Calcium aluminate cement tapes – Part I: Structural and microstructural characterizations

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 1017–1023

Calcium aluminate cement tapes – Part I: Structural and microstructural characterizations Y. El Hafiane a,∗ , A. Smith b,∗ , Y. Abouliatim a , T. Chartier c , L. Nibou a , J.-P. Bonnet b a

LMPEQ, Ecole Nationale des Sciences Appliqués, Route Sidi Bouzid, BP 63 Safi, 46000 Safi, Morocco GEMH (EA 3178), Ecole Nationale Supérieure de Céramique Industrielle, 12 rue Atlantis, 87068 Limoges Cedex, France c SPCTS (UMR 6638), Ecole Nationale Supérieure de Céramique Industrielle, 12 rue Atlantis, 87068 Limoges Cedex, France b

Available online 16 November 2013

Abstract This present paper concerns the structural and microstructural characterization of 1 mm thick calcium aluminate cement tapes prepared by tape casting. A study of the effect of environment, time and the consolidation temperature on the structural properties and microstructure has been undertaken. Consolidation environments studied are air, water and an environment saturated in humidity for storage durations of 1, 4 or 30 days at a temperature of 20 or 70 ◦ C. The structural characterization was carried out using XRD. The microstructural characterization was carried out through the distribution of pore volume by mercury porosimetry measurements and SEM micrographs. The effects of the various consolidation parameters were compared and discussed. The tape consolidated for 30 days in water at 70 ◦ C has the most advanced hydration with the formation of stable hydrates. This is evidenced by the reduction of the inter-granular pore size and the microstructure densification. © 2013 Elsevier Ltd. All rights reserved. Keywords: Aluminous cement; Hydration; Tape casting; Films; Structural and microstructural characterization

1. Introduction Tape casting is a process used in the manufacture of ceramic tapes from a ceramic slurry.1–3 The tapes produced can be used for a variety of purposes, including the manufacture of electronic elements such as ceramic capacitor, substrate and dielectric, piezoelectric and ferromagnetic materials.4–7 Suspensions of tape casting are usually developed in an organic environment (organic solvent).8,9 More and more work on aqueous suspensions has been developped.10–12 Our work concerns the use of tape casting to elaborate tapes, based on calcium aluminate cement (CAC), with a controlled thickness (from 100 ␮m to 2 mm).13–15 CAC are known for their high resistance to chemical attacks and for their refractory character.16 The originality of this work relies on the following facts: (i) the cast tape is consolidated at room temperature due to the hydration process of the anhydrous cement; (ii) the elimination of organic additives is carried out under the effect of osmotic pressure during setting of the samples, especially in

∗

Corresponding authors. Tel.: +33 5 87 50 23 00; fax: +33 5 87 50 23 01. E-mail addresses: [email protected] (Y. El Hafiane), [email protected], [email protected] (A. Smith). 0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.015

water to favor hydration.13 After a study about the choice of constituents, development of casting formula, adequacy of the rheological behavior and determination of optimal consolidation conditions,13 we have characterized the tapes. In part I, we present the structural and microstructural characteristics of the developed materials. In particular, we discuss how these properties vary according to the setting environment (water, air at 50% relative humidity or air saturated in humidity), setting time (1 day, 4 days or 30 days) and setting temperature (20 ◦ C or 70 ◦ C in the case of setting in water).

2. Materials and methods 2.1. Raw materials The tape casting suspension was elaborated from powder, water and organic additives. The used powder was a calcium aluminate cement (Secar 71, Lafarge, France) with chemical and mineralogical composition given in Table 1. A protocol for choosing and preparing the powder was established after the study of the cement freshness state and the initial particles sizes.13,15 We used batches of fresh cement. Once the cement bags were open, they were maintained in a dry atmosphere at

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Table 1 Chemical and mineralogical composition of the aluminous cement, Secar 71.

2.2. Tape casting

Constituent

Weight percentages %

Mineralogical phases CA CA2 C12 A7 A

56 38 <1 6

Oxide C A

28 72

In a previous paper,13 we developed a composition and a preparation protocol of suspensions for the elaboration of thick tapes by the tape casting process. The composition is given in Table 2. In Fig. 1, we summarize the process for preparing the suspension until the consolidation in various conditions. The preparation of the suspension starts by mixing the powder and the dispersant (Bevaloid 35L and HOAc) already dissolved in the water. Then the plasticizer (PEG 300) is introduced in a second time to avoid overlapping effects between the organic additives. After mixing until homogenization, the suspension is deaerated before casting the tapes. Homogenization is made in a laboratory blender with a vertical paddle and having an eccentric movement (Kenwood mixer). A tape of a thickness of e = 1 mm was cast with a velocity ˙ = 20 s−1 . V = 1.2 m min−1 corresponding to a shear rate, γ(V/e) The suspension apparent viscosity, during casting, is about 1 Pa s. The consolidation environments studied are water (noted W), air at 50% relative humidity (noted A) and air saturated in

20 ◦ C. The powder was sieved below 40 ␮m. The organic additives used in the preparation of the suspension are given in Table 2. Polyethylene glycol (PEG) 300 (Prolabo, France) was used as plasticizer, Bevaloid 35L (Rhodia, UK) was used as a dispersant and acetic acid (Prolabo, France) was used both as an accelerator and a dispersant.

Fig. 1. Preparation process of suspensions.

Y. El Hafiane et al. / Journal of the European Ceramic Society 34 (2014) 1017–1023

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Table 2 Functions and physicochemical properties of organic additives used in composition of calcium aluminate cement suspension for the tape casting process. Product

Type of product

Function

Molecular weight (g mol−1 )

Density

Weight ratio (×100)a

Secar 71 Demineralized water PEG 300 Acetic acid 90%

Cement Water Polyethylene glycol Carboxylic acid

– 18.016 300 60.05

2.9–3.05 1 1.13 1.05

1 33 4 2

Bevaloid 35L

Sodium salt of a condensed naphthalene sulphonate

Powder Solvent/binder Plasticizer Dispersant/accelerator of setting Dispersant

a

–

1.07–1.11

0.1

Each weight ratio is related to the mass of cement.

Table 3 Presentation of all tapes prepared from the composition developed and the different characterizations performed. Setting environment

Setting temperature

Setting time

Sample notation

Characterizations

Water

20 ◦ C

1 day

W (1, 20)

XRD

Air (50% RH) Humidity saturation Water

20 ◦ C

20 ◦ C 20 ◦ C

4 days 4 days 4 days

A (4, 20) S (4, 20) W (4, 20)

XRD, Merc. Poro., SEM XRD, Merc. Poro., SEM XRD, Merc. Poro., SEM

Air (50% RH) Humidity saturation Water

20 ◦ C 20 ◦ C 20 ◦ C

30 days 30 days 30 days

A (30, 20) S (30, 20) W (30, 20)

Merc. Poro., SEM Merc. Poro., SEM XRD, Merc. Poro., SEM

Water

70 ◦ C

1 day

W (1, 70)

XRD

Water

70 ◦ C

4 days

W (4, 70)

XRD

humidity, it corresponds to desiccators with the bottom filled with water (noted S). The experiments were carried at 20 ◦ C and in the particular case of water at 70 ◦ C. It is important to note that if the samples are stored in air partially or totally saturated in humidity directly after casting, the samples stored in water are first left 2 h in air (50% relative humidity) at 20 ◦ C and then immersed into water. This period of 2 h allows the tape to be sufficiently consolidated and thus prevents deterioration of the upper face in contact with water. The consolidation time reported in this paper represent the time between casting and characterization of samples. The samples are marked with a letter which indicates the consolidation environment (A: air, S: saturated humidity in a desiccator and W: water) followed by a pair of two digits, the first corresponds to the duration of consolidation (days: 1, 4 and 30) and the second to the temperature of the environment (20 ◦ C or 70 ◦ C). Finally, characterizations performed are: X-Ray Diffraction (XRD), Mercury Porosimetry (Merc. Poro.) and Scanning Electron Microscopy (SEM). The different samples are presented in Table 3.

2.3. Characterizations X-rays diffraction (XRD) was carried out with an INEL CPS 120-Curved Position Sensitive diffractometer apparatus, ˚ and a curved quartz using K␣1 radiation of Cu (λ = 1.5406 A) monochromator under a tension of 37.5 kV and an intensity of 28 mA. The samples are irradiated under a constant incidence angle and the diffracted beams are collected by the curved position sensitive detector.17 For each sample, in powder form, the acquisition time was equal to 45 min. Crystalline phases were

Table 4 JCPDS charts used for the identification of the crystalline phases. Phases

CA

CA2

CAH10

C2 AH8

C3 AH6

AH3

JCPDS

23-1036

23-1037

12-0408

11-0205

24-0217

29-0041

identified by comparison with Powder Diffraction Files (PDF) standards from the International Center for Diffraction Data (ICDD). JCPDS (Joint Committee on Powder Diffraction Standards) files were used for the identification of crystalline phases (Table 4). The evolution of the microstructure according to the environment, time and the consolidation temperature was characterized by mercury porosimetry.18 The pore radius, rp (which is assumed to be cylindrical) is given by Washburn’s equation (1): rp =

−4ν cos θ PHg

(1)

where PHg is the mercury pressure, ν (equal to 0.485 N m−1 ) is the surface tension of mercury, θ (equal to 140◦ ) is the contact angle between mercury and solid phase, and rp the pore radius where the pores are assumed to be cylindrical. The mercury intrusion curves represent the cumulative volume of mercury. It is also possible to represent the pore volume distribution as a function of pore radius (dVopen pores /drp = f(rp ) where dVopen pores is the intrusion differential volume). The apparatus used in the present work (Autopore II Micromeritics 9200) allows the analysis of pores ranging from 0.003 to 630 ␮m. The measurement principle consists to monitor the influence of the pressure on the amount of mercury

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Y. El Hafiane et al. / Journal of the European Ceramic Society 34 (2014) 1017–1023 Table 5 Intensities of the most intensive diffraction peaks for the crystalline phases in samples stored in the different consolidation environment; data corresponding to Fig. 2. Sample

CA

CA2

CAH10

C2 AH8

A (4, 20) S (4, 20) W (4, 20)

100 100 100

39 71 94

– – 43

– 6 14

Fig. 2. XRD spectra of cement tape stored 4 days in different environment: air A (4, 20), saturated humidity S (4, 20) and water W (4, 20).

which penetrates the porous material. The volume of open pores is equal to the volume of mercury penetration. The maximum pressure in the test is 413 MPa. The samples were subjected to a preliminary degassing pressure of 50 mmHg for 10 min. A scanning electron microscope (HITACHI SC 2500) with a secondary electron detector was used to follow the evolution of the microstructure of the studied samples. Before observation, the samples were coated with a gold layer in a vacuum chamber in order to ensure electronic conduction of samples when subjected to electron beam bombardment. The accelerating voltage can vary between 10 and 30 kV and the magnifications range between 200 and 30 000. 3. Results and discussion paper13

20 ◦ C

We have shown in a previous that water at is an interesting consolidation environment because it allows better hydration than in the two other media and permits elimination of PEG 300 and Bevaloid 35L. HOAc which has not been involved in the formation of calcium acetate hydrates is also washed out. We will consider in this work the influence of the setting conditions on the structure and microstructure of our tapes. 3.1. Structural characterization First of all, we focus on the nature of formed phases according to the environment, time and temperature of consolidation. 3.1.1. Effect of consolidation environment Fig. 2 shows the crystalline phases detected by XRD, in samples A (4, 20), S (4, 20) and W (4, 20). Table 5 shows the percentage of the most intensive peaks for the various crystallized phases detected by XRD. Whatever the consolidation environment, we find that the major phases are the anhydrous compounds CA and CA2 . In the case of sample stored in air A (4, 20) no hydrated crystallized phase was detected. For sample S (4, 20), C2 AH8 is present in a small amount. For W (4, 20) we record the presence of two hydrates, namely CAH10 and C2 AH8 . The intensity of the peak

Fig. 3. XRD spectra of cement tape stored in water at 20 ◦ C for increasing time consolidation: 1 day W (1, 20), 4 days W (4, 20) and 30 days W (30, 20). Table 6 Intensities of the most intensive diffraction peaks for the crystalline phases in samples stored in water for different consolidation time; data corresponding to Fig. 3. Sample

CA

CA2

CAH10

C2 AH8

W (1, 20) W (4, 20) W (30, 20)

100 100 100

91 94 78

29 43 37

– 14 19

corresponding to C2 AH8 is more important than its intensity recorded on S (4, 20). 3.1.2. Effect of consolidation time Fig. 3 shows the effect of time on the intensity of the peaks corresponding to the crystalline phases formed in samples stored in water at 20 ◦ C for 1, 4 or 30 days. The percentage of the most intensive peaks for the various crystallized phases detected by XRD is given in Table 6. For the three times, the major phases are anhydrous CA and CA2 . The crystallized hydrate CAH10 is present in the sample having one day consolidation. After 4 days or more (30 days) aging, we record the presence of CAH10 and C2 AH8 . The percentage of the most intensive peaks for CAH10 hydrate in W (4, 20) is greater than that in W (1, 20). It decreases substantially in the case of W (30, 20). Regarding C2 AH8 , no intensity is recorded after 1 day of consolidation and it increases significantly with time (Table 6). 3.1.3. Effect of consolidation at 70 ◦ C Fig. 4 shows the nature of crystallized phases detected by XRD, in samples W (1, 70) and W (4, 70) consolidated in water

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1021

a

Fig. 4. Formation of phases during hydration of tapes in water at 70 ◦ C depending on the consolidation time (1 day and 4 days).

b

Table 7 Intensities of the most intensive diffraction peaks for the crystalline phases in samples stored in water at 70 ◦ C for different consolidation time; data corresponding to Fig. 4. Sample

CA

CA2

C3 AH6

AH3

W (1, 70) W (4, 70)

13 11

17 16

100 100

73 76

at 70 ◦ C. The percentage of the most intensive peaks for the various crystallized phases detected by XRD is given in Table 7. For both times, the structural data are relatively similar. The importance of C3 AH6 and AH3 hydrates is marked. If we compare these results with those obtained on samples stored in water at 20 ◦ C (Table 6), we find that the percentage of the most intensive peaks for hydrated phases is high in this case. Consolidation in water at 70 ◦ C leads quickly to the formation of stable hydrates and also to a hydration rate significantly higher than in water at 20 ◦ C. 3.2. Microstructural characterization We examined the influence of consolidation environment at 20 ◦ C on the microstructure of tapes with 4 days or 1 month consolidation. Fig. 5 shows the porous volume distribution as a

Fig. 5. Pore size distribution for cement tape having: (a) 4 days and (b) 30 days consolidation in different environment (air, saturated humidity and water).

function of pore size after 4 days (Fig. 5a) and 1 month aging (Fig. 5b). In A (4, 20), the porous volume presents an important population between 0.15 and 2 ␮m and a dome around 0.03–0.1 ␮m. In S (4, 20) and W (4, 20) the population is within the 0.1–0.2 ␮m range. Concerning the submicronics population, its position is substantially the same whatever the consolidation environment is. The existence of two populations indicates the presence of two types of pores. The first one corresponds to the capillary pores which are the remains of the inter-granular spaces in the cement

Fig. 6. SEM micrographs of a fractured tape after 4 days consolidation in different environment: (a) air A (4, 20), (b) saturated humidity S (4, 20), and (c) water E (4, 20).

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Fig. 7. SEM micrographs of a fractured tape after 30 days consolidation in different environment: (a) air A (4, 20), (b) saturated humidity S (4, 20), and (c) water E (4, 20).

paste.19 The second pores, which have submicronic diameters, have a size that is little affected by the consolidation environment. They may be associated with porosity within crystallized or amorphous hydrates.18,19 The importance of capillary porosity in the samples which are stored in air may be related to the fact that the level of hydration is lower and the inter-granular spaces remain empty unlike the two other environments where hydrated phases are formed after 4 days consolidation.15 These phases, which develop from anhydrous cement and water, can fill the largest pores and lead to an overall decrease of pore size. SEM micrographs of these samples (Fig. 6) show the “filling” of empty space in these samples. In Fig. 6a, we distinguish micron-sized pores that are less visible as soon as the consolidation occurred in the saturated atmosphere (Fig. 6b) or in the water (Fig. 6c). After a month consolidation, we find that the diameter of the first population of pores has moved to smaller dimensions for both consolidation environments S (30, 20) and W (30, 20). This is due to the continued filling of capillaries by hydrates. Concerning A (30, 20), the position and the peak amplitude

relative to the capillaries did not practically change since the hydration has not progressed. SEM micrographs of these samples (Fig. 7) show the evolution of the microstructure between A (30, 20) (Fig. 7a) and W (30, 20) (Fig. 7b and c). Whereas for the samples stored in air, A (30, 20), there is an agglomeration of the anhydrous grains consisting essentially of crystalline phases CA and CA2 , the sample stored in water W (30, 20) present a microstructure with large crystallites of hydrates. These crystals can be attributed to the phases CAH10 and C2 AH8 , detected by XRD, that can develop when consolidation takes place in water (Fig. 3). 4. Conclusion Depending on the processing conditions of calcium aluminate cement tapes, we can get a range of structural and microstructural characteristics: - With consolidation in air, no crystalline hydrates have been detected.

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- Hydration for 4 days at 20 ◦ C is more advanced in water than in saturated humidity. - Tapes consolidation for 30 days at 20 ◦ C in water leads to an important hydration. - A consolidation in water at 70 ◦ C leads to the formation of C3 AH6 stable hydrates. - The analysis of microstructure shows two pore sizes. The first one ranges between 0.03 and 0.1 ␮m and does not seem to be affected by the conditions of consolidation. This population may be related to the porosity between the hydrates. The second population is due to the inter-granular porosity and depends on the consolidation conditions. It is very important in the case of consolidation in air and tends to move toward smaller sizes when consolidation is carried out in water or saturated humidity environment. Acknowledgments The authors thank Lafarge for supplying the raw materials. References 1. Dhanesh T, Pulanchiyodan A, Mailadil TS. Casting and characterization of LiMgPO4 glass free LTCC tape for microwave applications. J Eur Ceram Soc 2013;33(1):87–93. 2. Boch P, Chartier T, Huttepain M. Tape casting of Al2 O3 laminated composites. J Am Ceram Soc 1986;69(8):191–2. 3. Blugan G, Morawa K, Koebel S, Graule T, Kuebler J. Development of a tape casting process for making thin layers of Si3 N4 and Si3 N4 +TiN. J Eur Ceram Soc 2007;27(16):4789–95. 4. Mistler RE. Tape casting: the basic process for meeting the needs of electronic industry. Am Ceram Soc Bull 1990;65(6):1022–6. 5. Chartier T. Tape casting. In: Bloor D, Brook RJ, Flemings MC, Mahajan S, Cahn RW, editors. The encyclopedia of advanced materials. Cambridge: Pergamon; 1994. p. 2763–7.

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