borosilicate glass buoyance material

borosilicate glass buoyance material

Materials Science & Engineering A 674 (2016) 604–614 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: w...

2MB Sizes 0 Downloads 28 Views

Materials Science & Engineering A 674 (2016) 604–614

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Mechanical properties and thermal conductivity of a temperature resistance hollow glass microspheres/borosilicate glass buoyance material Sue Ren, Jiachen Liu, Anran Guo, Wenjie Zang, Haitao Geng, Xin Tao, Haiyan Du n Key Laboratory of Advanced Ceramics and Mechanical Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

art ic l e i nf o

a b s t r a c t

Article history: Received 8 January 2016 Received in revised form 3 August 2016 Accepted 4 August 2016 Available online 8 August 2016

A temperature resistance buoyancy material was fabricated through a tert-butyl alcohol gelcasting process with borosilicate glass (BG) and hollow glass microspheres (HGMs) as the matrix and filler, respectively. The effects of the mass ratio of HGMs to BG and sintering temperature on the microstructure, thermal conductivity, and mechanical properties of the composite were studied. The results show that HGMs were bonded together by the BG, and the sample sintered at 750 °C exhibited a broad pore size distribution, from several microns to more than one hundred microns. The thermal conductivity experimental values of all the samples were less than that of Hashin-Shtrikman upper bound (HS þ ) prediction but agreed well with that predicted from effective medium percolation theory. The relationship between compressive strength and relative density was predicted by the Gibson-Ashby model, with the calibration factor phi below 0.7. Young′s modulus values obtained from the experiment were below that of HS þ prediction. The modulus values of the four types of samples sintered at 650 °C agreed well with the Pabst model prediction, while the values of the samples sintered at 700 °C and 750 °C were distributed in a zone between Ashby-Gibson model and HS þ prediction. & 2016 Elsevier B.V. All rights reserved.

Keywords: Hollow glass microspheres Borosilicate glass Mass ratio Thermal conductivity Mechanical properties Temperature resistance

1. Introduction Epoxy resin syntactic foams (SFs) with closed cell structure prepared by dispersing hollow microspheres in a resin exhibit excellent properties containing low density, high compressive strength, low moisture absorption, and low shrinkage. The SFs, as a core material for sandwich structures, are commonly used in the field of aerospace and deep sea [1–6]. In case of carrying out antiship missile experiment under the deep-sea environment, SFs as one of the important parts of the submarine would be subjected to high temperature (from several hundred degrees Celsius to thousands of degree Celsius) erosion generated during the launching process. Meanwhile, the mechanical performance of the SFs may be also affected under these conditions. Thus, the fabricated SFs foams should possess excellent temperature resistance behavior in order to be used in the high temperature environment. At present, literatures on improving the temperature resistance of foams materials were quite limited, and injecting some temperature resistance filler into the matrix may be a possible way of n

Corresponding author. E-mail address: [email protected] (H. Du).

http://dx.doi.org/10.1016/j.msea.2016.08.014 0921-5093/& 2016 Elsevier B.V. All rights reserved.

improving the temperature resistance. The fillers can be fly ash cenospheres, hollow silica microspheres, and hollow carbon microspheres [7,8]. However, the temperature resistance of the foams prepared with the above fillers was not largely improved. The main reason is that the researchers may not identify the factor influencing the temperature resistance of the SFs. The temperature resistance of the epoxy syntactic foams was dependent on the epoxy resin (the matrix) and hollow glass microspheres (the filler). Epoxy resin, a type of organic compound, composed of a series of epoxy groups with low molecular weight, exhibited poor temperature resistance. On contrary, the filler of HGMs possessed excellent temperature resistance because of the chemical composition of soda-lime-borosilicate glass [9]. In view of the above factor influencing the temperature resistance of buoyance material, it is not hard to see that the matrix plays a rather importance role in determining the temperature resistance of the SFs. Therefore, we should seek for a new matrix in order to obtain the foams with excellent temperature resistance. Compared to metal or polymer matrix composite, the ceramic matrix composite possessed outstanding temperature resistance [10,11], and if the matrix was replaced by the ceramics, the temperature resistance of the buoyance material would be greatly

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

improved. Borosilicate glass (BG), a type of ceramic material, possessed distinguished temperature resistance and mechanical properties, and superior wettability. Besides, the chemical composition and softening temperature of the BG was similar to that of the HGMs, which may be beneficial to enhance the bonding strength among the HGMs. In previous study [12], we have successfully prepared a novel temperature resistance hollow glass microspheres/borosilicate glass (HGMs/BG) buoyance material through a tert-butyl alcohol (TBA)-based gelcasting process. During the experiment process, we found that the mass ratio of HGMs to BG played a significant role in the mechanical properties and thermal conductivity of the HGMs/BG composite. The effect of hollow particles addition on the mechanical property and microstructure of the cellular material has been reported in previous study. Kim et al. [13] found that the porosity of the cellular glasses can be adjusted from 42% to 62% with hollow glass microspheres ranging from 30% to 100%. In Su′s study, the results [14] indicated that the porosity of the Al2O3 ceramic varied from 22.3% to 60.1% with the Al2O3 hollow sphere from 30% to 50%. The results of the Dr. Zhao′s study [15] revealed that the amount of hollow ceramic microspheres would have an effect on the mechanical properties and porosity of Al matrix syntactic foams. However, less study has been performed on the influence of the mass ratio of HGMs to BG on mechanical property and thermal conductivity of the HGMs/BG composite. The aim of this study was to investigate the effect of mass ratio of HGMs to BG on the mechanical and thermal properties of the HGMs/BG composite. The composite with excellent temperature resistance and mechanical property would be used in marine and aerospace in the near future.

suspension. The samples were prepared through a TBA-based gelcasting process and the solid loading of 45 wt% remains unchanged during the experimental process. To investigate the effect of the mass ratio of HGMs to BG on the mechanical performance, microstructure, and thermal properties of the HGMs/BG composite, four types of composites were prepared with mass ratios of HGMs to BG of 3:7, 4:6, 5:5, and 6:4, denoted as HGMs/BG-37, HGMs/BG-46, HGMs/BG-55, and HGMs/BG-64, respectively. Then the suspension was de-aired under vacuum to remove air bubbles introduced during ball milling. After the addition of initiation solution (10 wt%, 2 ml), the suspension was poured into the mold with a size of 20  20  20 mm. Finally, the mold was heated at 40 °C for 0.5 h, and after drying the samples were sintered at 650, 700, and 750 °C, respectively, at heating rate of 1 °C/min, holding for 2 h. Fig. 2 showed a process scheme for the preparation of HGMs/BG composites by a TBA-based gelcasting process. 2.3. Characterization The morphology of the HGMs and HGMs/BG composites was observed by scanning electron microscope (SEM, SU1510, Hitachi, Japan). The pores size distribution of closed cells of the composite was determined by quantitative image analysis of cross-section SEM micrographs. The linear shrinkage of the samples was calculated using Eq. (1),

⎛l −l ⎞ 1 lshrinkage = ⎜ 0 ⎟× 100% ⎝ l0 ⎠

(1)

where, l is the length (mm) of the sample, the subscript 0 and 1 represent before and after sintering, respectively. The pore volume fraction υp (the sum of open porosity and closed porosity) is calculated from Eq. (2),

2. Experimental procedure 2.1. Raw materials Hollow glass microspheres (softening temperature of 600 °C) as raw materials were supplied by 3 M Co., USA, and the basic parameter was list in Table 1. The particle size distribution and phase composition of the HGMs were shown in Fig. 1. Borosilicate glass (softening temperature 650 °C [16], sieved with 200-mesh screen) was purchased from Xuanyang Co., Ltd., Zhuhai, China and their properties were list in Table 2. Tert-butyl alcohol (C(CH3)3OH, TBA), acrylamide (CH2 ¼CH CNH2, AM), and N, N′-methylene bisacrylamide ((CH2 ¼CHCONH)2CH2, MBAM) were used to prepared the premixed solution. The procedure was carried out as described previously [17] and ammonium persulfate (APS) and citric acid (2 g per 100 g slurry) acted as the initiator and dispersant in the gelation reaction, respectively. All the chemicals purchased from Kemiou Chemical Reagent Co., Ltd., Tianjin, China were analytical regent. 2.2. Preparation procedure The homogeneous suspension was prepared through a highenergy ball milling of a mixture of BG, TBA, AM, MBAM and CA at 600 rpm for 4 h, and then the HGMs were added into the Table 1 Properties of the hollow glass microspheres.

Hollow glass microspheres

605

Isostatic crush strength (MPa)

Typical density (g/cm3)

Weight composition (%)

37.9

0.38

B O Na Al Si Ca 36.57 28.83 3.11 0.21 24.28 7.00

⎛ ρ ⎞ υP = ⎜⎜ 1− bulk ⎟⎟× 100% ρtrue ⎠ ⎝

(2)

where, ρtrue and ρbulk is the true density (g/cm ) and bulk density (g/cm3) of the sample measured by a pycnometer and weight-tovolume ratio of the samples, respectively. The open porosity was measured using Archimedes method, according to National Standard of the People′s Republic of China (GB/T 1966-1996) and to ensure the samples completely sunk in the bottom of water, a heavy object was tied to the samples and the weight and density of the object was given in the experiment. The thermal conductivity at room temperature was carried out on a Thermal Properties Analyzer (XIATECH TC 3000), according to the National Standard of the People′s Republic of China (GB/T 10297-2015). Uniaxial compressive test was performed by using an electronic universal testing machine (CSS-44100) with a crosshead speed of 0.5 mm/min, according to National Standard of the People′s Republic of China (GB/T 4740–1999) and Young′s modulus can be calculated from the elastic stage of the stress-strain curves. The specific strength (ssc) was calculated using Eq. (3), 3

σsc =

σc ρbulk

(3)

where, sc and ρbulk is maximum compressive strength (MPa) and bulk density (g/cm3) of the composites, respectively.

3. Results and discussion 3.1. Effect of the mass ratio of HGMs to BG on the green body To keep the integrity of the HGMs, TBA-based gelcasting

606

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

Fig. 1. Particle size distribution (a) and XRD pattern (b) of the HGMs. Table 2 Properties of borosilicate glass. True denWeight composition (%) sity (g/cm3) Borosilicate glass

3.2

B O Al Si Ca Ba Fe Zn 47.08 19.51 0.24 1.37 0.56 6.82 0.57 23.86

temperature. Table 3 presents the properties of the green bodies with different mass ratios of HGMs to BG. The results show that there was a small linear shrinkage for the four composites, from 1.17% to 0.39%. Accordingly, the bulk density and compressive strength decreased from 0.61 to 0.41 g/cm3 (higher than of that of HGMs, 0.38 g/cm3) and from 12.35 to 7.53 MPa, respectively. 3.2. Effect of the mass fraction of HGMs on the sintered samples

technique as an in situ consolidation process was used in the study. Fig. 3 shows the microstructure variation of HGMs before (Fig. 3a) and after gelcasting process (Fig. 3b), corresponding to the schematic diagram of Fig. 4a and b, respectively. As shown in Fig. 3a, the outer surface of the HGMs was smooth. After the initiator was dropped into the suspension (containing of HGMs, BG, and organics), the polymerization reaction started, resulting in the formation of a strong macromolecular network structure in the composite (Fig. 4b). HGMs were wrapped in the network, leading to the green body a high compressive strength at room

3.2.1. TG-DSC Fig. 5 shows the TG-DSC curves of the HGMs/BG composite. It can be seen that there were mainly three weight loss stages, corresponding to the three exothermic peaks in the curves. The rate of thermal decomposition of the composite was very slow, from room temperature to 220 °C and the weight loss (only about 3.05 wt%) may be due to the physical-absorbed water evaporation [18]. The first weight loss (about 13.42%) in the range of 210– 420 °C, corresponding to the narrow endothermic peak at 376 °C, was attributed to the pyrolysis of polyacrylamide generated in the

Fig. 2. The flowchart of preparing the HGMs/BG composite.

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

607

Fig. 3. SEMs images of the microstructure variation of HGMs before (a) and after (b) gelcasting process.

Fig. 4. The diagrammatic sketch of HGMs before (a) and after (b) gelcasting process.

Table 3 The characteristic of the green body of the HGMs/BG composite. The mass ratio of HGMs to BG

Composite nomenclature

Bulk density (g/ cm3)

Compressive strength (MPa)

Linear shrinkage (%)

3:7 4:6 5:5 6:4

HGMs/BG-37 HGMs/BG-46 HGMs/BG-55 HGMs/BG-64

0.61 0.56 0.48 0.41

13.16 11.85 10.58 7.53

0.83–1.17 0.72–0.95 0.52–0.75 0.39–0.55

Fig. 5. TG-DSC curves of the green body.

green body [19], which was in good agreement with the total proportion of AM and MBAM in the preparation process. The second weight loss (approximate 9.95%) between 420 °C and 600 °C resulted from the pyrolysis of the residual small molecules. The organics was totally decomposed in the third stage (in the range of 600–900 °C). 3.2.2. Liner shrinkage, bulk density, compressive strength and specific strength Fig. 6 shows the linear shrinkage, bulk density, compressive strength, and specific strength of the four types of composites with different mass ratios of HGMs to BG as a function of sintering temperature. It shows that the linear shrinkages were below 5% for the four types of samples sintered at 650 °C and the shrinkages increased up to 15% or even higher with the sintering temperature increasing to 700 °C or 750 °C. In case of the HGMs/BG-37 sample, the shrinkage increased from 2.76% to 40.60% with sintering temperature from 650 °C to 700 °C. The results suggested that the sintering temperature played a great effect on the liner shrinkage of the HGMs/BG composite. This large shrinkage at 700 °C or 750 °C may be related to the densification process during the sintering process. Besides, the mass ratio of HGMs to BG also affected the shrinkage of the composite and the sample with large mass ratio tended to exhibit low shrinkage. For the HGMs/BG-37 and HGMs/BG-64 samples sintered at 750 °C, the shrinkage was 40.60% and 29.36%, respectively. As shown in Fig. 6b and 6c, all the samples sintered at 650 °C with different mass ratios exhibited low density and low compressive strength, and for the HGMs/BG-64 composite the values were 0.36 g/cm3 and 3.13 MPa, respectively. The density and compressive strength gradually enhanced with an increase of temperature, and for the HGMs/BG-64 composite sintered at

608

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

750 °C, the density and compressive strength increased to 0.86 g/ cm3 and 12.80 MPa, respectively, indicating that the sintering temperature exerted a great effect on physical properties of the composite. The softening temperature of BG was 650 °C and therefore, the BG presented a high-viscosity state when the sintering temperature was below 650 °C. In this case, the outer surface of HGMs could not be completely coated by the liquid BG after the organics decomposition, leading to some void among the HGMs. Correspondingly, all the samples exhibited low density and low compressive strength at the sintering temperature of 650 °C. With increasing of sintering temperature to 700 °C or 750 °C, higher than the softening temperature of BG (  650 °C), the BG started to soften and gradually transformed into the liquid glass, with the characteristic feature of low viscosity. The outer surface of the HGMs could be totally coated by the liquid glass and the HGMs were bonded together by the liquid glass with no obvious voids in the samples. When the liquid BG cooled to room temperature, the samples exhibited high compressive strength. Fig. 6b and 6c also shows that bulk density and compressive strength decreased with increasing the mass ratio from 3:7 to 6:4, which may be related to the decrease of BG content in the sample. The specific strength of the four types of samples sintered at different temperatures is shown in Fig. 6d. The specific strength (ssc) was defined by the ratio of compressive strength to bulk density and the high value means that the composite could withstand high compressive strength under the same bulk density. The results indicate the four composites sintered at 700 °C or 750 °C exhibited

specific strength of above 15 MPa/(g cm  3), higher than that of the one sintered at 650 °C. 3.2.3. Microstructure To investigate the microstructure of the composite under different sintering temperature, the HGMs/BG-64 composite was selected as the typical example. Fig. 7a1, 7a2, and 7a3 shows SEM images of the HGMs/BG-64 composite sintered at 650 °C, 700 °C, and 750 °C, respectively, and Fig. 7b1, 7b2, and 7b3 presents the corresponding pore size distribution of the closed cells. The morphology of the closed cells (formed by the HGMs) is nearly spherical, indicating that the shape of the HGMs was retained during the sintering process. The pore size distribution of the open cells and closed cells was nonuniform, from a few microns to several tens of microns, mainly depending on the length of the flaws and the particle size of the HGMs, respectively. As shown in Fig. 7a1, HGMs were bonded together with some gaps in the sample, which may be closely related to the lessflowability of BG. Meanwhile, the composite exhibited low strength and high porosity, 3.53 MPa and 73.38% (see Fig. 6), respectively. The pore size distribution of closed cells was in the scope of 20–60 mm (Fig. 7b1), similar to the particle size distribution of HGMs. Koopman [20] reviewed that an observation of unbroken microspheres in a foam indicating that the weak interfacial adhesion characteristics may be present in the system and this phenomenon was consistent with that of HGMs in our composite. Fig. 7a2 shows that HGMs were bonded together without

Fig. 6. The linear shrinkage (a), bulk density (b), compressive strength (c), and specific strength (d) of the four composites sintered at different temperature.

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

609

Fig. 7. The morphology feature and the corresponding pore size distribution of closed cells of the HGMs/BG-64 composite sintered at 650 °C (a1 and b1), 700 °C (a2 and b2), and 750 °C (a3 and b3), respectively.

gaps, which may be due to the formation of liquid glass at sintering temperature of 700 °C. It is known that the formed liquid glass exhibit low viscosity and excellent flowability and the HGMs were bonded together by the liquid glass, resulting in the sample with high compressive strength of 10.56 MPa and large density of 0.69 g/cm3. It is also noted that the shell of the closed cells slightly deformed and the composite exhibited a wide range of pore size distribution, from several microns to tens of microns (see Fig. 7b2). As sintering temperature increased to 750 °C, the shell of the closed cells severely deformed (Fig. 7a3) and accordingly, the pore

size distribution was broad, from several microns to more than one hundred microns. The results revealed that the temperature may exert significant influence on microstructure and pores size distribution of the closed cells and the optimum temperature may be at 700 °C. Fig. 8 shows microstructures of the HGMs/BG composites sintered at 700 °C with the mass ratio of 3:7, 4:6, 5:5, and 6:4, respectively. It shows that the samples presented a homogeneous microstructure without large voids in the materials. These results suggest that the processing method is capable of avoiding

610

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

Fig. 8. SEM images of the HGMs/BG-37 (a), HGMs/BG-46 (b), HGMs/BG-55 (c), and HGMs/BG-64 (d) composite sintered at 700 °C.

significant agglomeration of the HGMs, and at the same time, allows for a uniform distribution of the HGMs among the glass powders. Besides, the dense struts in the cellular structure were produced and the thickness of the strut decreased as the content of the HGMs increased. Some “large holes” with smooth inner surface were also observed, which may be related to the liquid glass. Besides, the number of the holes gradually decreased with the mass ratio increase from 3:7 to 6:4, as shown in Fig. 8a, b, c, and d, respectively. It can be deduced that the mass ratio of HGMs to BG would have a great influence on both mechanical and thermal properties of the composite.

composite occurs in three major ways: (1) thermal conduction through solid and gas, (2) natural thermal convection of gas in the HGM, and (3) thermal radiation on the surface between HGMs. The radiation and convection effect were both neglected and conduction is the main way in the composite. Predicting effective thermal conductivity of the composite can be assimilated to a twophase system, constituted by a dense solid skeleton and the pores. Several models were used for thermal conductivity predictions. The parallel and series models give the highest and lowest possible limits of the effective thermal conductivity, calculated by Eq. (4) and Eq. (5), respectively.

3.2.4. Thermal conductivity Fig. 9 shows schematic diagram of the heat transfer in the HGMs/BG composite. The heat transfer process in the HGMs/BG

Parallel model λ eff = 1−υp λ s + υpλ p

(

Series model λ eff =

)

(4)

λ sλ p

(

λ sυp + λ p 1−υp

)

(5)

Where, λ and υ is thermal conductivity (W/m K) and pore volume fraction (%), respectively. The subscript eff, s, and p represent the effective thermal conductivity, solid phase, and pore, respectively. The Hashin-Shtrikman approach considers isolated spherical inclusions placed in a continuous matrix and in the upper bound expression, Eq. (6), the conducting phase is the matrix.

⎡ λ + 2λ + 2υ λ − λ p s p p s λ eff = λ s⎢ ⎢⎣ λ +2λ − υ λ − λ p s p p s

( (

Fig. 9. Heat transfer model of the HGMs/BG composite.

) ⎤⎥ ) ⎥⎦

(6)

Another approach is the effective medium percolation theory (EMPT), which considers a random mixture of the two homogeneous phase and continuous paths through the pore. Landauer′s

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

expression [21], Eq. (7), is used to predict the effective thermal conductivity.

λ eff =

1⎡ ⎢ λ p 3υp −1 + λ s( 3υs −1) 4⎣

(

+

)

⎤ ⎡ λ 3υ −1 + λ 3υ −1 ⎤2 +8λ λ ⎥ )⎦ p s( s s p ⎣ p ⎦

(

)

(7)

Values of 0.026 W/m K and 1.4 W/m K were used for the thermal conductivity of air and dense borosilicate glass solid, respectively. Fig. 10a shows the measured effective thermal conductivity (λeff) as a function of porosity and the experimental results were compared with that of from the predictions models. The same information is presented in Fig. 10b with an expanded logarithm scale for the Y-axis. As expected, the thermal conductivity decreased with porosity increasing from 42.54% to 79.38% and the HGMs/BG-37 and HGMs/BG-64 composite exhibited the maximum and the minimum λeff value of 0.56 W/m K and 0.06 W/m K, respectively. It is known that the thermal conductivity of borosilicate glass was 1.40 W/m K, higher than that of HGMs, 0.005–0.017 W/ m K, which means that thermal conductivity would gradually decrease with decreasing of BG content. Therefore, the sample with high proportion of BG tend to exhibit large thermal conductivity than that of the one with low content of BG and the results also indicated that the BG had great effect on thermal conductive of the composite. Fig. 10a also shows that all the data were distributed in the zone between series and parallel model prediction and the results were consistent with the Tessier-Doyen′ s study [22]. Moreover, for a binary phase system, the experimental values in this study were below the HS þ prediction, in accordance with previous study [22–27]. Additionally, it′s worth noting that the thermal conductivity of the four types of samples agreed well with the predictions from EMPT except for the HGMs/BG-55 sample with thermal conductivity of 0.15 W/m K (see Fig. 10b) and all the experimental values were still lower than that of some ceramic foams [22]. The lower thermal conductivity may be attributed to the following two reasons: firstly, the HGMs as the filler possessed low thermal conductivity; secondly, the pores separated by the HGMs walls (see Figs. 7 and 8) may also influence thermal conductivity of the composite, consistent with the scope of EMPT model. According to the percolation theory, applied to heat conduction, above the percolation threshold, the heat transfer was more and more difficulty through the material and the λeff is governed by the gas phase. We can deduce that for obtaining a low thermal

611

conductivity material with greater than 70% porosity, the nature of the solid phase seems to be rather unimportant. The porosity and thermal conductivity of the HGMs/BG-64 composite sintered at 650 °C was 79.38% and 0.06 W/m K, respectively, and the λeff was close to thermal conductivity of air (0.026 W/m K), consistent with the percolation theory. 3.2.5. Mechanical strength The mechanical property of the composite is illustrated in Fig. 11a. The classical model for the compressive strength of the cellular material is generally expressed by Gibson-Ashby [28] equation, as follow:

⎡ σc = σbendf ϕ, ρrel = σbend⎢ C ϕρrel ⎣

(

)

(

3/2

)

⎤ + ( 1−ϕ)ρrel ⎥ ⎦

(8)

where, sbend is the bending strength of the used borosilicate glass which is typically given as 70 MPa [29]; C is a dimensionless constant, being  0.2 [28]; ρrel is the relative density (the ratio between the measured density of the composite and the density of the used glass). The calibration factor was called phi (Φ), and the quality (1  Φ) expresses the fraction of glass in the cell walls. If the foam is open-cell, the pores are fully interconnected, with no solid in the hypothetical wall (1  Φ ¼0); on the contrary, for a closed-cell foam, the calibration factor is typically lower than 1. In practice, (1  Φ) represents the contribution to the mechanical strength given by the cell faces in the cellular structure [28]. Additionally, larger pores may correspond to greater Φ values, due to the low ratio between thickness of cell walls (tf, μm) and length of cell edges (l, μm) owing to the relation as follows [28].

tf l

=1. 4( 1−ϕ)ρrel

(9)

Fig. 11a shows the compressive strengths as a function of relative density and the results were compared with that of predicted from the Ashby-Gibson model with the parameter Φ values from 0 to 0.7, respectively. It is noted that the compressive strength of the samples with different mass ratio sintered 650 °C was in the range of 3.53–10.32 MPa, corresponding to the relative density ranging from 0.11 to 0.14. For the sample sintered at 750 °C, the compressive strength and bulk density increased from 12.8 to 20.55 MPa and from 0.21 to 0.43 g/cm3, respectively, with the mass ratio increasing from 3:7 to 6:4. The compressive strength values were below the prediction with the parameter of Φ ¼0, and all the data points lay in a zone for Φ values being between 0 and 0.7, indicating that the open and closed pores

Fig. 10. Effective thermal conductivity as a function of porosity for analytical predictions and experimental measurements. Calculation Parameters: λs ¼ 1.4 W/(m K) and λair ¼0.026 W/(m K)-(a) linear scale and (b) logarithm scale.

612

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

Fig. 11. Compressive strength of the composite with different relative density and the fitting results with Eq. (8) (a) and the strain-stress curves of the HGMs/BG composite (b).

Table 4 Analytical models for Young's modulus prediction. Models

The type of pores

Scope of porosity Expression

Pabst

Open, interconnected

0.15o υp o 0.65

E=E se

( −2υp / ( 1 −υp))

(10)

[35,36] Ashby-Gibson

Open

0.65 o υp o 0.95 2

E=E s( 1−υ P)

(11)

[27] Hashin-Shtrikman upper bound

Closed, spherical and unconnected

0o υp o 1

⎛ 1−υ p ⎞ ⎟⎟ E=E s⎜⎜ ⎝ 1+υ p ⎠

(12)

[23]

simultaneously existed in the sample. Correspondingly, the relation between compressive strength and relative density was fitted well by a straight line with the parameter of Φ ¼0.3. Though the sample exhibited the characteristic of open-cell foams, the compressive strength was still higher than that of previous reported glass foams (0.5–5 MPa) [30–32]. Kim et al. [13] discussed that the pores with dense strut in the material was a possible reason for the material with high compressive strength. Therefore, it can be deduced that the high compressive strength of the composite may be due to the small pores with the dense struts which was formed by the hollow structure of microspheres (Fig. 7) and the result was consistent with the Kim′s study. Fig. 11b is the stress-strain curves for the HGMs/BG composites sintered at 700 °C under the compression testing. It should be noted that the compressive behavior of the composite demonstrated a typical characteristic of cellular materials [33] and two stages containing elastic stage and fracture stage were observed during the process. In the initial stage, after the cross-head contacted the sample, the load increased linearly. When the applied load exceeded a certain value, the sample stared crushing and the load maintained a constant value. The nominal stress deduced from the load can be defined as the compressive strength. Further, the curve exhibited a horizontal plateau stage and, during this

stage, the sample started to rupture with zig-zag appearance emerging in the strain-stress curves, which is similar to that of the cellular epoxy [34]. Finally, in the fracture stage, the stress decreased until the sample completely failure. The wave oscillations of stress may be due to the HGMs-BG interfacial debonding or/and the HGMs fracture under the compressive loading conditions. Gupta et al. [35] discussed that under compressive loading condition, debonding does not play an important role because the matrix is compressed on the particle and the separation occurs only in a small region in the direction transverse to the applied load. The wall of the HGMs would be easily destroyed during the compressing process, leading to the compaction of HGMs/BG composite. Therefore, it is supposed that the possibility of HGMs fracture under compressive loading conditions may be an importance reason for the compressive strength decrease. 3.2.6. Young's modulus Young′s modulus was calculated through the elastic stage of the stress-strain curves. The experimental values of the HGMs/BG-37, HGMs/BG-46, HGMs/BG-55, and HGMs/BG-64 composite sintered at 700 °C was1.46 GPa, 1.34 GPa, 1.24 GPa, and 1.01 GPa, respectively, suggesting that the mass ratio may exert a significant influence on modulus of the composite. The relation between Young′

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

Fig. 12. Young's modulus at room temperature of the HGMs/BG composite as a function of porosity. Comparison with the analytical predictions of the HashinShtrikman upper model (Eq. (12)), Ashby's expression (Eq. (11)) and Pabst′s expression (Eq. (10)).

s modulus and porosity can be investigated through the following three models: Hashin-Shtrikman upper bound (HS þ ), Ashby-Gibson, and Pabst, respectively. Table 4 is the range of porosity, relations, and porosity type of the above three models, respectively, and when we used the relations, the individual microstructure of the sample should be in accordance with the given model parameters. υp (%) and Es (GPa) are the volume fraction of pores and the Young′s modulus of the solid skeleton of the composite (here refer to the borosilicate glass), respectively. Fig. 12 represents the comparison between the experimental values and three analytical expressions describing the Young's modulus as a function of porosity. As expected, Young′s modulus decreased with the increase of the porosity, and all the experimental values were smaller than that of HS þ prediction, consistent with majority studies [32]. The calculated average value of Es was 5.359 GPa, significantly lower than that of the dense borosilicate glass (50–100 GPa [37]) shaped by uniaxial compression test. Such a small value also gives an indication of the weak state of cohesion of HGMs in the solid skeleton of the foams. Despite this, the values was used to plot the HS þ prediction with Eq. (12) and with Eq. (10) derived by Pabst et al. The sequence in term of Young's modulus for νp 40.5 of the three prediction curves: Eq. (10) oEq. (11) oEq. (12), can be related to the amount of connectivity of the pore volume. The composites with high pore volume fraction possessed low modulus values, agreed well with the Pabst model. These results indicated that the composites sintered at 650 °C possessed high open porosity, consistent with the SEM images results. In Bourret's study [24], the experimental values of moduli were above Pabst model prediction, may be related to the pore structure of kaolin-based foams. In contrast, the modulus values of the composite sintered 700 °C or 750 °C lay in the zone between the Ashby-Gibson and HS þ prediction, suggesting that there is a restricted connectivity among the pores (Figs. 7 and 8).

4. Conclusion In this study, the temperature resistance HGMs/BG buoyancy materials with different mass ratio of HGMs to BG were successfully prepared through a TBA-based gelcasting technique. BG with low softening temperature and similar chemical composition to

613

the HGMs was used as the matrix. The results show that the compressive strength and bulk density increased with the increase of the mass of HGMs to BG from 3:7 to 6:4, and the sample with the mass ratio of 3:7 sintered at 750 °C exhibited maximum compressive strength and density, 20.55 MPa and 1.38 g/cm3, respectively. What′s more, the sample exhibited a broad pore size distribution, from several microns to more than one hundred microns. The composite exhibited low thermal conductivity, ranging from 0.56 and 0.06 W/m K, and compared with the values from the prediction models, the experimental values were close to the EMPT prediction, lower than that of HS þ prediction. The relationship between compressive strength and relative density was predicted by the Ashby-Gibson model and the experiment values of compressive strength were scattered in the region for the parameter Φ values being between 0 and 0.7, indicating that open cells and closed cells were in the composite. The compressive properties of the HGMs/BG composite demonstrated a typical behavior of cellular materials, with two well defined stages, elastic stage and fracture stage in the stress-strain curves. Young's modulus values were calculated from elastic stage of the stress-strain curves. The HGMs/BG-37 and HGMs/BG-64 composite possessed the maximum and minimum modulus of 1.62 and 0.13 GPa, respectively. Young′s modulus obtained from the sample sintered at 650 °C, were closed to the values predicted by the Pabst model, suitable for cellular materials with interconnected pores. In case of the materials sintered at 700 °C or 750 °C, the value points were distributed in the region between the HS þ and Ashby-Gibson model prediction. The HGMs/BG composite shows high closed porosity, uniform structure, low thermal conductivity and excellent temperature resistance. The composite with these superior properties may potentially use in aerospace and deep sea in the near future.

Acknowledgment The authors would like to acknowledge the National Natural Science Foundation of China (Project no. 51472176) for financial support.

References [1] B. John, C.P. Reghunadhan Nair, Syntactic foams, in: H. Dodiuk, S.H. Goodman (Eds.), Handbook of Thermoset Plastics, Elsevier Inc, Oxford, 2014, pp. 511–554. [2] S.N. Patankar, Y.A. Kranov, Hollow glass microsphere HDPE composites for low energy sustainability, Mater. Sci. Eng. A 527 (2010) 1361–1366. [3] C. Swetha, R. Kumar, Quasi-static uni-axial compression behavior of hollow glass microspheres/epoxy based syntactic foams, Mater. Des. 32 (2011) 4152–4163. [4] E.M. Wouterson, F.Y.C. Boey, X. Hu, S.C. Wong, Specific properties and fracture toughness of syntactic foam: effect of foam microstructures, Compos. Sci. Technol. 65 (2005) 1840–1850. [5] F. Awaja, B.D. Arhatari, X-ray micro computed tomography investigation of accelerated thermal degradation of epoxy resin/glass microsphere syntactic foam, Composites Part A 40 (2009) 1217–1222. [6] A. Trofimov, L. Pleshkov, H. Back, Hollow glass microspheres for high strength composite cores, Reinf. Plast. 50 (44–46) (2006) 48–50. [7] O.L. Ferguson, R.G. Shaver, Syntactic foams of hollow carbon microspheres in resin matrix, J. Cell. Plast. 6 (1970) 125–130. [8] K. Okuno, R.T. Woodhams, Mechanical properties and characterization of phenolic resin syntactic foams, J. Cell. Plast. 10 (1974) 237–244. [9] 3M Glass Bubbles, 2007. 〈http://www.3m.com.cn/〉. [10] G.J. Zhang, J.F. Yang, T. Ohji, Fabrication of porous ceramics with unidirectionally aligned continuous pores, J. Am. Ceram. Soc. 84 (2001) 1395–1397. [11] T. Ohji, M. Fukushima, Macro-porous ceramics: processing and properties, Int. Mater. Rev. 57 (2012) 115–131. [12] S. Ren, A. Guo, X. Dong, X. Tao, X. Xu, J. Zhang, H. Geng, J. Liu, Preparation and characteristic of a temperature resistance buoyancy material through a gelcasting process, Chem. Eng. J. 288 (2016) 59–69.

614

S. Ren et al. / Materials Science & Engineering A 674 (2016) 604–614

[13] D.H. Jang, Y.W. Kim, I.H. Song, H.D. Kim, Processing of Cellular glasses using glass microspheres, J. Am. Ceram. Soc. 89 (2006) 3262–3265. [14] Z. Su, X. Xi, Y. Hu, Q.F., S. Yu, H. Li, J. Yang, A new Al2O3 porous ceramic prepared by addition of hollow spheres, J. Porous Mater. 21 (2014) 601–609. [15] X.F. Tao, L.P. Zhang, Y.Y. Zhao, Al matrix syntactic foam fabricated with bimodal ceramic microspheres, Mater. Des. 30 (2009) 2732–2736. [16] H. Xu, J. Liu, H. Du, A. Guo, Z. Hou, Preparation of porous silica ceramics with relatively high strength by a TBA-based gel-casting method, Chem. Eng. J. 183 (2012) 504–509. [17] The Glass powder. 〈http://zhuhaixuanyang.glass.cn/〉. [18] I. Ganesh, D.C. Jana, S. Shaik, N. Thiyagarajan, An aqueous gelcasting process for sintered silicon carbide ceramics, J. Am. Ceram. Soc. 89 (2006) 3056–3064. [19] Z.L. Lu, J.W. Cao, S.Z. Bai, M.Y. Wang, D.C. Li, Microstructure and mechanical properties of TiAl-based composites prepared by stereolithography and gelcasting technologies, J. Alloy. Compd. 633 (2015) 280–287. [20] M. Koopman, K.K. Chawla, K.B. Carlisle, G.M. Gladysz, Microstructural failure modes in three-phase glass syntactic foams, J. Mater. Sci. 41 (2006) 4009–4014. [21] R. Landauer, The electrical resistance of binary metallic mixtures, J. Appl. Phys. 23 (1952) 779–784. [22] N. Tessier-Doyen, X. Grenier, M. Huger, D.S. Smith, D. Fournier, J.P. Roger, Thermal conductivity of alumina inclusion/glass matrix composite materials: local and macroscopic scales, J. Eur. Ceram. Soc. 27 (2007) 2635–2640. [23] Z. Živcová, E. Gregorová, W. Pabst, D.S. Smith, A. Michot, C. Poulier, Thermal conductivity of porous alumina ceramics prepared using starch as a poreforming agent, J. Eur. Ceram. Soc. 29 (2009) 347–353. [24] J. Bourret, N. Tessier-Doyen, B. Naït-Ali, F. Pennec, A. Alzina, C.S. Peyratout, D. S. Smith, Effect of the pore volume fraction on the thermal conductivity and mechanical properties of kaolin-based foams, J. Eur. Ceram. Soc. 33 (2013) 1487–1495. [25] B. Naït-Ali, K. Haberko, H. Vesteghem, J. Absi, D.S. Smith, Thermal conductivity of highly porous zirconia, J. Eur. Ceram. Soc. 26 (2006) 3567–3574. [26] B. Nait-Ali, C. Danglade, D.S. Smith, K. Haberko, Effect of humidity on the

[27]

[28] [29]

[30]

[31] [32] [33]

[34]

[35]

[36] [37]

thermal conductivity of porous zirconia ceramics, J. Eur. Ceram. Soc. 33 (2013) 2565–2571. B. Nait-Ali, K. Haberko, H. Vesteghem, J. Absi, D.S. Smith, Preparation and thermal conductivity characterization of highly porous ceramics Comparison between experimental results, analytical calculations and numerical simulations, J. Eur. Ceram. Soc. 27 (2007) 1345–1350. L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, Cambridge University Press, Cambridge, 1997. E. Bernardo, G. Scarinci, A. Maddalena, S. Hreglich, Development and mechanical properties of metal-particulate glass matrix composites from recycled glasses, Composites Part A 35 (2004) 17–22. D.U. Tulyaganov, H.R. Fernandes, S. Agathopoulos, J.M.F. Ferreira, Preparation and characterization of high compressive strength foams from sheet glass, J. Porous Mater. 13 (2006) 133–139. E. Bernardo, R. Cedro, M. Florean, S. Hreglich, Reutilization and stabilization of wastes by the production of glass foams, Ceram. Int. 33 (2007) 963–968. E. Bernardo, F. Albertini, Glass foams from dismantled cathode ray tubes, Ceram. Int. 32 (2006) 603–608. S. Meille, M. Lombardi, J. Chevalier, L. Montanaro, Mechanical properties of porous ceramics in compression: on the transition between elastic, brittle, and cellular behavior, J. Eur. Ceram. Soc. 32 (2012) 3959–3967. Y. Yamada, K. Shimojima, M. Mabuchi, M. Nakamura, T. Asahina, T. Mukai, K. Higashi, Effects of load direction on the mechanical properties of opencellular epoxy with a cubic prism structure, Philos. Mag. Lett. 80 (2000) 215–220. N. Gupta, R. Ye, M. Porfiri, Comparison of tensile and compressive characteristics of vinyl ester/glass microballoon syntactic foams, Composites Part B 41 (2010) 236–245. W. Pabst, E. Gregorová, Mooney-type relation for the porosity dependence of the effective tensile modulus of ceramics, J. Mater. Sci. 39 (2004) 3213–3215. K.M. Prewo, J.J. Brennan, Silicon carbide yarn reinforced glass matrix composite, J. Mater. Sci. 17 (1982) 1201–1206.