Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions

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JECS-10965; No. of Pages 10

Journal of the European Ceramic Society xxx (2016) xxx–xxx

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Surface modification of ␣-Al2 O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions Shuai Zhang a , Na Sha b,∗ , Zhe Zhao a,c,∗ a Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China b School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai, 201418, China c School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai, 201418, China

a r t i c l e

i n f o

Article history: Received 26 October 2016 Received in revised form 7 December 2016 Accepted 8 December 2016 Available online xxx Keywords: Ceramic stereolithography Alumina Surface modification Dicarboxylic acids Rheology

a b s t r a c t Stereolithography of UV-curable ceramic suspensions can benefit from the preparation of stable, low viscosity and high solid loading ceramic suspensions without yield stress. Appropriately adding dispersants could optimize the rheological behavior to meet the requirements of stereolithography. In this work, short-chain dicarboxylic acids were utilized to modify the alumina particles and achieve well dispersed ceramic suspensions. The maximum adsorption capacities of dicarboxylic acids were determined by the method of High Performance Liquid Chromatography and the mechanism of surface modification and dispersion was also discussed. Dicarboxylic acids’ influence on the rheology behavior was systematically studied. When doses of dicarboxylic acids reach their maximum adsorption capacities, the alumina suspensions would achieve their lowest viscosities and yield stresses. 45 vol% alumina suspension with a viscosity <2 Pa s at shear rate 30 s−1 was successfully formulated. A sintering density of 96.5% can be achieved for the sebacic acid-modified alumina UV-curable suspension. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Stereolithography (SL) is the most promising freeform rapid prototyping (RP) technique to fabricate the ceramic parts with elaborate 3D shapes layer by layer which can be attributed to its better precision in size and site control than other RP methods [1–3]. The principle of stereolithography is vividly depicted by Fig. 1. UV-curable ceramic suspensions applied to traditional stereolithography apparatus (SLA) with a scraper (see Fig. 1(a)) should have a viscosity <5 Pa s (30 s−1 ) [3]. Then, with the scraper utilized to spread ceramic suspensions, the suspensions with appropriate rheological behavior, especially shear-thinning, is preferable [4,5]. However, for the SLA without a scraper, the ceramic suspensions must own a much lower viscosity without an obvious yield stress to obtain an excellent capability of self-leveling [1]. And a better

∗ Corresponding authors can be contacted at: Na Sha, School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai, 201418, China. Zhe Zhao, Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China. E-mail address: [email protected] (Z. Zhao).

adhesion between cured green parts and build platform is critical and crucial for the improved apparatus (see Fig. 1(b)) [6]. These problems can be successfully solved by careful selection of monomers and dispersants. Previous studies concerning UV-curable ceramic suspensions in aqueous and organic disperse media have been systematically implemented and their utilization for stereolithography is widely known [1–5,7–13]. With the wide choice of monomers in organic systems, it is available to adjust the viscosity, curing rate, refractive index, degree of crosslinking and adhesion via elaborate selection of monomers for the successful stereolithography process. To lower the sintering temperature and achieve high precision and surface quality, submicron and nanometer ceramic powders are favorable to be utilized [2]. Furthermore, in high solid loading ceramic suspensions, the scattering phenomenon is at the mercy of interparticle space and the smaller particle size could reduce the scattering and achieve a thicker cure depth, which is decided by the rayleigh scattering for dilute suspensions [1,12]. However, there exist many hydroxy groups on these small ceramic particles’ surface (such as alumina, zirconia and silica etc.) which make alumina particles hydrophilic. Consequently, few ceramic particles could be successfully and efficiently dispersed

http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.013 0955-2219/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: S. Zhang, et al., Surface modification of ␣-Al2 O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.013

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Fig. 1. The principle of stereolithography apparatus (SLA) rapid prototyping: (a) Traditional SLA (b) Improved SLA.

into nonpolar organic UV-curable resins, appropriate dispersants are frequently required to decrease the viscosity, improve stability and then increase solid loading to fulfill the critical requirement for ceramic sintering and densification (solid loading ≥ 40 vol%). Previous studies about dispersant selection in UV-curable systems were always focused on the commercial polymeric dispersants while micromolecule surfactants with relative short-chain have been scarcely investigated [1–5,7–13]. Furthermore, most of the existing formulations of submicron alumina suspensions (40 vol%–50 vol%) own a higher viscosity about 3–5 Pa s (at 30 s−1 ) which can’t be successfully used in the improved common stereolithography apparatus like Fig. 1(b). Adake etc [7]. have formulated 40 vol% alumina suspensions using stearic acid (nc = 18)and oleic acid(nc = 18)as dispersants with much higher viscosities of 15.4 Pa s and 5.03 Pa s (at 31.6 s−1 ) which could be attributed to the low reactivity of carboxyl groups on long-chain fatty acids. In this work, dicarboxylic acids with short carbon chain length (3 ≤ nc ≤10) were selected as dispersants to formulate alumina UVcurable suspensions because they are a kind of common, cheap and easily available surfactants with excellent surface modification effect. The feasibility of dicarboxylic acids acting as effective dispersants applied to non-polar organic system was further verified. Dicarboxylic acids’ modification mechanism and dispersion mechanism were both discussed. The stability of the suspensions dispersed in 1,6-hexanediol diacrylate (HDDA) using different carbon chain length dicarboxylic acids was determined by sedimentation test. Rheology behavior was systematically investigated by changing dispersants’ dose and solid loading. Highly dense sintered alumina samples were successfully fabricated by utilizing 45 vol% solid loading suspension after stereolithography, debinding and sintering.

2. Experimental procedure 2.1. Materials Commercially available Al2 O3 powder (purity ≥ 99.99%), Sumitomo AA04, was purchased from Sumitomo Chemical Co., Ltd. The submicron particles of AA04, with average particle size of 400 nm and specific surface area of 4.1 m2 /g, have spherical shape (Fig. 2(a)). Three common monomers with different degrees of functionality were selected as dispersion media for the Al2 O3 powder to prepare UV-curable ceramic suspensions. All the monomers were obtained from Aladdin Industrial Corporation. The stereolithography of ceramic suspension was performed on the common improved stereolithography apparatus with a 405 nm laser source. Consequently, (2,4,6-Trimethylbenzoyl) diphenylphosphine oxide (TPO, Adamas Reagent Co., Ltd.) was chosen as the photoinitiator. The photoinitiator TPO was obtained from Adamas Reagent Co., Ltd. The properties and molecular structures of monomers and photoinitiator are listed in Table 1. In this paper, dicarboxylic acids with different carbon chain length (3 ≤ nc ≤ 10) were added to serve as dispersants. Propandioic acid, succinic acid, glutaric acid, adipic acid, heptanedioic acid, suberic acid, azelaic acid and sebacic acid, all eight dicarboxylic acids (their molecular structures are shown in Table 2), with mass purity ≥ 99%, were purchased from Aladdin Industrial Corporation. HPLC grade methanol and phosphoric acid were obtained from Adamas Reagent Co., Ltd. 2.2. Determination of adsorption quantity To determine the adsorption quantities of acids on the surface of alumina, 10 g Al2 O3 powder were dried for 8 h at 200 ◦ C, then immersed into 50 ml ethanol with different concentrations

Fig. 2. SEM image of (a) alumina as received and (b) modified alumina by succinic acid.

Please cite this article in press as: S. Zhang, et al., Surface modification of ␣-Al2 O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.013

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Table 1 The properties of monomers and photoinitiator in this paper. Refractive index

Density (g/cm3 ) (20 ◦ C)

Functionality

2-Hydroxyethyl acrylate (2-HEA)

1.445

1.011

1

1,6-Hexanediol diacrylate (HDDA)

1.456

1.010

2

Trimethylolpropane triacrylate (TMPTA)

1.474

1.089

3

(2,4,6-Trimethylbenzoyl) diphenylphosphine oxide (TPO)

–

1.17

–

Monomers or photoinitiator

Molecular structure

of dicarboxylic acids and mixed by roller mixer for 48 h at room temperature. The different concentrations were utilized to obtain a systematic overview of adsorption behavior for these acids with different carbon chain length and 6 mg/ml was set to the maximum concentration for all the acids. Finally, the solution samples were obtained after the centrifugation of ceramic slurries and the adsorption quantities can be calculated through determination of acids concentrations remaining in these samples via the method of High Performance Liquid Chromatography (HPLC) (UltiMate3000, Thermo Fisher, USA). To determine the remaining acids concentrations, the chromatographic conditions were optimized. The methanol and phosphoric acid solution were selected as the mobile phase. The injection volume was 10 ␮l and the flow rate was set to 1.0 ml/min. The column (Hypersil GOLD Dim.250 × 4.6 mm) was maintained at 30 ◦ C and the detection was carried out at a wavelength of 210 nm. The phosphoric acid solution’s pH was adjusted to 2.1 to inhibit the ionization of dicarboxylic acids, and the mobile phase ratio was optimized (see Table 2) to adjust the time of peak appearance and

gain symmetrical peaks. All the liquid supernatant samples should be diluted by the mobile phase. 2.3. Preparation of UV-curable suspensions The preparation method of UV-curable suspension utilized in this work just contains several simple steps. For the unloaded UVcurable mixtures, 0.5 wt% (with respect to the mass of monomers) photoinitiator TPO was added to the monomers. Through ultrasonic treatment for 3 min to dissolve the photoinitiator, the mixture was under stirring at room temperature and in the dark during 4 h. The Al2 O3 powder, after dried for 8 h at 200 ◦ C, was added to ethanol with different concentrations of dicarboxylic acids (with respect to the mass of alumina powder) dissolved and mixed by roller mixer for 48 h as mentioned before. Then the suspension was dried at 60 ◦ C for 24 h to remove ethanol. Before re-disperse in the monomers, the dry powder with adsorbed acids was ground with mortar and pestle. As shown in Fig. 2, the modified alumina particles reflect no obvious difference from the unmodified

Table 2 The optimized chromatographic conditions to determine the concentrations of all dicarboxylic acids and their molecular structures. Dicarboxylic acids

Molecular structure

Mobile phase ratios: methanol/phosphoric acid solution (pH 2.1)

Propandioic acid

2:98

Succinic acid

45:55

Glutaric acid

30:70

Adipic acid

30:70

Heptanedioic acid

60:40

Suberic acid

70:30

Azelaic acid

70:30

Sebacic acid

70:30

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Fig. 3. The sedimentation of UV-curable ceramic suspension.

alumina in shape and dimension. Finally, UV-curable suspensions were obtained by ball milling at 300 r/min for 60 min to break down the possible agglomerates and achieve a good homogeneity. The Fourier transform infrared (FTIR) spectra of oven-dried alumina powder modified by dicarboxylic acids were obtained by using a FTIR spectroscope (FTIR, TENSOR27, Germany). 2.4. Characterization of the UV-curable suspensions Sedimentation test was conducted in this work to evaluate stability of UV-curable ceramic suspensions. Fig. 3 is the schematic diagram for sedimentation test. To evaluate the dicarboxylic acids’ role in the suspension, alumina modified by 0.3 wt% dicarboxylic acids (nc = 3, 7, 10) was dispersed into HDDA (containing 0.5 wt% TPO) to prepare 15 vol% alumina suspensions by the method described in Section 2.3. All the suspension samples were poured into measuring cylinder with a maximum height value of 10 cm. The measuring cylinder with suspension was kept in dark place for several days. Then, the sedimentation curves were plotted by the sedimentation distances (H-h) in Fig. 3 in a series of given amounts of time for the alumina suspensions. Rheological behavior of all suspensions was performed on rotational rheometer (Kinexus pro+, Malvern, England). The shear stress and viscosity as a function of shear rate from 1 s−1 to 100 s−1 , viscosity at shear rate 30 s−1 , yield stress and thixotropy were measured. 2.5. Stereolithography of high solid loading ceramic suspensions and sintering of the green body To prepare high solid loading suspensions, three common monomers, HDDA, HEA and TMPTA, were utilized with a volume ratio of 6:3:1 which is an optimum ratio. According to the method described at Section 2.3, the monomers mixture with 0.5 wt% TPO dissolved and alumina modified by 0.4 wt% sebacic acid were formulated into 45 vol% solid loading suspension for stereolithography. The debinding of green body was performed with a heating rate of 5 ◦ C/min up to 150 ◦ C, then of 0.5 ◦ C/min up to 600 ◦ C with a plateau of 2 h to avoid deformation or cracking and reduce the porosity. The green body were finally sintered with a heating rate of 5 ◦ C/min up to 1550 ◦ C with a plateau of 2 h. The microstructure of sintered samples were observed by scanning electron microscopy (SEM) (Nanosem 430, FEI, USA) and the density was measured by immersion method with a high precision analytical balance.

Fig. 4. Chromatograms for dicarboxylic acids with different carbon chain length.

the chromatograms for dicarboxylic acids with different carbon chain length. The peaks of these acids presented very good symmetry and the data can be repeated with good reliability. The standard curves of dicarboxylic acids were constructed by measuring peak areas of corresponding concentrations. Good linearities were obtained in the concentration range with correlation coefficients between 0.9998 and 0.9999. The concentrations of supernatant samples were determined via measurement of the areas of peaks, then the absorption quantities were obtained. Fig. 5(a) shows quantities of three acids with different carbon chain length (nc = 3, 6 and 9) adsorbed on the alumina surface as a function of equilibrium concentrations of acids. The adsorption curves can be depicted by Langmuir’s isotherm [14,15]:  = m KCeq /(1+KCeq )

(1)

 is the adsorption quantity, m is the maximum adsorption capacity, K is a constant and Ceq is the equilibrium concentration. The adsorption isotherm indicates the possible mechanism by which the acids absorbed to the surface of alumina particles. Langmuir’s isotherm assumes that there is a finite number of adsorption sites with no lateral interactions between the adsorbates. The adsorption process is homogeneous, which means all the sites have the same affinity for the adsorbates and only one adsorbate can be adsorbed on each site [14,15]. The maximum adsorption capacities (m ) were achieved by fitting of the experimental data with Langmuir’s isotherm. Fig. 5(b) shows the maximum adsorption capacities of eight dicarboxylic acids with different carbon chain length (3≤ nc ≤10). With the carbon chain length increase, m appears a downward trend. The saturated adsorption of these acids depend on the several factors, functioning of carbon chain length, pKa values and possible structures formed on the alumina surface. With the carbon chain length increase, pKa values of carboxyl groups would increase, therefore reduce the charge of acids in solution and lead to lower adsorption of acids due to electrostatic interaction with the alumina surface[16]. 3.2. Fourier transform infrared (FTIR) spectroscopy

3. Results and discussions 3.1. Adsorption quantity With the optimized chromatographic conditions, chromatograms of all the dispersants were achieved. Fig. 4 shows

The alumina samples (with 0.1 wt, 0.3 wt% and 0.5 wt% glutaric acid) after adsorption were washed by ethanol in centrifugal machine at 8000 rpm for three times to remove the remaining unabsorbed dicarboxylic acids. In comparison with the unmodified sample (see Fig. 6), the dicarboxylic acids are absorbed onto

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Fig. 5. (a) Adsorption quantities corresponding to equilibrium concentrations (b) m corresponding to different carbon chain length.

Fig. 6. FTIR spectroscopy of modified alumina samples with different concentrations of glutaric acid.

The characteristic vibrational peaks of −COO− are observed at 1420 cm−1 and 1580 cm−1 which confirm formation of carboxylate complex. The stronger symmetric stretching vibration ␯sym (COO-) appears at 1580 cm−1 , the weaker asymmetric stretching vibration asym (COO-) is located at 1420 cm−1 , and also these characteristic peaks replace strong −COOH vibrational peak at about 1710 cm−1 . The remaining little peaks at about 1710 cm−1 could be attribute to the formation of nonbridging structures (Fig. 7a, c and e). The free carboxyl groups in the nonbridging structures could be connected with the alumina surface via hydrogen bonding. With the glutaric acid’s dose increasing until its maximum adsorption capacity (about 0.5 wt%), the sym (COO-) becomes stronger while the little peaks at 1710 cm−1 show no obvious difference, which indicates that the bridging linkage structures have been reinforced. The strong polar free carboxyl groups have poor affinity with weak polar monomers. Consequently, the bridging linkage structures (Fig. 7b, d and f) are favorable to reduce the poor affinity with monomers and then achieve a lower viscosity of alumina suspension. Furthermore, we can infer that the modified alumina suspensions would achieve their lowest viscosities when the doses of dicarboxylic acids reach their m . 3.3. Stability and rheological behavior of ceramic suspensions

the surface of alumina by chemical adsorption. According to the adsorption curves shown in Fig. 5(a), the adsorption process of dicarboxylic acids could possibly be dominated by the monolayer chemical adsorption. All the possible structures formed by chemical adsorption are presented in Fig. 7. The carboxyl groups can react with Al3+ via chemical bond formation with a bidentate bridging linkage (Fig. 7a and b), a monodentate linkage (Fig. 7c and d) or a bidentate chelating linkage (Fig. 7e and f) [17–19]. Consequently, the covered area of a single molecule adhered to the surface may be influenced by the formed structures shown in Fig. 7, thus the decrease in m with the increasing carbon chain length is not linear (see Fig. 5(b)).

3.3.1. Stability The Al2 O3 particles dispersed in the dispersion media with lower solid loading always have a finite settling velocity which can be described by Stokes’ law: 0 = (s −0 )d2 g/18

(2)

Parameter ␳s refers to the density of solid particles, parameter ␳0 refers to the density of media, parameter g is gravitational acceleration constant, ␮ is the viscosity of dispersion media, and d is the average diameter of solid particles.

Fig. 7. Possible structures by chemisorption on the surface of alumina.

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Table 3 Rheological parameters for the 20 vol% alumina suspensions by fitting the experimental data at different acid doses with Yasuda’s equation. Acids

Acid doses (wt%)

Rheological parameters

R-squared value

K(Pa sn ) n

Succinic acid (nc = 4)

Adipic acid (nc = 6)

Fig. 8. The sedimentation curves of 15 vol% suspensions formulated by 0.3 wt% dicarboxylic acids-modified alumina.

In this Section 3.3.1, 15 vol% alumina suspensions were prepared by the formulation provided in Section 2.4 and the method described in Section 2.3. The gravitational settling velocity of ␣Al2 O3 particles is mainly dominated by the particle diameter. According to Stokes’ law, for the submicron alumina particles dispersed into HDDA, the settling velocity caused by gravity is not obvious and can be further counteracted by Brownian movement to some extent. However, with the particle size decreasing, many uncoordinated atoms and hydroxy groups appear on the surface which dramatically increase the surface energy and result in aggregation of powders. Therefore, the suspension formulated by alumina powder as received shows obvious sedimentation behavior, which can be attributed to particles’ poor affinity to HDDA and particles aggregation caused by Van der Waals attractive interaction. To achieve well dispersed and stable alumina suspensions, dicarboxylic acids were added to serve as dispersants and diminish interparticle attractive interaction. The influence on sedimentation behavior of alumina suspensions is summarized in Fig. 8 for three dicarboxylic acids with different carbon chain length (nc = 3, 7, 10). The sedimentation curves representing the sedimentation distances as function of time show a linear behavior. The sedimentation distances decrease obviously after addition of dicarboxylic acids. The dispersion mechanism of these acids mainly relies on the surface modification to lower surface energy and the value of Hamerker constant. Besides, the adsorbed layer of dicarboxylic acids can also introduce steric barriers between alumina particles and dispersing media. In organic system, the electrostatic forces contribution is not obvious. This mechanism can interpret the colloidal stability and the lower viscosity of the modified alumina UV-curable suspension. 3.3.2. Dispersants’ influence on rheological behavior The evolution of stress (␴) as a function of shear rate (␥) characterises the steady state rheological behavior of the suspension. In steady-state regime, stress and shear rate are related by the following relationship: ␴ = ␩.␥

(3)

where ␩ = ␩(␥) is the sample viscosity. In this Section 3.3.2, 20 vol% alumina suspensions were utilized to study the rheological behavior, and the unloaded dispersing medium was HDDA containing 0.5 wt% TPO as mentioned in Section 2.3. The alumina powder was modified by succinic acid, adipic

Suberic acid (nc = 8)

0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.2 0.3 0.4 0.5 0.6

1.8834 1.6411 1.5381 1.2612 1.1874 1.3488 1.8530 1.6775 1.5261 1.1987 1.1493 1.3939 1.7768 1.5294 1.3922 1.2457 1.0424 1.3818

0.247 0.255 0.259 0.274 0.282 0.277 0.252 0.254 0.268 0.282 0.292 0.270 0.277 0.278 0.294 0.296 0.314 0.284

0.9872 0.9888 0.9868 0.9859 0.9856 0.9900 0.9888 0.9891 0.9898 0.9857 0.9891 0.9893 0.9929 0.9840 0.9857 0.9828 0.9810 0.9888

acid and suberic acid (nc = 4, 6 and 8), and their doses changed from 0.1 wt% to 0.6 wt%. The fluid type of alumina suspensions, which is generally described by the curve of shear stress (␴) versus shear rate (␥), is influenced by the doses of dispersants. Fig. 9 summarizes the influences of doses of three dicarboxylic acids with different carbon chain length. When a small dose of acids is added, the suspensions show a pseudoplastic behavior while a yield stress existed. The fluid type of these suspensions falls in between plastic fluid and pseudoplastic fluid. This behavior is attributed to the presence of network microstructure due to interaction of particles to particles and particles to monomers at static state and large rheological units at low shear rates including particles and monomer molecules bonded to them. Therefore, the microstructure destruction of ceramic suspension before flow results in a yield stress existed and the units being disrupted at high shear rates dominate the pseudoplastic behavior with shear thinning. With the dose increasing until 0.5 wt% (close to m for these three acids), the fluid type is gradually transforming into plastic fluid with increasingly weaker shear thinning behavior shown in Table 3, which is attributed to the more effectively coated layer of dicarboxylic acids, the reduced solvation effect and the weaker microstructure of the system. When the dose of the acids exceeds their maximum adsorption capacities, more free carboxyl groups would promote the microstructure formation of alumina suspensions and increase the interactions between particles by the bridging. For the concentrated ceramic slurries, a power-law equation can be applied to compare the rheological behavior determined by the final colloidal suspension stability, which is expressed like this according to Yasuda’s equation[20,21]:  = K n-1

(4)

where ␩ is the viscosity, n is the flow behavior index, and k is the consistency index, both k and n are the parameters which set the equilibrium rheology. A higher value for the consistency index, k, for an identical flow behavior index, n, denotes higher viscosity. At the same time, the flow behavior index, n, is closely related with the shear-thinning behavior of the alumina suspensions. A lower value for the flow behavior index, n, indicates the more obvious behavior of shear-thinning. As is shown in Fig. 10, experimental data mainly conforms to Yasuda’s equation at usual shear rate from 1 s−1 to 100 s−1 and the rheological parameters are summarized in Table 3.

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Fig. 9. Shear stress (␴) versus shear rate (␥) from 1 s−1 to 100 s−1 for 20 vol% alumina suspensions with different doses of dicarboxylic acids: (a) Succinic acid (nc = 4)(b) Adipic acid (nc = 6) (c) Suberic acid (nc = 8).

When doses of these three acids reach 0.5 wt%, parameter k achieves its minimum value and n reaches its maximum value for each acid. And for each acid, the values of n are close with each other, while the values of k have a great difference. Therefore, appropriate addition of dicarboxylic acids could reduce the viscosity of the ceramic suspensions, while may weaken slightly the behavior of shear thinning. This could be attributed to the weaker interaction of the particles and monomers and the more fragile microstructure of the suspension as discussed before. As a result, there exist more free monomer molecules in the system which promote the flow of the suspension. And with the carbon chain length increasing, the values of k decrease and the values of n increase, which could be attributed to the better affinity for longer carbon chain length. As discussed above in this paper, acids’ dose and carbon chain length have an obvious influence on the suspension’s rheological behavior. For the improved stereolithography apparatus like Fig. 1(b), the viscosity at low shear rate and the yield stress seriously influence the self-leveling capability of the UV-curable suspensions. Fig. 11(a) shows the influence of all eight dicarboxylic acids’ doses on the samples’ viscosities at shear rate 30 s−1 , which corresponds to the variation of k displayed in Table 3. It is obvious that the viscosities of all the samples with different acids reach their minimum values when the doses approach their maximum adsorption capacities. By plotting of shear viscosity versus shear stress and finding the maximum value, the yield stress of suspension, the tipping point between deformation and flow, was obtained. And the yield stresses of the suspensions are summarized in Fig. 11(b) which are in accordance with the viscosity variation. Consequently, the optimal dispersant selection and concentration of a dicarboxylic acid were determined by the viscosity at shear

rate 30 s−1 in this study. The suspensions containing sebacic acid as a dispersant own the lowest viscosity value at 30 s−1 among these acids and the optimal concentration is 0.4 wt% (close to sebacic acid’s m). Combining sedimentation data in Fig. 8, for the modified alumina suspensions, the lower the viscosity is, the greater the sedimentation distance. But the sedimentation behavior of 45 vol% alumina suspension with 0.4 wt% sebacic acid would not seriously influence the homogeneity of the green body achieved by our SLA process in Section 3.4. 3.3.3. Different solid loading suspensions To fabricate the alumina ceramic parts with complicated 3D shapes and higher density, the higher solid volume content of ceramic green parts is a prerequisite that must be met for the successful debinding and sintering process. Consequently, it is necessary to study the influence of solid loading on the rheological behavior. In this Section 3.3.3, different solid loading suspensions (5 vol%, 10 vol%, 15 vol%, 20 vol%, 30 vol% and 40 vol%) were prepared, the alumina particles utilized were modified by 0.4 wt% sebacic acid (nc =10) and HDDA (containing 0.5 wt% TPO)was selected for dispersion medium. Influence of solid loading on viscosity at a shear rate 30 s−1 is summarized in Fig. 12. As expected, the shear viscosity increases as the alumina loading increases and undergoes a steep rise at high solid loadings (>30 vol%). Different models have been built up to describe the relationship between solid loading and viscosity[22–24]. The Krieger-Dougherty model gives a good estimation of the suspensions’ behaviors : r = s /d = (1 − ˚/˚m )

−B˚m

(5)

where B is the Einstein coefficient, r is the relative viscosity between the suspension (s ) and dispersion medium (␩d ). This

Fig. 10. Shear viscosity (␩) versus shear rate (␥) from 1 s−1 to 100 s−1 for 20 vol% alumina suspensions with different doses of dicarboxylic acid: (a) Succinic acid (nc = 4) (b) Adipic acid (nc = 6) (c) Suberic acid (nc = 8).

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Fig. 11. Influence of dispersant doses on (a) viscosity at shear rate 30 s−1 and (b) yield stress for dispersions of 20 vol% alumina in HDDA.

relationship is a popular choice for fitting to experimental data, in which case B and m are considered to be fitting parameters. The fitting parameter m is 0.483 and the R-squared value is 0.9984. Fig. 13 shows the rheological behavior of alumina suspensions with different solid loading. It can be seen from Fig. 13 (a) that the alumina suspensions flow like Newtonian fluid when solid loading is under 20 vol% and these suspensions can’t be well fitted by Yasuda’s equation. And when the solid loading reaches and exceeds 20 vol%, the suspensions begin to reflect obvious yield stress and pseudoplastic and can be perfectly depicted in higher solid loading by Yasuda’s equation (with a R-squared value of 0.9978 for 40 vol% suspension) (see Fig. 13(b)). The rotational rheometer was also utilized to determine the thixotropic character of the alumina suspensions by successive applications of loading (increasing shear rate from 1 s−1 to 100 s−1 in 5 min) and unloading (decreasing shear rate from 100 s−1 to 1 s−1 in 5 min) cycles. Interparticle forces, which are derived from the interatomic and intermolecular interactions on surface of particles, provide the essential quality for thixotropic phenomenon [25]. The rheological behavior of suspensions strongly depends on the solid content. The plots of the shear stress versus shear rate from 0.1 s−1 to the maximum shear rate 100 s−1 for the different solid loading suspensions are shown in Fig. 13(c). The hysteretic area is formed by upwards and downwards shear rates, and the time dependence behavior change over solid loading. With the

Fig. 12. Influence of solid loading on viscosity at single shear rate 30 s−1 .

increasing of solid loading, the interparticle space decreases significantly and particle interactions and the shear stress become dramatically higher. The enlargement of the thixotropy loop may be closely related to the more stronger interaction of particles and the more solid microstructure. And with increasingly obvious shear thinning behavior shown in Fig. 13(b), the hysteretic loops are becoming larger, which may be caused by the shear-thinning behavior and disrupting microstructure of the suspensions. However, it is difficult to rebuild the original solid microstructure of the suspensions in a short time by shear induced aggregation and Brownian build-up of aggregates [26].

3.4. Stereolithography and sintering To ensure the successful process of stereolithography of UVcurable ceramic suspensions in common SLA apparatus, HDDA, HEA and TMPTA (containing 0.5 wt% TPO) were mixed by the volume ratio of 6:3:1 by the method described in Section 2.3. HEA, with smaller viscosity and functionality, was added to further decrease the viscosity of the suspension and shrinkage of green body during UV-curing then to promote the adhesion with build platform, but in another hand, to decrease the degree of crosslinking and curing rate. The purpose of addition of TMPTA was to increase the green bodies’ crosslinking degree and curing rate without much influence on viscosity. In this Section 3.4, 45 vol% alumina suspension with 0.4 wt% sebacic acid was formulated to guarantee the successful debinding process and densification of the green body during sintering. A relative density of 96.5% was finally achieved for 45 vol% alumina suspensions with a lower viscosity of 1.62 Pa s at shear rate 30 s−1 , which can be successfully used in the improved apparatus like Fig. 1(b). It is still possible to further decrease the viscosity by combination of different types of dispersants and careful selection of monomers to wet the ceramic particles more perfectly. The microstructure of the sintered alumina part by SEM is shown in Fig. 14. Some residual pores were still observed in the sintered alumina sample, but the sintered body possesses relatively fine grain size of 2–4 ␮m which indicated homogeneous green body achieved by our SLA process. With aspects of density and microstructure, the material obtained by this advanced 3D printing technique is quite similar to the standard high purity alumina products from industry. However, we believe that further increase of solid loading by further development in dispersants and better UV-curable resins, it is possible to reduce the sintering temperature and so realize better ceramic devices with complex shapes.

Please cite this article in press as: S. Zhang, et al., Surface modification of ␣-Al2 O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.013

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Fig. 13. Rheological measurements for alumina suspensions with different solid loading: (a) Shear stress (␴) versus shear rate (␥) from 1 s−1 to 100 s−1 (b) Shear viscosity (␩) versus shear rate from 1 s−1 to 100 s−1 (c) The thixotropy loops for these suspensions by increasing of shear rate up to 100 s−1 .

References

Fig. 14. The microstructure of the sintered alumina part by SEM.

4. Conclusions The feasibility of dicarboxylic acids (3 ≤ nc ≤ 10) applied to organic system has been demonstrated and the mechanism of surface modification, dispersion and stability was discussed in this work. These short chain dicarboxylic acids depend on the chemical bonding with the alumina surface to realize the purpose of dispersion. Appropriate addition of dicarboxylic acids could influence the fluid type, weaken the pseudoplastic, increase the stability and reduce shear thinning behavior and viscosities of the suspensions. The viscosities of the alumina suspensions can reach their minimum values when the doses of the dicarboxylic acids approach their maximum adsorption capacities. 0.4 wt% sebacic acid was selected as the best choice to modify the alumina powder for use in stereolithography because of the lowest viscosity at shear rate 30 s−1 . High solid loading suspensions of submicrometer alumina particles dispersed in UV-curable monomers suitable for stereolithography apparatus were successfully formulated. A sintering density of 96.5% can be achieved for the 45 vol% sebacic acidmodified alumina UV-curable suspension with a lower viscosity of 1.62 Pa·s at shear rate 30 s−1 . Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jeurceramsoc. 2016.12.013.

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Please cite this article in press as: S. Zhang, et al., Surface modification of ␣-Al2 O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.013