modulus because of the porous nature of the particles. The specific compressive strength and modulus of SiCHS/VE syntactic foams was observed to be lower in comparison to GMB/VE syntactic foams. The CTE of SiCHS-VE was observed to be lower than compositions of GMB/VE, indicating better thermal stability for the SiCHSsyntactic foams at high temperatures than for GMB/VE. A decrease of 65.1% and 79.3% in the CTE was observed for the two types of syntactic foams, in comparison to the CTE of the neat resin. The modulus of the hollow sphere material predicted using the modified Turner's model was observed to be significantly lower than the Young's modulus of bulk SiC, due to the presence of porosity in the particle walls. The estimated porosities in the walls of the two types of SiCHSwere 16.6% and 24.8% for S1 and S2, respectively.

Tuning CTE and Dielectric Constant

A combination of wall thickness and volume fraction was used to simultaneously tailor the dielectric constant and CTE of a syntactic foam. The CTE may be varied between 30×10−6° C. and 70×10−6° C. for syntactic foams having a dielectric constant of 3 in vinyl ester matrix glass microballoon filled syntactic foams. To obtain these CTE values, the hollow particle volume fraction and density may be varied. Similar calculations may be conducted for other desired values of dielectric constant and CTE.

Glass hollow particles were employed in a vinyl ester matrix to fabricate syntactic foams. However, the method is generic and may be applied to any combination of hollow particles and matrix material. The produced syntactic foams may be suitable for electronic packaging applications.

A syntactic foam composite material was developed that may be simultaneously tailored for a desired dielectric constant and coefficient of thermal expansion (CTE). A range of compositions may be developed for syntactic foams that have the same dielectric constant but have different CTE values and vice versa. The selection of an appropriate material composition based on the proposed approach may provide syntactic foams with both the desired dielectric constant and the desired CTE.

A combination of wall thickness and volume fraction may be employed to simultaneously tailor the dielectric constant and the CTE.FIGS. 43A and 43Bdepict a scheme for the selection of an appropriate hollow particle volume fraction and wall thickness for obtaining the desired properties in a syntactic foam. A line is marked inFIG. 43Arepresenting the syntactic foam compositions that have a dielectric constant of 3. The same line indicating a dielectric constant of 3 is projected onFIG. 43B, to indicate the CTE values of the syntactic foam compositions. It was found that the CTE may be varied between 30×10−6° C. and 7010° C. for syntactic foams having a dielectric constant of 3. To obtain these CTE values, the hollow particle volume fraction and density may be varied, as provided in Table 15 for compositions with a dielectric constant of 3. Similar calculations may be conducted for other desired values of dielectric constant and CTE.

As used herein, ρmbis the density of hollow particle, Φmbis the hollow particle volume fraction, η is the radius of internal to outer radius of hollow particle, ρcis the syntactic foam density, and α is the CTE of syntactic foam.

The dielectric constant of syntactic foams (a) decreases with increasing Φmbof GMBs, (b) decreases with increasing test frequency, and (c) increases with increasing temperature. Impedance exhibits behavior similar to that of the dielectric constant with respect to Φmband frequency.

Additional Notes

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” Any ranges cited herein are inclusive.

The terms “substantially” and “about” used throughout this Specification are used to describe and account for small fluctuations. For example, they may refer to less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%.

As shown inFIG. 4, e.g., a computer-accessible medium120(e.g., as described herein, a storage device such as a hard disk, floppy disk, memory stick, CD-ROM, RAM, ROM, etc., or a collection thereof) can be provided (e.g., in communication with the processing arrangement110). The computer-accessible medium120may be a non-transitory computer-accessible medium. The computer-accessible medium120can contain executable instructions130thereon. In addition or alternatively, a storage arrangement140can be provided separately from the computer-accessible medium120, which can provide the instructions to the processing arrangement110so as to configure the processing arrangement to execute certain exemplary procedures, processes and methods, as described herein, for example.

Software and web implementations of the present invention could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various database searching steps, correlation steps, comparison steps and decision steps. It should also be noted that the words “component” and “module,” as used herein and in the claims, are intended to encompass implementations using one or more lines of software code, and/or hardware implementations, and/or equipment for receiving manual inputs.