Cold sintering Na2Mo2O7 ceramic with polyetherimide (PEI) polymer

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Cold sintering Na2Mo2O7 ceramic with polyetherimide (PEI) polymer to realize high performance composites and integrated multilayer circuits Jing Guo, Neal Pfeiffenberger, Allison Beese, Alicyn M. Rhoades, Lisheng Gao, Amanda Baker, Ke Wang, anne bolvari, and Clive A. Randall ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00609 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Applied Nano Materials

Cold sintering Na2Mo2O7 ceramic with polyetherimide (PEI) polymer to realize high performance composites and integrated multilayer circuits Jing Guo1*, Neal Pfeiffenberger2, Allison Beese3, Alicyn Rhoades4, Lisheng Gao1, Amanda Baker1, Ke Wang1, Anne Bolvari2, and Clive A. Randall1* 1. Materials Research Institute and Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802 USA. 2. SABIC, 475 Creamery Way, Exton, PA 19341 USA 3. Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802 USA 4. School of Engineering, Penn State Behrend, 4701 College Drive, Erie, PA 16563, United States Ceramic-polymer composites, low temperature processing, microwave dielectrics, breakdown strength, multilayers.

ABSTRACT

The cold sintering process is utilized to fabricate ceramic-polymer (Na2Mo2O7-polyetherimide, PEI) composites and integrated multilayer circuits. The Na2Mo2O7-PEI bulk composites cold sintered at 120 °C show high densities (>90% theoretical). The permittivity at microwave

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frequencies decreases with increasing PEI content, following the classical logarithmic mixing law, and Q×f values show no deterioration with the addition of PEI. Furthermore, the characteristic dielectric breakdown strength of the ceramic-polymer composite obtained from a Weibull plot increases dramatically from 55.1 MV/m to 107.5 MV/m with 10-20 vol. % PEI additions. In the case of high PEI content where there is more segregation of the polymer within the ceramic matrix, there is a gradual decrease in the dielectric breakdown strength. Na2Mo2O7PEI-Ag bulk ring resonators can be obtained by post screen printing, and the mixing laws are used to calculate the permittivity of the ring resonators. As a prototype of integrated multilayer circuits, Na2Mo2O7-PEI-Ag multilayer ring resonators with good microwave dielectric properties can be successfully densified by cold sintered co-fired ceramic-composite technology at 120 °C without delamination or warping, demonstrating the feasibility of cold sintering in the ceramicpolymer composite integrated multilayer circuits.

INTRODUCTION Ceramic-polymer composites have attracted extensive attention for electronic device development due to the unique electrical, mechanical and thermal properties that result from the advantageous combination of ceramics and polymers, such as low dielectric loss, high dielectric breakdown strength, and high flexibility.1-3 The feasibility of adopting ceramic-polymer composites for a wide variety of applications has been studied, such as electronic packaging, capacitors, sensors, radio-frequency devices, and substrates.2, 4 However, most current research has focused on polymer-based composites, in which polymers comprise the major material constituent and ceramics are fillers in polymer matrices.2,5,6 In the case of higher-volume fraction ceramic-based composites, multiple processes are usually needed to sinter ceramic skeletons with a subsequent polymer infiltration into these skeletons.7 Densifying ceramic-based


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composites without destroying the polymer phase in a simple one-step process remains challenging due to the different processing temperatures of ceramics (high temperature, ~1000 °C) and polymers (low temperature, ~250 °C). Alternative ceramic sintering techniques have been reported in recent years: Li2MoO4 bulk ceramic has been fabricated at room temperature with deionized water,8 A low temperature sintering technique termed the cold sintering process (CSP) has been developed to sinter ceramics with the assistance of a transient liquid phase, such as aqueous solutions.9 Using this cold sintering process, we have succeeded in densifying a diverse range of ceramics with various compositions and crystal structures at temperatures ≤300 °C.9-12 More recently, microwave dielectric

ceramic

Al2SiO5-NaCl,13

temperature

stable

microwave

dielectric

ceramic

Na0.5Bi0.5MoO4-Li2MoO4,14 and structural material CaCO3,15 have been reported to have been densified using the cold sintering process at 120 °C, 150 °C and room temperature, respectively, demonstrating the versatility of the cold sintering process. There have also been reports on ZnO ceramic densification at low temperatures using water or acetic acid solution assisted flash and FAST/SPS sintering.16,17 One key advantage of cold sintering process is the reduction of ceramic sintering temperature from >500 °C (typically >1000 °C) to ≤300 °C, enabling the co-sintering of ceramics with polymers and metals, which is typically limited in the traditional processing methods. Mechanistically, the transient liquid phase controls the dissolution and precipitation processes that densifies the ceramic.9,10 The applied pressure and the capillary force plastically deform the particulate polymer phases. The dissolution-precipitation together with the applied pressure and capillary force collectively drives the sintering and densification of the composite.9,18 Several ceramic-polymer and ceramic-metal composites have been demonstrated to densify at low


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temperatures, specifically in the range of 120-180 °C, by the cold sintering process and have resulted in unique material properties, including Li2MoO4-PTFE (Polytetrafluoroethylene), Li1.5Al0.5Ge1.5(PO4)3-PVDF-HFP (Poly(vinylidene fluoride-co-hexafluoropropylene), V2O5PEDOT:PSS (Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)), and LiFePO4-CNF (carbon nano fibers) composites, and Li2MoO4-Ag (silver) multilayers.11,18,19 Polyetherimide (PEI) is a high temperature (glass transition temperature, Tg: 217 °C) thermoplastic polymer with excellent dielectric and mechanical strength. In previous work, the dielectric ceramic Na2Mo2O7 (NM) has achieved >90% of theoretical density when cold sintered at temperatures as low as 120 °C.9 In this work, PEI is selected to cold co-sinter with Na2Mo2O7 to form a dielectric composite with improved electrical properties over pure NM ceramic. The densities, microstructures, microwave dielectric properties, and dielectric breakdown strength of the cold sintered NM-PEI composites are reported and discussed in this work. The continuum property mixing laws are used to predict the permittivity of the composites as a function of the connectivity of the respective phases and also their volume fractions. From these results, we propose cold sintering process as a means to develop electronic devices based on ceramicpolymer composites. (NM-PEI)-Ag bulk ring resonators with post screen printing and (NMPEI)-Ag multilayer ring resonators with cold sintered co-fired ceramic-composite (CSCC) technology were investigated for this purpose. EXPERIMENTAL SECTION

Powder Preparation. Na2CO3 (99.95%), and MoO3 (99.5%) were purchased from Alfa Aesar. PEI (ULTEM 1000) with a glass transition temperature (Tg) of 217 °C was provided by SABIC. Stoichiometric amounts of Na2CO3, and MoO3 were mixed and ball-milled (Norton Chemical


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Process Products Division, Akron, OH USA) with ZrO2 balls in ethanol for 24 h. Afterwards, the powder was calcined in air at 500 °C for 5 h, and ball-milled again in ethanol for 24 h. Pure Na2Mo2O7 powder with an average particle size of 1.9 µm was obtained as seen in Figure S1 (Supporting Information). Then, the Na2Mo2O7 powder with theoretical density of 3.682 g/cm3 was mixed with 10, 20, 30, 40, and 50 vol. % PEI powder (bulk PEI density of 1.29 g/cm3), ballmilled in ethanol for 24 h and dried.

Bulk Composite Cold Sintering. In the case of bulk (1-x)NM-xPEI (x= 0, 10, 20, 30, 40, 50 vol. %) composites, 8-11.9 wt.% deionized water was added into the (1-x)NM-xPEI powders with a pipette. After thorough mixing with a mortar and pestle, the moistened powders were placed in a steel die. Then, a heater jacket was wrapped round the die as shown in Figure S2 (Supporting Information). Afterwards, the die was pressed under a uniaxial pressure of 175-350 MPa, and heated up to 120-240 °C. The ramping (Figure S2, Supporting Information) and holding times were 20-40 and 20 min, respectively. Finally, all the cold sintered samples (Diameter: 12.7 and 25.4 mm; Thickness: 1-5 mm) were dried in an oven at 120 ºC for 6 hours. Silver paste was post screen printed on some of the cold sintered (1-x) NM-xPEI composites to achieve bulk ((1-x)NM-xPEI)-Ag ring resonators.

Multilayer Composite Cold Sintering. In the case of (1-x)NM-xPEI (x=0, 10, 20 vol. %) and Ag electroded multilayer composites, tape casting and cold sintered co-fired ceramic-composite technology were employed to fabricate samples. At first, the (1-x)Na2Mo2O7-xPEI powders were mixed with a solution of 95 wt.% methylethylketone (MEK) and 5 wt.% QPAC 40 resin (Empower Materials, Newark, DE USA) and ball-milled for 12-24 h. Afterwards, a solution of 66.3 wt.% methylethylketone (MEK), 28.4 wt.% QPAC 40 resin, and 5.3 wt.% butyl benzyl


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phthalate S-160 (Tape Casting Warehouse, Morrisville, PA USA) was added into the slurry, followed by another ball-milling for 24 h and rolling for 1-2 h (MX-T6-S Analog Tube Roller, Scilogex, Rocky Hill, CT USA). Then, the (1-x)Na2Mo2O7-xPEI green tapes were prepared by a tape casting procedure using a laboratory tape casting machine (A.J. Carsten Co., Inc, San Diego, CA USA) with a doctor blade casting head and a carrier film (silicone-coated polyethylene terephthalate). After drying at room temperature, the (1-x)Na2Mo2O7-xPEI green tapes were cut into 1 inch diameter circles using a CO2 laser (Laser Systems, Scottsdale, Arizona USA). Then, some of (1-x)Na2Mo2O7-xPEI tapes were printed with silver ink (DuPont 5029, Wilmington, DE USA) using a screen printer (Model 645, AMI Presco, North Branch, NJ USA). Afterwards, one (1-x)Na2Mo2O7-xPEI layer with the ring electrode, six (1-x)Na2Mo2O7-xPEI layers without electrodes and one (1-x)Na2Mo2O7-xPEI layer with the whole electrode were stacked together, and laminated at 75 °C for 20 min with an isostatic pressure of 21 MPa (Isostatic Laminator, IL4004 Pacific Trinetics Corporation, Carlsbad CA USA). The binder burnout was performed at 200-240 °C for 2-3 hours with a heating rate of 0.5 °C/min. Then, the (1-x)Na2Mo2O7-xPEI-Ag multilayers were wetted by a water vapor in a sealed beaker at 60-75°C. Afterwards, the moistened layers were put into a die and cold sintered at 120 °C for 20 min (ramp time: 20-25 min) under uniaxial pressures of 175 MPa, followed by drying in an oven at 120 ºC for 6 hours. The multilayer samples (8 layers) show a thickness of 350-400 µm and 250-300 µm before and after cold sintering, respectively.

Characterization. The structures and phase purities of cold sintered composites were measured by X-ray diffraction (XRD) system (PANalytical Empyrean) operated at 45 kV and 40 mA with Cu Kα radiation. The density was measured using Archimedes’ methods with ethanol as the liquid medium. In the calculation of the theoretical density of Na2Mo2O7-PEI-Ag


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multilayer ring resonators, the volume fraction of Ag was estimated using the thickness and area of Ag. The microstructures were analyzed using an environmental scanning electron microscope (ESEM, FEI, Quanta 200) and transmission electron microscopy (TEM, FEI Talos @ 200kV) with a SuperX energy dispersive spectrometer (EDS) system, which has four silicon drift detectors surrounding the sample. All the EDS maps were collected under scanning transmission electron microscopy (STEM) mode by using a high angle annular dark field (HAADF) detector. The TEM specimens were prepared by in situ lift out via milling in a FEI Helios NanoLab DualBeam 660 focused ion beam (FIB). A thick protective amorphous carbon layer is deposited over the region of interest and Ga+ ions (30 kV then stepped down to 1 kV to avoid ion beam damage to the sample surfaces) are used in the FIB to make the samples electron transparent for TEM imaging. Based on the Hakki-Coleman method, the microwave permittivities and Q×f values of (1-x)NM-xPEI composites were identified using the TE011 mode with a vector network analyzer (Anritsu 37369D). The microwave dielectric properties of ((1-x)NM-xPEI)-Ag composite ring resonators were obtained from S21 spectrum with the vector network analyzer. To perform the dielectric breakdown measurement, the samples were thinned and polished down to ~200 µm. The polished samples were placed in Galden HT-200 fluid and a DC voltage was applied to the samples at a rate of 500 V/s with Trek Model 30/20 as the high voltage source at room temperature. The dielectric breakdown strength was calculated by dividing the breakdown voltage by the sample thickness. To obtain the Weibull distribution of the dielectric breakdown strength, 27-48 samples for each composition were used. The bulk and multilayer ring resonators (Diameter: 25.4 mm) were prepared by cold sintering with a pressure of 175 MPa and all the other samples (Diameter: 12.7 mm) were cold sintered under a pressure of 350 MPa.

RESULTS AND DISCUSSION


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Densification and Electrical Properties of NM-PEI Composites. The cold sintering of a NM ceramic and PEI polymer composite is demonstrated with the density trends in Fig. 1a. The relative densities of all the samples cold sintered at 120 °C under a pressure of 350 MPa for 20 min are higher than 90%. Since the density of PEI is lower than that of NM, the numerical density of (1-x)NM-xPEI composite decreases systematically with the PEI addition. It is noted that the pressure plays an important role in the density of the cold sintered (1-x)NM-xPEI composite. With a pressure of 77 MPa (120 °C for 20 min), the relative density is lower than 90%, while with a pressure in the range of 175 and 350 MPa (120 °C for 20 min), the relative density is higher than 90%, as seen in Fig. 1b. Figure 1c presents the XRD patterns of (1-x)NMxPEI composite with different volume fractions of PEI after cold co-sintering at 120 ºC and 350 MPa. There are no observed impurities in the cold sintered (1-x)NM-xPEI composite, indicating that NM is chemically compatible with PEI polymer under such cold sintering condition.

Figure 1. (a) Densities of (1-x)NM-xPEI composites cold sintered at 120 °C under a pressure of 350 MPa for 20 min as a function of PEI content. (b) Densities of cold sintered (1-x)NM-xPEI


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composites with different pressures. (c) XRD patterns of cold sintered (1-x)NM-xPEI composites (x = 10-50 Vol%). PEI and standard Na2Mo2O7 PDF profiles are provided for better comparison. Back scattered electron images of cross-sections of (d) 90NM-10PEI and (e) 50NM-50PEI composites cold sintered at 120 °C. The direction of applied pressure during CSP is provided in the SEM images. Additional evidence of (1-x)NM-xPEI composite sintering can be found in the SEM micrographs, as shown in Fig. 1d-e and Fig. S3 (Supporting information). Due to the different atomic weight (Z contrast) of Na/Mo (22.99/95.94) and C/H/O/N (12.01/1.01/16/14.01), the grains with a bright color are rich in Na/Mo (NM phase) and the regions with a dark color are rich in C/H/O/N (PEI phase). It is seen that PEI is dispersed homogeneously and the porosity is low in the (1-x)NM-xPEI composites cold sintered at 120 °C, which is in accordance with the density data. It is noted that PEI is a soft material, and thus the polymer phase in the composite shows a preferential orientation after cold sintering, which is perpendicular to the pressure. In the case of (1-x)NM-xPEI composites with high amounts of PEI, particularly higher than 20%, there is some agglomeration for PEI phase, as seen in Fig. 1e and Fig. S3. The detailed microstructures of the grain boundaries of cold sintered (1-x)NM-xPEI composites are shown in TEM images along with the corresponding elemental distribution in Fig. 2 and Fig. S4 (Supporting information). Since 80NM-20PEI is a ceramic matrix composite, Na2Mo2O7-Na2Mo2O7 grain boundaries can be observed in most regions. There are also boundaries of Na2Mo2O7 and PEI observed in some regions, and even thin layers of PEI with nano-meter size located in the grain boundary of Na2Mo2O7, which may play an important role in the electrical properties of the composites. It is noted that most of the pores of the (1-x)NM-xPEI composites locate between the Na2Mo2O7 grains, as seen in Fig. S4.


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Figure 2. (a-b) TEM images of cold sintered 80NM-20PEI composite at two magnifications. (c) HAADF STEM image and (d-f) EDS maps showing the related elemental distributions in (c). Figure 3a presents the permittivities and Q×f values of cold sintered (1-x) NM-xPEI composites at microwave frequencies (10-14 GHz). Compared with Na2Mo2O7, PEI has a lower permittivity of ~3, and thus the permittivity of the (1-x) NM-xPEI composites decreases from 13.4 to 7.1 with increasing the amount of PEI from 0 to 50 vol. %. There does not appear to be a chemical reaction between Na2Mo2O7 and PEI under the cold sintering process. This is further supported with the permittivity trends of the (1-x)NM-xPEI composites that follow the classic mixing laws based solely on volume fractions. In this study, parallel (Equation 1), series (Equation 2) and Lichtenecker logarithmic (Equation 3) mixing laws were all considered to capture the trend in relative permittivity of the composites: ε = V1 ε1 + V2 ε2

(1)

1/ε = V1/ε1 + V2/ε2

(2)


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lgε = V1 lgε1 + V2 lgε2

(3)

where ε1 and ε2 are respective relative permittivities of material 1 and material 2; V1 and V2 (V1+V2=1) are respective volume fractions of material 1 and material 2. The measured permittivity of (1-x)NM-xPEI composites is lower than that calculated from parallel law, higher than that from series law, and as would be expected in good agreement with mixed connectivity between these extremes as given with the logarithmic law, as shown in Fig. 3a. When increasing the amount of PEI, the Q×f values of (1-x)NM-xPEI composites fluctuate in the range of 10,000 and 15,000 GHz, indicating that the addition of PEI has no obvious deleterious effects on the loss of the (1-x)NM-xPEI composite, although there is some agglomeration for PEI phase with high x values. For dielectric materials, there is also concern over the stability under an applied electric field. There are different mechanisms that can limit the critical field strengths, and one of the most important mechanisms is thermal runaway. During thermal runaway local conduction and its associated local heating creates a positive feedback, more localized conduction is generated and eventually creates a breakdown in the material, which is in ceramics an irreversible decrease in electrical resistivity. After breakdown, the materials no longer can withstand high field, and the dielectric materials are no longer functional. Given the statistical nature of the breakdown strength distributions across an number of samples, it is necessary to conduct multiple breakdown studies and assess the trends within appropriate statistical approaches such as through Weibull statistics.20 Fig. 3b shows the Weibull plots of the room temperature dielectric breakdown strength of (1-x)NM-xPEI composites cold sintered at 120 °C. For the dielectric breakdown, Weibull distribution shows the probability of the dielectric failure under electric


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field. Typically, the two-parameter Weibull distribution (Equation 4) is used to describe the probability distribution function (P) of breakdown electric field strength (E):3 P = 1-exp[-(E/α)β]

(4)

where α is the scale parameter, at which point 63.2% of the samples have the dielectric failure, and β is the shape parameter, which is also known as Weibull modulus. In the case of cumulative probability (Pi), the Weibull plot can be described as follows: ln[-ln(1-Pi)] = β(lnEi-lnα)

(5)

Pi=i/(n+1)

(6)

E1 ≤ E2 ≤ …≤ Ei ≤…≤ En

(7)

where Ei is the specific breakdown strength of the ith sample and n is the total number of the samples. The characteristic dielectric breakdown strength, α, and Weibull modulus, β, can be readily determined from the intercept and slope of the plot of ln[-ln(1-Pi)] vs lnEi, respectively. Accordingly, the Weibull parameters of cold sintered (1-x)NM-xPEI composites (α and β), were obtained from the Weibull plot (Fig. 3b). The Weibull modulus is in the range of 5~8 and the dielectric breakdown strength shows an increasing trend with the addition of PEI, as shown in Fig. 3c. In this work, all the samples have high densities without obvious impurities and the dielectric breakdown strength tests were performed under the same conditions using samples with similar thickness; therefore, the trend of the characteristic dielectric breakdown strength is dominated by the compositions of cold sintered (1-x)NM-xPEI composites. Typically, ceramics have a low dielectric breakdown strength, while, polymers exhibit high dielectric breakdown strength. The reported breakdown strength of a pure PEI is in the range of 460 ~ 661 MV/m,21,22 which is much higher than that of Na2Mo2O7. However, it is important to note that the dielectric failure is initiated by the weakest links, and therefore, the details of local field distribution and


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the associated conduction and dissipation of thermal energy all contribute to the ultimate dielectric strength. In our investigation, the dielectric breakdown strength can be improved by adding polymers into ceramics, as schematically shown in Fig. 3d. With 10-20 vol. % of PEI, the breakdown strength of (1-x)NM-xPEI composite increases dramatically in a statistically significant jump from 55.1 MV/m to 107.5 MV/m, as depicted in Fig. 3c. In the case of high PEI content, there is a gradual decrease in the breakdown strength, likely resulting from the agglomeration of PEI and heterogeneous separation of Na2Mo2O7 and PEI, which limit the uniformity of field distribution and enhance the local electric field.23

Figure 3. Room temperature electrical properties of (1-x)NM-xPEI composites cold sintered at 120 °C as a function of x value. (a) Microwave dielectric properties. (b) Weibull plot of the dielectric breakdown strength. (c) The dielectric breakdown strength as a function of PEI content. (d) Schematic of dielectric breakdown phenomenon of ceramics and ceramic-polymer composites, showing the improvement of dielectric breakdown strength with polymer additions.


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Prototypes of Electronic Device Development Based on Ceramic-Polymer Composites. To assess the practical applications of this new dielectric composite category, we have assessed ceramic-polymer composites with electrodes by post screen printing and multilayer ceramicpolymer composites with processing similar to a low temperature co-fired ceramic (LTCC) packaging approach.24-26 A prototype of ((1-x)NM-xPEI)-Ag bulk ring resonators is shown in Fig. 4. The Ag microstripline patterns with a 0.254 mm line width and a 10 mm diameter ring were printed on cold sintered (1-x)NM-xPEI composites with 1 inch diameter, as shown in Fig. 4a. Fig. 4b-c shows the backscattered scanning electron images of the ((1-x)NM-xPEI)-Ag bulk ring resonators. There are three types of regions in the sample, where the grey grains belong to Na2Mo2O7, the darkest regions belong to PEI, and the brightest ones belong to Ag, indicating that three phase system with Na2Mo2O7, PEI and Ag can be readily integrated.

The microwave permittivity of the ((1-x)NM-xPEI)-Ag bulk ring resonators decreases from 12.7 to 6.9 with an increasing amount of PEI from 0 to 50 vol. %, as shown in Fig. 4d, which is similar to the results in Fig. 3 for the (1-x)NM-xPEI bulk composites. The parallel and series mixing laws provide the upper and lower bounds, and the Lichtenecker logarithmic (Equation 3) mixing law closely predicts the permittivity of the bulk ring resonators. Compared with the (1x)NM-xPEI bulk composites, the Q values (f = 3.5-4.5 GHz) of the ((1-x)NM-xPEI)-Ag bulk ring resonators are smaller, which results from the Ag patterns. For example, 80NM-20PEI-Ag resonator (with Ag patterns) has a Q×f value of 190 GHz, while 80NM-20PEI resonator (without Ag patterns) prepared by the same method shows a Q×f value > 9,500 GHz, revealing that Ag patterns plays an important role in the Q×f value of ring resonators. However, with varying amounts of PEI, the Q values of the ((1-x)NM-xPEI)-Ag ring resonators show small changes, which is similar to the trend of the Q×f values of (1-x)NM-xPEI composites.


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Figure 4. Microstructures and room temperature electrical properties of ((1-x)NM-xPEI)-Ag bulk ring resonators cold sintered at 120 °C. (a) Photograph of a 50NM-50PEI-Ag bulk ring resonator. Backscattered scanning electron micrographs of surfaces of (b) 50NM-50PEI and (c) 90NM-10PEI bulk ring resonators. (d) Permittivities and Q values of ((1-x)NM-xPEI)-Ag composite ring resonators as a function of x value. Figure 5a-g shows a schematic of preparing ((1-x)NM-xPEI)-Ag multilayer ring resonators by the cold sintered co-fired ceramic-composite technology. Poly(propylene carbonate) (QPAC 40) and butyl benzyl phthalate S-160 were used as the binder and plasticizer, respectively, to enable low temperature binder burnout.27 The (1-x)NM-xPEI green tapes were obtained by a tape casting procedure as shown in Fig. 5a. After cutting (Fig. 5b) and Ag printing (Fig. 5c), the composite tape layers were stacked together, and laminated with isostatic pressure (Fig. 5d). The QPAC binder in the (1-x)NM-xPEI composites decomposes at a low temperature in the range of 200-240 °C without damaging PEI in the composites, as shown in Fig. 6. After binder burnout


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(Fig. 5e), the (1-x)Na2Mo2O7-xPEI-Ag multilayers were wetted by exposing them to a water vapor in a sealed beaker (Fig. 5f), and cold sintered at 120 °C for 20 min under an uniaxial pressure of 175 MPa (Fig. 5g).

Figure 5. Schematic illustration of ((1-x)NM-xPEI)-Ag multilayered ring resonator preparation by cold sintering, which includes (a) tape casting, (b) cutting, (c) printing, (d) stack and lamination, (e) binder burnout, (f) wetting, and (g) sintering. (h) Photograph and SEM images of surfaces of (80NM-20PEI)-Ag multilayered ring resonator cold sintered at 120 °C.


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Figure 6. The thermal gravimetric analysis (TGA) of 80NM-20PEI composite with QPAC binder. The cold co-sintered 80Na2Mo2O7-20PEI-Ag multilayer ring resonator has a high relative density of ~95%, which is in good agreement with the microstructure in Fig. 5h. It was observed that the three phase multilayer ring resonators can be densified without delamination or warping. The microwave dielectric properties of cold sintered multilayer ring resonators are similar to that of bulk ring resonators. For example, 80Na2Mo2O7-20PEI-Ag multilayer ring resonators show a permittivity of 8.3 and a Q of 73 (4.06 GHz), and 80Na2Mo2O7-20PEI-Ag bulk ring resonators show a permittivity of 8.5 and a Q of 46 (4.12 GHz). CONCLUSIONS The cold sintering process is introduced to fabricate high performance ceramic-polymer composites and develop electronic devices. NM-PEI composites can be cold sintered to >90% of theoretical density at temperatures as low as 120 °C. PEI shows a good dispersion in the cold sintered NM-PEI composites. The microwave permittivity of the composite decreases from 13.4 to 7.1 with increasing amounts of PEI, and follows the classic logarithmic mixing law. The Q×f value of the composite has no obvious deterioration with the addition of PEI, which is in the


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range of 10,000 and 15,000 GHz. The two-parameter Weibull distribution has been used to describe the probability distribution function of the dielectric breakdown strength. With 10-20 vol. % of PEI, the breakdown strength of the composite is increased dramatically from 55.1 MV/m to 107.5 MV/m, while with high PEI content where there is more segregation of the polymer within the ceramic matrix, the breakdown strength of the composite decreases. The dielectric breakdown strength is initiated by the weakest links. The details of local field distribution, the associated conduction and dissipation of thermal energy all contribute to the ultimate dielectric strength. (NM-PEI)-Ag bulk ring resonators with a 10 mm diameter Ag ring have been successfully prepared by post screen printing. When increasing the amount of PEI, the microwave permittivity of the (NM-PEI)-Ag ring resonators show similar results with that of the NM-PEI composites. Na2Mo2O7-PEI-Ag multilayer ring resonators with good dielectric properties can be obtained by cold sintered co-fired ceramic-composite technology. The three phase multilayer ring resonator shows no delamination or warping. ASSOCIATED CONTENT Supporting Information. The following file is available free of charge. Additional XRD pattern, SEM and TEM images. The heating rate of the inside of the die. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J. Guo); [email protected] (C. A. Randall) Funding Sources


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This work is supported under an industrial project with SABIC (Saudi Basic Industries Corporation). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors would like to thank Steven E Perini at Materials Characterization Laboratory of Pennsylvania State University for his help with the electrical breakdown measurement and dielectric tests of ring resonators, Haiying Wang for her help on TEM sample preparation by FIB. The authors gratefully acknowledge technical assistance from Christopher Grabowski at SABIC. REFERENCES 1.

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Cold sintering Na2Mo2O7 ceramic with polyetherimide (PEI) polymer

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