Dual gradient direct ink writing for formation of kaolinite ceramic functionally graded materials

Journal of Alloys and Compounds 814 (2020) 152275

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Dual gradient direct ink writing for formation of kaolinite ceramic functionally graded materials Danna Tang a, b, Liang Hao a, b, *, Yan Li a, b, **, Zheng Li a, b, Sasan Dadbakhsh c a

Gemological Institute, China University of Geosciences, Wuhan, 430074, PR China Hubei Jewelry Engineering Technology Research Center, Wuhan, 430074, PR China c €gen 68, SE-100 44, Stockholm, Sweden KTH Royal Institute of Technology, Department of Production Engineering, Brinellva b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 June 2019 Received in revised form 11 September 2019 Accepted 12 September 2019 Available online 13 September 2019

Direct ink writing (DIW) technique has emerged as a powerful tool to create specific functionally graded materials (FGMs) products with macroscopic and microscopic porous architectures and mechanical properties. In order to explore the DIW process control method of ceramic composite FGMs, several additives (e.g., polysorbate, liquid paraffin and water) were mixed with kaolinite and barite powders to print the gradient materials with difference in both material compositions and structures. A stable ceramic slurry with a viscosity of 2.66e3.66 GP s at 5 MPa atmospheric pressure has been formulated by uniformly mixing 2 mm and 10 mm particles. Besides, the optimized flow rate of 150 mml/s and thickness of 0.5 mm were well proved to obtain good stacking of the slurry, whereas, the change of velocity shows little effect on the forming quality. The meso/macro pores of the gradient component can be achieved by adjusting the printing and sintering processes. The dual-extrusion DIW method presented here is versatile to be adapted to a wide range of biomimetic ceramic materials for the fabrication of FGMs objects with unprecedented properties. © 2019 Elsevier B.V. All rights reserved.

Keywords: Ceramic composite Direct ink writing Functionally graded materials Pore distribution

1. Introduction The ceramic represented in the various forms such as concrete, mineral clay, particle is the world's biggest materials industry in terms of the quantity of materials produced [1]. Ceramic composite is one of the most ancient material used by human and a kind of green material that is used extensively in buildings, infrastructure and personal goods today [2]. Ceramic materials can be designed with the best use of their structures to maximize the advantages of their natural functions [3]. The use of kaolinite, barite and dolomite-based ceramic composite to achieve sustainable development in personal consumable products (e.g. cosmetics, decorative) and adsorbents (to purify water and also for waste treatments) is popular in various manufacturing fields [4,5]. These materials are often of importance to functional devices including electronic packaging devices [6]. In the fabrication of geological models,

* Corresponding author. Gemological Institute, China University of Geosciences, Wuhan, 430074, PR China. ** Corresponding author. Gemological Institute, China University of Geosciences, Wuhan, 430074, PR China. E-mail addresses: [email protected] (L. Hao), [email protected] (Y. Li). https://doi.org/10.1016/j.jallcom.2019.152275 0925-8388/© 2019 Elsevier B.V. All rights reserved.

kaolinite and barite composites are also used in order to simulate natural properties and phenomena, such as fracture analysis of rocks and slip prediction of landslides, which utilize structural characteristics of these mineral materials to achieve a variety of designed mechanical properties [7,8]. The simultaneous design of ceramic composite compositions and structures could effectively manipulate their mechanical properties and offer better sustainable performance. Additive manufacturing (AM) is a technique that can produce very complex geometries. Its application for the fabrication of nearnet shapers is gradually increased [9]. As one of AM technique, direct ink writing (DIW) of ceramic materials shows great promise in controlling the pore structure and enhancing physical and chemical functionalities by mixing with other materials, like metal fiber and polymer materials, etc. Recently, the DIW ceramic composites have attracted great attention because it can enable strategic design with the use of a novel AM process to enhance the value of ceramic materials and broaden their applications [10,11]. In order to achieve multiple functions, ceramic is often mixed with a variety of materials as specific formats for AM, such as graphene added to functional ceramic parts to enhance its electrical and thermal conductivity properties, and carbon/glass fiber to reinforce

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structures [12,13]. Hence, there are increased research activities investigating structural design of multi-materials in recent years. In despite of these efforts, AM of ceramic composites printing has yet achieved the precise control of spatial distribution and orientation of multiple material compositions. As an initial attempt in previous study, a deposition-based 3D printing was proposed as a novel method for constructing the geological model, especially its porous slide zone, to closely mimic equivalent material properties and porous structures of natural landslide materials [14]. With such a capability, it is proposed a new development of forming gradient or hierarchical parts with spatially varying ceramic materials to achieve synchronous controlling of stringent material properties. This means that Functionally graded materials (FGMs) of ceramic composites are to be constructed through layer-by-layer forming principle. This can form complex structure with multi-material which is suitable for digital processing models with sophisticated shape and performance [15]. Components of various ceramic materials with predetermined material gradients have been reported in recent studies [16,17]. Leu et al. [18] developed an extrusion method to control the fabricate CaCO3 composite with a graded color. A green part with graded composition of alumina (Al2O3) and zirconia (ZrO2) was also fabricated. In a work carried out by Singh et al. [19], Al/Al2O3 FGMs were prepared by combining DIW with investment casting to reinforce the density of formed material. The above work in related to DIW FGMs has mainly investigated the geometric forming and material properties and little research has studied the meso/macro pores distribution in related to the material graduation and sintering mechanisms. The AM of gradient kaolinite composites has also not yet investigated. The main scope of this research is to evaluate kaolinite and barite FGMs as AM printing materials using the DIW technique. Several DIW process parameters were quantified and analyzed in order to understand the effect of the AM printing process on the forming of FGMs samples. It firstly investigates the material system in order to precisely adjust the material characteristics of the slurry. Following on this, the five DIW process parameters (movement speed, flowrate, layer thickness, nozzle height and retraction distance) are investigated in order to optimize the DIW forming of clay ceramic composite. With suitable materials and process parameters, this research demonstrates the feasibility of forming sophisticated FGMs objects with varying colors in 1D, 2D and 3D, and graded barite/kaolin particles distribution concentration in 3D.

2. Experimental

2.2. DIW and sintering ceramic composites The barite and the kaolinite were respectively mixed with the additives to form two kinds of slurry of A (kaolinite, with pink essence) and B (barite, with skylight blue essence). The computer modeling Rhino3D NURBS software (America Robert McNeel Ltd.), slicing Cura software (Version 14.07, Netherlands Ultimaker Ltd.) and a dual-extrusion mixing head (Animation ViscoDuo-VM 03, ViscoTec Ltd, Germany) were used to form a material gradient from bottom-up B (barite slurry) material to A (kaolinite slurry) material as can be seen in Fig. 1. The computer modeling software (Rhino3D NURBS, America Robert McNeel Ltd.) and slicing software (Cura, Version 14.07, Netherlands Ultimaker Ltd.) were used to form a material gradient from bottom-up B (barite slurry) material to A (kaolinite slurry) material as can be seen in Fig. 1. All the printing parameters setted in the slicing software were presented in details (Tables S1 and SI). A dual-extrusion head (Animation ViscoDuo-VM 03, ViscoTec Ltd, Germany) was attached to the three-axis moving device. An external air compressor was used to provide the extrusion pressure of the material dispensing head, and its maximum air pressure can be up to 12 MPa. The pressure usually used was 5e6 MPa in order to avoid damage to the dual-extrusion head. The gradient specimen was sintered in the air using muffle sintering furnace (FR-1236, China). In previous study, we proved additive binders were volatile at 200
C and gradually stabled at 350
C. The temperature was eventually stabilized at 1000
C without weight change [14]. 350
C was set to the additive volatilization temperature and the heating rate was 5
C/min in the whole process. The sintering cycles were: (1) heating at 350
C for 60 min; (2) ramping up to 1000
C with the heat rate of 20
C/min; (3) heating at 1000
C for 60 min; and (4) gradually ramping down to room temperature. 2.3. Characterization techniques 2.3.1. Particle characterization Particle size study was carried out using Laser particle size analyzer (Rise-2002, Rise, China) with a measurement range of 0.1e1200 mm. Barite powder and kaolinite powder were dispersed in deionized water during the test. Morphological study was carried out using scanning electron microscopy (SEM) (FE-SEM, SU8010, Hitachi, Japan) with an acceleration voltage of 20 kV. The morphology of barite powder and kaolinite powder after gold coating was investigated by imaging fracture surfaces.

2.1. Materials Kaolinite (2SiO2$Al2O3$2H2O) is an important and widely assessable ceramic material in nature with a 1:1 layered silicate structure, supplied by China Chu Xiong Ltd., Barite (BaSO4) resources are also abundant in the world, and barite often presents thick plate or columnar crystals, and exhibits as dense block, platelike or granular aggregates with Moh's hardness of 3e3.5 and density of 4.5 g/cm3, which was supplied by China Hua Senda Ltd. The wax-based additive is selected to mix with ceramic powder due to the plasticity and stable mechanical properties in the specimen. Additionally, surfactants and auxiliaries need to be added to inhibit the delamination of liquid wax during printing. The bonding force provided by the additive is critical to the formation. Water is a mixing medium between the surfactant and liquid paraffin [20]. In this experiment, the surfactant of Polysorbate (Tween-80, C24H44O6(C2H4O)n) and liquid paraffin (CAS 8020-83-5) were produced by GongQin Chemical Ltd (Guangzhou, China) and used as received.

2.3.2. Slurry characterization The viscometer (LV DV-II þ Pro, America) was utilized to measure the viscosity of the additive (polysorbate, liquid paraffin, water). The rotation speed was between 18 and 36 RPM and the temperature was kept at room temperature (25
C). Rotational Rheometer (Kinexus labþ, Malvern Panalytical, England) was used to measure the rheological properties of slurry A (kaolinite slurry) with the change of additives’ (polysorbate, liquid paraffin, water) content (100 wt %, 50 wt %, 30 wt %). 2.3.3. Specimen characterization Microscopic test was carried out using Optical microscope (M205C, Leica, Germany) to observe the morphology of the extrusion part. Atomic force microscope (AFM, SPM-9700HT, Shimadzu, Japan) was utilized to measure the pore size of the extrusion part with a measurement range of 2e50 nm. Automatic surface area and porosity analyzer (TRISTAR II3020,

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Fig. 1. Gradient printing process for kaolinite and barite ceramic.

Micromeritics, America) was used to compare the pore changes of the barite and kaolinite powder within the extrusion specimen. 3D X-ray microscope (XRM, Xradia 510 Versa, Zeiss, Germany) was used to test the pore distribution and characteristics of dualgradient sintered parts of kaolin and barite specimen. FEI Avizo software (Version 9.0, America) was then performed to adjust the accurate 3D pore reconstruction using the sliced data.

3. Results and discussion 3.1. Extrudability and suspensibility of the slurry The barite powder exhibited a granular combination and its particle shape and size was presented with relative uniform distribution in Fig. 2a. While the kaolinite particles were non-uniform and the size was unevenly dispersed (Fig. 2b). This finding was verified by the particle size test that the particle size of barite powder was 2 mm in average, while the particle size of kaolinite powder was ranging from 2 to 50 mm in Fig. 2c. The rheological performance of mixed slurry with 50 wt% additive content was similar to that of pure additive, and the viscosity significantly increased when the additive was reduced to 30 wt % (Fig. 3a). This indicates that 30 wt% is a turning point of the additive content in the slurry, which causes the catastrophic change in rheological properties. As it is shown in Fig. 3bec, the single raster width of both the kaolinite slurry and the barite slurry were in the range of 250e350 mm and apparently less than the nozzle diameter (0.4 mm) when the slurry with 30 wt% additive content was printed. The rheological property of kaolinite slurry is represented as a typical non-Newtonian fluid, similar to many medical and food grade gels [21,22]. With high viscosity, slurry materials could maintain good consistency of small single channel deposition width even at relatively fast-moving speeds. The uniform dispersion of barite and kaolinite powder in the slurry is critical to the stabilization and extrudability of the slurry, especially under the relatively large shear force. The energy of

hydration and repulsion of the suspended particles are depended on the distance between these particles (Eq. (1)). [23]. When the distance between particles decreases, the attraction force between particles increases. The high attraction force could lead in the agglomeration of particles. When high pressure is applied, the liquid phase of the slurry is forced to flow from the particles to the pressure direction. This could result in the less liquid phase content in the slurry remaining at the top of syringe and often cause the aggregation. The easier the particles are attracted, the more likely they are to form blockage. The aggregation rate in this force-field can be calculated by the Fuchs formula (Eq. (2)) [24]. The aggregation of particles was mainly controlled by the rate of diffusion at high viscosity, since the van der Waals forces and hydrodynamic interactions have only minor effects. Small particles have a faster diffusion rate than large particles, because small particles have a larger specific surface area and are easier to attract each other (Fig. 4a). This is the reason why the wide particle size distribution of small and large kaolinite particles has less surface energy to absorb each other, which results in a low diffusion rate under extrusion force (Fig. 4b).

VHR ¼ 2p


R1 R2 H h0 V 0HR exp  h0 R1 þ R2

(1)

where R1 and R2 are the neighboring powder particle radius respectively, V 0HR is the energy constant, h0 is the attenuation length, H is the Interaction distance.

2kT k¼ 3hReff

(∞ ð 0

 )1  UðhÞ dh exp kB T ðRþ þ R þ hÞ2 BðhÞ

(2)

where k is the aggregation rate coefficient, T is the absolute temperature, h is the viscosity of paste, Reff is half the radius of R, BðhÞ is the hydrodynamic resistance function, Rþ and R are the radius of the two particles involved, h is the surface separation, UðhÞ is the interaction free energy of the two particles, kB is the Boltzmann

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Fig. 2. Microscopic morphology of particles, a) 1.00 mm scale of barite powder, b) 50.0 mm scale of kaolinite powder. c) Particle size distribution of kaolinite powder and barite powder.

Fig. 3. a) Rheological properties of the mixed slurry with different additive ratios (100%, 50%, 30%). Morphology of multi-channel paste, b) kaolinite, c) barite.

Fig. 4. Suspension of particles, a) narrow particle size distribution with fast diffusion rate, b) wide particle size distribution with low diffusion rate. c) Particle packing phenomenon.

constant. A multi-modal particle size distribution of the kaolinite particles (2e50 mm) was also selected to provide better forming quality and sintering properties. The graded range of particle size is regarded as a compromising method which could improve the density of sample without significant reduction of the flowability. The positive effect of suitable particle range on the performance of printing and sintering processes has been investigated in many studies [25,26]. The appropriate particle size range is also conducive to the sintering of densified metals [27]. According to the particle size distribution principle, the barite powder particle distribution should also

have a certain particle size range. However, barite powder is the main component of the rock, using as a weighting agent with a density of 4.3 g/cm3. Multi-scale particle size distribution of the barite powder is likely to cause nozzle blockage and specimen collapse situation due to particle precipitation. Sun et al. reported that mixing two kinds of ceramic particle size can avoid separation and misalignment between layers appeared in the samples, leading to enough strength and high bulk density [28]. If the complexity of the process is not considered in the experiment, the material A mixed by two kinds of particle size gradation kaolinite particles (e.g. 45e100 mm and 0e25 mm) would be optimal

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for the ceramic density. Since the material A and B do not have particle size gradation, the sintered part may exhibit a significant top-down change in density and pore distribution. Larger pores should appear in the place where there are relatively large kaolinite particle and no barite particle located after sintering. In order to reduce the extrusion viscosity while maintaining stability, the pneumatic pump was used to obtain a relatively large sheering force of 5e6 MPa. Particle discontinuity is prone to occur despite the appropriate pressure. Fig. 4c shows discontinuous stacking in AM, resulting in a significant reduction in structural accuracy and density. The spacing between droplets is less than the height of one droplet, bringing in non-uniform material concentration of droplets and particle packing phenomenon (Eqs. (3) and (4)) [29]. When the particle size is smaller than 10 mm, the effect of gravitational potential energy can be ignored [30]. The presence of kaolin particles larger than 10 mm will be more conducive to slurry deposition. As a result, the discontinuous accumulation of droplets would result in defects in the product, directly leading to voids and collapse in crucible sintering. When the porosity exceeds a critical range, the densification process could not fill up these defects.

" Wk < W ¼ D

Wk ¼

4

Table 2 Physical properties of barite, kaolinite and mixed slurry. Ra (mm)

Surface Area (m2g1)

Barite powder Kaolinite powder Mixed slurry

12.78 14.45 4.68

3.04 4.29 0.45

3.2. AM and adjustment of parameters Movement speed, flowrate and layer thickness are the three main parameters in the adjustment of gradient specimen. In the experiment, three parameters were adjusted synchronously in order to identify their effect on the additive forming of 3D specimens. As can be seen from Fig. 5a, the forming quality kept stable at a high level with the moving speed at the range of 6e12 mm/s. The volumetric material flow rate of ceramic slurry directly depends on the extrusion rate. Meantime, the critical moving rate determines the length and the bead width of the deposited slurry track [31]. According to this relationship, the extrusion rate and axis movement rate can be decided in order to give a stable bead diameter. However, 0.5 mm was the best layer thickness and the forming quality was independent from the change of moving speed (Fig. 5b). Fig. 5c showed that the neat mesh filling is the standard for evaluating the quality of the component. A typical continuity occurred in the gradient color part when printing at the layer thickness of 0.5 mm and the flow rate of 150 mml/s, because the size of the nozzle (0.4 mm) was adaptable for the better forming quality of gradient specimen (Fig. 5d) (from pink to blue). Franchin et al. used the same size nozzle (~460 mm) to produce a stable printing [25]. This is the size commonly used for DIW of ceramic in case of plug. In term of the manufacturing efficiency, these parameters provide the fastest rate for gradient specimen building with appropriate geometry accuracy. Few articles have summarized the correlation of movement speed, flowrate and layer thickness (Table 3). [32e40]. Only two articles mentioned that the layer thickness is mainly related to the size of the diameter of the nozzle, moreover, it can be slightly controlled by flow rate and moving speed [41,42]. Obviously, this

(3)

#2=3 " 2pD 4 3ðq  sinqcosqÞ ð1  cosqÞ2 ð2 þ cosqÞ

Particles and slurry

roughness was significantly reduced from 12.78-14.45 mme4.68 mm after the slurry was formed according to AFM test. The surface areas of barite powder and kaolinite powder were 3.04 and 4.29 m2g-1, respectively. It is apparent that the roughness reduced after the forming of the specimen, and the surface area synchronously reduced to 0.45 m2g-1. The smaller roughness of the slurry results in less friction when the slurry is extruded at the nozzle. When the surface roughness is less than 1.6 mm, it cannot even be observed with the naked eye (ISO 1302:2002).

#1=3

ð1  cosqÞ2 ð2 þ cosqÞ

5

(4)

where Wk is the spacing between two droplets, D is the diameter of the droplet, q is solidification angle of ball lock. The dispersion of the slurry depends on the ratio of powders and additives. In Table 1, the content of the three additives was changed respectively. It can be seen in Group 1 that the slurry condition changed from milky to viscous when the content of the water exceeded 40 g, indicating that a slight decrease in water content can cause a sharp change to the viscosity of slurry. Polysorbate was an important factor affecting dispersibility as a slight decrease in polysorbate content can lead to slurry stratification (Group.2). The slurry was uniformly dispersed in Group 3 whose content of liquid wax was more than 40 g, though the viscosity of the slurry sharply decreased. These results indicate that an increased content in water and liquid wax can significantly reduce the viscosity, conversely, a growing content in polysorbate can significantly increase the viscosity. The addition of additive into the slurry shows a considerable effect on the pore characteristics and surface morphology in term of roughness. Smooth surface morphology of slurry with smaller roughness would facilitate extrusion, in conjunction with the stable slurry dispersion playing an important role. In Table 2, the average

Table 1 Viscosities of the mixed additives. No

Water (g)

Tween 80 (g)

Liquid wax (g)

Value (MPa$s)

Slurry condition

1

46 40 34 28 40 40 40 40 40 40 40 40

40 40 40 40 46 40 34 28 40 40 40 40

40 40 40 40 40 40 40 40 46 40 34 28

2660 e Over range Over range 7950 e 240 276 268 e 3600 5100

Stable/Milky Unstable Stable/Viscous Stable/Viscous Stable/Viscous Unstable Stratified/Floc Stratified/Floc Stratified/Floc Unstable Stable/Milky Stable/Milky

2

3

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D. Tang et al. / Journal of Alloys and Compounds 814 (2020) 152275

Fig. 5. a) Forming quality related to moving speed and flow rate, b) forming quality related to moving speed and layer thickness, c) quality grading standard, d) model printing at a nozzle size of 0.4 mm and flow rate of 150 mml/s.

Table 3 Summary of movement speed, flowrate and layer thickness adjustment rules. Materials

Parameters

Control law

Temperature: 60
c Flowrate: 0.3 mL/min Layer thickness: 0.3e0.5 mm Temperature: 55e65
C temperatures (2e250
C) Pressures: 0.3e0.8 bar Layer thickness: 0.3e0.5 mm Moving speed: 1.5e12 mm/s Potato flakes (Atlantic)/Xanthan gum Temperature: 25
C (XG)/k-carrageenan gum (KG) Layer thickness: 1.2 mm Moving speed: 25 mm/s Medical-grade polycaprolactone Size: 50 mm  50 mm  2 mm (mPCL) Temperature: 110
C Layer thickness: 0.33 mm Polyurethane (PU)/Al2O3 Size: 250 mm  250 mm  150 mm Rotation speed: 0 to 100 r/s Moving speed: 0.5e4 mm/s Poly (ethylene glycol) diacrylate Pressures: 4 to 26 psi (PEGDA)/Silver nanoprisms (AgNPs) Optimized print pressure and speed N-acryloyl glycinamide/N[tris(hydroxymethyl)methyl] acrylamide Photocurable methacrylate/acrylate Layer thickness: 0.3 mm monomers and oligomers Moving speed: 5 mm/min Nozzle diameter: 410 mm Interpenetrating polymer network (IPN)/sodium alginate and gelatin Moving speed: 10e80 mm/s (SA/G) Flowrate: 70e120 mml/s Poly-Lactic Acid (PLA)/Acrylonitrile String width depends mainly on the diameter of the nozzle but can be slightly Butadiene Styrene adjusted through parameters such as the nozzle movement speed and extrusion flow rate Alginate/poly(acrylamide) ionic Moving speed: 80 mm/min covalent entanglement hydrogel The moving stage was controlled independently. The flow rate and print speed were then optimized to extrude material at the appropriate speed to maintain a consistently thick layer Gentamycin sulfate (GS)/ desferoxamine (DFO) GS/polyvinyl alcohol (PVA) Poly (propylene fumarate) (PPF)/HA nanoparticles

rule is not applicable to the formation of ceramic materials, and the moving speed has little influence on the layer thickness of ceramic materials. The possible reason is that the fluidity of the two highviscosity materials is stable and almost independent of speed.

Ref.

Temperature demand, single parameter [32] control, irregular [33]

[34]

Size demand, single parameter control, [35] irregular [36] Single parameter control, irregular

[37]

[38]

[39] [40]

Layer thickness depends on the nozzle diameter, moving speed and flowrate

[41]

[42]

Thus, the formation of ceramic composites can avoid the influence of irrelevant factors. This could help to reduce the experiment workload to adjust three parameters and identify suitable processing parameters.

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Fig. 6. a) Forming quality related to nozzle height, b) Forming quality related to retraction distance.

The height of the nozzle from the platform is key factor that affects the print quality, especially for real-time deposition of dualgradient materials. Four sets of nozzle distances of 1, 2, 3 and 4 mm were tested, and 2 mm was found to be the optimum nozzle distance (Fig. 6a). It is noted that when the nozzle height is less than hc, there is a ‘squeezing’ effect (Eq. (5)) [43]. It is inversely proportional to the nozzle moving speed and the nozzle diameter. The proper moving speed of the nozzle is related to the flow rate and the nozzle diameter (Eq. (6)) [44]. Therefore, apart from the discontinuity of droplets caused by uneven material, the nozzle parameters can also lead to this phenomenon. At the distances of 3 and 4 mm, sinking effect formed by filling discontinuities, and 1 mm was the worst quality due to excessive mass of the slurry.

hc ¼ Vd =ðvn Dn Þ vn ¼ 4Q

.

pD2n

(5) 

(6)

where Vd ; vn Dn Q are the volume of the material extruded per unit time, the nozzle moving speed, the nozzle diameter, and the volumetric material flow rate, respectively. A lower height than the optimum height resulted in thicker extrusion lines than expected due to the uniform mixing speed of the two materials. Instead, a height higher than 2 mm contributed to the slurry not coming into contact with the surface at the turning point, which affected the continuity of subsequent process. This phenomenon concurred with previous similar work, which had unstable printing in the corner area of the part [43]. The optimal build height through the experimental comparison was determined to be 2 mm in this work, which was in good agreement with the calculation result of formula. According to Eq. (5), the critical nozzle height would be 1.88 mm and very close to the experimental finding. As the formula is customized designed for porcelain dental slurries rather than ceramic slurry, it is likely that the difference in materials cause a slightly different extrusion height. The retraction distance was a parameter related to the nozzle height and size. For the nozzle set up, it is found that stacking occurred at the retraction distance of 0.2 and 0.4 mm, and 0.6 mm was proved to be the best retraction distance (Fig. 6b). The printing quality dropped sharply at 0.8 mm as there was no slurry in the front part of the head, and slurry breakage occurred during the dispensing interval. Although the optimal processing parameters can be identified, it must be noted that these parameters would be different when the various material compositions are used. During the real-time variation of the mixed slurry during the dispensing of

the graded materials, the print parameters cannot be changed in real time during printing to accommodate material changes. Printing parameters can be versatile due to the use of the same additive. Therefore, it is necessary to find appropriate parameters that are generally suitable for the graded materials.

3.3. Properties of the sintered porous gradient ceramic specimen Sintering process play another key effect to the forming of micropores in the gradient part in addition to parameter adjustment. When the specimen was not sintered, the AFM image of the surface was uneven with many dents (Fig. 7a). The surface had a concentrated protrusion after the specimen was sintered with the height between 1.52 and 1.78 mm (Fig. 7b). The concentrated discharge of air created a single track of protrusions, indicating the directional mobility of the exhaust process. But overall, the process of sintering flattened the surface and recombined the grains, possibly further refining the macroscopic pores into micro-sized pores. The gradient macro pore distribution (>1 mm) caused by material gradient can be seen in the sintered sample. The bottom part of kaolinite parts appeared more fractures and cracks in the single channels than the upper part of barite material (Fig. 8a). The kaolinite part had large connected pores, while the barite part had

Fig. 7. a) AFM image of unsintered surface, b) AFM image of sintered surface.

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Fig. 8. a) Morphology of the Sintered model, b) Crack and pore distribution.

Fig. 9. a) Morphology of the Sintered model, b) the sintered barite part with flat surface, c) the sintered kaolinite part with uneven surface.

small pores. The middle part mixed different sizes of voids (Fig. 8b). The large and widely distributed particles of kaolinite caused such continuous pores. Particle size has an important influence not only on the sintering performance, but also the morphological features. Some researchers proved that as the particle size of ceramic powder reduced the density increased. It can even reach full density with an average particle size of <0.5 mm [45,46]. This phenomenon of pore inhomogeneity can also be proved by surface microstructure in Fig. 9a. The sintered barite part was relatively flat and had no significant micron pores (Fig. 9b). The middle part began to undulate, having uneven particles on the surface. It was more obvious that the grain boundary in the kaolinite part had a grainy shape with uneven size. The surface is also more uneven, with many small particles attached (Fig. 9c). It is apparent that the scale-distributed kaolinite particles result in a more pronounced grain boundary after sintering, while the barite part is relatively flat. This is also consistent with the previous estimation, the top-down change in density is related to the distribution of the particles. In previous study, the gradient pore design of ceramic composites has showed great potential in the enhancement of biomimetic properties and mechanical properties [47,48]. Obviously, the curve was not closed due to the coexistence of mesopores and macropores (Fig. 10a). The reason why the low

pressure phase curves were open is that the balance time should be shorter. The larger mesopores and macropores in the ceramic structure present at the same time, owing to rapidly increasing of the nitrogen adsorption amount at the relative pressure close to the saturation vapor pressure [49]. The pore distribution of the mixed powder was dispersed between 7 and 44 nm, and 17 nm was the most distributed pore size (Fig. 10b). The coexistence of such macropores and mesopores is also closely related to the distribution of kaolinite and barite particles. The gas escapes from the particle boundaries during the sintering process. An inherent inhibition force of the component is required to prevent the gas from escaping in order to increase the density. The force of non-uniform particle distributed place is much smaller than that of uniform particle distributed place [50]. The gas will escape when the sintered specimen reaches a critical density. This appearance is well shown in Fig. 9c due to gas escaping from the particle boundaries. Therefore, macropore mainly appears in kaolinite part because of the gradient distribution of pore in the whole sintered part. The meso/macro pores of ceramic materials are the main factors affecting their performance, and the microstructure is depended on the species of ceramic materials. In order to obtain the required performance, previous researches use various manufacturing methods to obtain the ideal pore distribution. For example, many

Fig. 10. a) Adsorption isotherm curve of sintered model, b) Pore size distribution.

D. Tang et al. / Journal of Alloys and Compounds 814 (2020) 152275

shell pores in the ceramic materials form a porous structure, and its specific surface area and adsorption capacity are very superior [51]. In the level of macroscopic structure, the influence of pore structure is also considered in the study of geology and engineering, and porous ceramic materials have been widely used as environmental and biological functional materials [52e54]. When the properties of the gradient materials such as mechanics and thermal properties need to be better explored, their range of values depend on the microstructure and pore distribution. This connection has been well studied in many gradient metal materials [55e57]. This study is to achieve the gradient printing of kaolinite ceramic composite, and illustrate the relationship between micro/macro pore distribution and gradient material distribution, as well as the sintering process. With the precise geometry fabrication capability of the AM, it is also expected that the design of graded macro pores can be applied as effective approach to manipulate the structure performance and property of the graded ceramic materials, but this would be subjected to further study. 4. Conclusion This study presented the feasibility of using direct ink writing (DIW) technique to create specific functionally graded materials (FGMs) specimens with macroscopic and microscopic porous architectures. In order to explore the FGMs process control method of ceramic composite, several additives (e.g., polysorbate, liquid paraffin and water) were mixed with kaolinite and barite powders to print the gradient difference of both materials and structures. A stable ceramic slurry with a stable viscosity of 2.66e3.66 GP s at 5 MPa atmospheric pressure has been achieved by uniformly mixing 2 mm and 10 mm particles. The kaolinite and barite particles were stably dispersed in slurry by adding additives Water and polysorbate were the two main factors affecting dispersibility, resulting in slurry stratification. The optimized flow rate of 150 mml/s and thickness of 0.5 mm were well proved to obtain good stacking of the slurry, however, the change of velocity has little effect on the forming quality. The roughness (4.68 mm) reduced after the forming of the component, and the surface area synchronously decreased to 0.45 m2g-1. The forming quality was independent of moving speed variation (6e12 mm/s). The meso/macro pores of the gradient component can be achieved by adjusting the printing and sintering processes. The sintered part had both mesoporous between 7 and 44 nm and macropores with a gradient distribution. The dual-extrusion DIW method presented here is versatile to be adapted to a wide range of biomimetic ceramic materials for fabrication of FGMs objects with unprecedented properties. These findings would provide a basis of new methodologies to design and manufacture superior clay FGMs for bionic engineering applications, or even a universal regulation for bionic soft materials, facilitating the bionic application and functional design in the future. Acknowledgments The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 51675496, No. 51671091, No. 51902295), China Scholarship Council (No. 201906410014), and Nature Science Foundation of Hubei Province (No. 2019CFB264). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152275.

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Compliance with ethical standards Conflict of interest All the authors declare that they have no conflict of interest. Ethical approval This chapter do not contain any studies with human participants or animals performed by any of the authors. Informed consent Informed consent was obtained from all individual participants in the study. References [1] P.C. Aitcin, Concrete structure, properties and materials, Can. J. Civ. Eng. 13 (4) (1986) 499. [2] A. Moropoulou, A. Bakolas, S. Anagnostopoulou, Composite materials in ancient structures, Cement Concr. Compos. 27 (2) (2005) 295e300. [3] Z. Zhang, J.L. Provis, A. Reid, H. Wang, Geopolymer foam concrete: an emerging material for sustainable construction, Constr. Build. Mater. 56 (3) (2014) 113e127. [4] Q.L. Sun, Z.Z. Yang, H. Cheng, Y. Peng, Y.Q. Huang, M.X. Chen, Creation of threedimensional structures by direct ink writing with kaolin suspensions, J. Mater. Chem. C 6 (2018) 11392e11400. [5] A. Sarı, Fabrication and thermal characterization of kaolin-based composite phase change materials for latent heat storage in buildings, Energy Build. 96 (2015) 193e200. [6] J.J. Liao, H.H. Chen, H. Luo, X.F. Wang, K.C. Zhou, D. Zhang, Direct ink writing of zirconia three-dimensional structures, J. Mater. Chem. C 5 (2017) 5867e5871. [7] S. Das, M. Aguayo, V. Dey, R. Kachala, B. Mobasher, G. Sant, N. Neithalath, The fracture response of blended formulations containing limestone powder: evaluations using two-parameter fracture model and digital image correlation, Cement Concr. Compos. 53 (10) (2014) 316e326. [8] J. Ma, H. Tang, X. Hu, A. Bobet, R. Yong, M.A.M.E. Eldin, Model testing of the spatialetemporal evolution of a landslide failure, Bull. Eng. Geol. Environ. 76 (1) (2016) 1e17. [9] S. Singh, S. Ramakrishna, R. Singh, Material issues in additive manufacturing: a review, J. Manuf. Process. 25 (2017) 185e200. [10] C.F. Revelo, H.A. Colorado, 3D printing of kaolinite clay ceramics using the Direct Ink Writing (DIW) technique, Ceram. Int. 44 (5) (2018) 5673e5682. [11] Y. Zhou, A. Marie LaChance, A. Smith, H. Cheng, Q. Liu, L. Sun, Strategic design of clay-based multifunctional materials: from natural minerals to nanostructured membranes, Adv. Funct. Mater. 1807611 (2019) 1e18. [12] S. Kim, S.H. Ku, S.Y. Lim, J.H. Kim, C.B. Park, Grapheneebiomineral hybrid materials, Adv. Mater. 23 (17) (2011) 2009e2014. [13] S. Christ, M. Schnabel, E. Vorndran, J. Groll, U. Gbureck, Fiber reinforcement during 3D printing, Mater. Lett. 139 (2015) 165e168. [14] D. Tang, L. Hao, Y. Li, W. Xiong, T. Sun, X. Yan, Investigation of wax-based barite slurry and deposition for 3D printing landslide model, Compos, Part A: Appl. S 108 (2018) 99e106. [15] M. Yin, L. Xie, W. Jiang, G. Yin, Design and fabrication of integrated micro/ macrostructure for 3D functional gradient systems based on additive manufacturing, Opt. Commun. 414 (2018) 195e201. [16] Y. Cai, N. Wang, L. Cheng, X. Yin, H. Yin, Y. Wang, X. Ren, X. Li, X. Fan, Electrical conductivity and electromagnetic shielding properties of Ti3SiC2/SiC functionally graded materials prepared by positioning impregnation, J. Eur. Ceram. Soc. 39 (2019) 3643e3650. [17] X. Wang, K. Wei, Y. Tao, X. Yang, H. Zhou, R. He, D. Fang, Thermal protection system integrating graded insulation materials and multilayer ceramic matrix composite cellular sandwich panels, Compos. Struct. 209 (2019) 523e534. [18] M.C. Leu, B.K. Deuser, L. Tang, R.G. Landers, G.E. Hilmas, J.L. Watts, Freeze-form extrusion fabrication of functionally graded materials, CIRP Ann. 61 (1) (2012) 223e226. [19] N. Singh, R. Singh, I. Ahuja, On development of functionally graded material through fused deposition modelling assisted investment casting from Al2O3/ SiC reinforced waste low density polyethylene, Trans. Indian Inst. Met. 71 (10) (2018) 2479e2485. [20] X.C. Xia, D.D. Xie, Y.H. Huang, M.B. Yang, Formation of the three-dimensional (3D) interlinked hybrid shish-kebabs in injection-molded PE/PE-g-CNF composite by “structuring” processing, Compos. Sci. Technol. 157 (2018) 209e216. [21] D. Whyte, M. Walmsley, A. Liew, R. Claycomb, G. Mein, Chemical and rheological aspects of gel formation in the California Mastitis Test, J. Dairy Res. 72 (1) (2005) 115e121. [22] E. Weber, M. Moyers-Gonz alez, T.I. Burghelea, Thermorheological properties of a Carbopol gel under shear, J. Non-newton. Fluid 183e184 (2012) 14e24.

10

D. Tang et al. / Journal of Alloys and Compounds 814 (2020) 152275

[23] H. Ohshima, Approximate analytic expression for the stability ratio of colloidal dispersions, Colloid Polym. Sci. 292 (9) (2014) 2269e2274. [24] S. Shrotri, P. Somasundaran, Particle deposition and aggregation, measurement, modeling and simulation, Colloids Surf., A 125 (1) (1997) 93e94. [25] G. Franchin, H.S. Maden, L. Wahl, A. Baliello, M. Pasetto, P. Colombo, Optimization and characterization of preceramic inks for direct ink writing of ceramic matrix composite structures, Materials 11 (4) (2018) 515. [26] A.C.I.A. Peters, G.C. Overbeek, T. Annable, Bimodal particle size distribution polymer/oligomer combinations for printing ink applications, Prog. Org. Coat. 38 (3) (2000) 137e150. [27] C.F. Revelo, H.A. Colorado, 3D printing of kaolinite clay ceramics using the Direct Ink Writing (DIW) technique, Ceram. Int. 44 (2018) 5673e5682. [28] C. Sun, X. Tian, L. Wang, Y. Liu, C.M. Wirth, J. Günster, D. Li, Z. Jin, Effect of particle size gradation on the performance of glass-ceramic 3D printing process, Ceram. Int. 43 (2017) 578e584. [29] H.C. Le, Y.P. Luo, J. Zhou, J.M. Hou, X.H. Li, J. He, A novel selection method of scanning step for fabricating metal components based on micro-droplet deposition manufacture, Int. J. Mach. Tool Manuf. 56 (2) (2012) 50e58. [30] S. Hwang, E.I. Reyes, K.S. Moon, R.C. Rumpf, N.S. Kim, Thermo-mechanical characterization of metal/polymer composite filaments and printing parameter study for fused deposition modeling in the 3D printing process, J. Electron. Mater. 44 (3) (2015) 771e777. [31] L. Hao, S. Mellor, O. Seaman, J. Henderson, N. Sewell, M. Sloan, Material characterisation and process development for chocolate additive layer manufacturing, Virtual Phys. Prototyp. 5 (2) (2010) 57e64. [32] Y.Y. Liu, H.C. Yu, Y. Liu, G. Liang, Q.X. Hu, Dual drug spatiotemporal release from functional gradient scaffolds prepared using 3D bioprinting and electrospinning, Polym. Eng. Sci. 56 (2) (2016) 170e177. [33] J.E. Trachtenberg, J.K. Placone, B.T. Smith, J.P. Fisher, A.G. Mikos, Extrusionbased 3D printing of poly(propylene fumarate) scaffolds with hydroxyapatite gradients, J. Biomater. Sci. Polym. Ed. 28 (6) (2017) 23. [34] Z. Liu, M. Zhang, C.-h Yang, Dual extrusion 3D printing of mashed potatoes/ strawberry juice gel, LWT - Food Sci. Technol. (Lebensmittel-Wissenschaft -Technol.) 96 (2018) 589e596. [35] L.E. Visscher, H.P. Dang, M. Knackstedt, D.W. Hutmacher, P.A. Tran, Novel 3D printed Polycaprolactone scaffolds with dual macro-microporosity for applications in local delivery of antibiotics, Mat. Sci. Eng. C-Mater 87 (2018) 78. [36] L. Ren, Z. Song, H. Liu, Q. Han, C. Zhao, B. Derby, Q. Liu, L. Ren, 3D printing of materials with spatially non-linearly varying properties, Mater. Des. 156 (2018) 470e479. [37] A.P. Haring, A.U. Khan, G. Liu, B.N. Johnson, 3D printed functionally graded plasmonic constructs, Adv. Opt. Mater. 5 (18) (2017), 1700367. [38] G. Fei, Z. Xu, Q. Liang, L. Bo, W. Liu, Direct 3D printing of high strength biohybrid gradient hydrogel scaffolds for efficient repair of osteochondral defect, Adv. Funct. Mater. (2018), 1706644. [39] D. Kokkinis, F. Bouville, A.R. Studart, 3D printing of materials with tunable failure via bioinspired mechanical gradients, Adv. Mater 30 (19) (2018), 1705808. [40] S. Sultan, A.P. Mathew, 3D printed scaffolds with gradient porosity based on a cellulose nanocrystal hydrogel, Nanoscale 10 (2018) 4421e4431. [41] J. Giannatsis, A. Vassilakos, V. Canellidis, V. Dedoussis, Fabrication of graded structures by extrusion 3D Printing, in: 2015 IEEE International Conference on

[42]

[43]

[44] [45]

[46]

[47]

[48]

[49]

[50] [51]

[52] [53]

[54]

[55]

[56]

[57]

Industrial Engineering and Engineering Management (IEEM), 2015, pp. 175e179. S.E. Bakarich, R.G. Iii, R. Gately, S. Naficy, M.I.H. Panhuis, G.M. Spinks, 3D printing of tough hydrogel composites with spatially varying materials properties, Addit. Manuf. 14 (2017) 24e30. J. Wang, L.L. Shaw, Rheological and extrusion behavior of dental porcelain slurries for rapid prototyping applications, Mat. Sci. Eng. A-struct 397 (1) (2005) 314e321. S. Khalil, W. Sun, Biopolymer deposition for freeform fabrication of hydrogel tissue constructs, Mat. Sci. Eng. C-bio. S 27 (3) (2007) 469e478. W.G. Fahrenholtz, G.E. Hilmas, S.C. Zhang, S. Zhu, Pressureless sintering of Zirconium diboride: particle size and additive effects, J. Am. Ceram. Soc. 91 (5) (2010) 1398e1404. M. Rahimian, N. Ehsani, N. Parvin, H.R. Baharvandi, The effect of particle size, sintering temperature and sintering time on the properties of AleAl 2 O 3 composites, made by powder metallurgy, J. Mater. Process. Technol. 209 (14) (2009) 5387e5393. X. Guo, Z. Zhou, S. Wang, S. Zhao, Q. Zhang, G. Ma, A novel method for preparation of interconnected pore-gradient ceramic foams by gelcasting, J. Porous Mater. 19 (5) (2012) 853e858. A. Di Luca, A. Longoni, G. Criscenti, C. Mota, C. van Blitterswijk, L. Moroni, Toward mimicking the bone structure: design of novel hierarchical scaffolds with a tailored radial porosity gradient, Biofabrication 8 (4) (2016), 045007. Y. Javadzadeh, R. Bairami Atashgah, M. Barzegar-Jalali, F. Soleimani, G. Mohammadi, A. Sabzevari, K. Adibkia, Inclusion of piroxicam in mesoporous phosphate glasseceramic and evaluation of the physiochemical characteristics, Colloids Surf., B 116 (2014) 751e756. I.W. Chen, Mobility control of ceramic grain boundaries and interfaces, Mat. Sci. Eng. A-Struct. 166 (1993) 51e58. M.A. Al-Ghouti, M.A.M. Khraisheh, S.J. Allen, M.N. Ahmad, The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth, J. Environ. Manag. 69 (3) (2003) 229e238. W. Wang, A. Wang, Nanoscale clay minerals for functional ecomaterials: fabrication, applications, and future trends, Springer Int. Publ. (2018) 1e82. A. Biswas, T.C. Ovaert, C. Slaboch, H. Zhao, I.S. Bayer, A.S. Biris, T. Wang, Mineral concentration dependent modulation of mechanical properties of bone-inspired bionanocomposite scaffold, Appl. Phys. Lett. 99 (1) (2011), 013702. P.H. Kuncoro, K. Koga, N. Satta, Y. Muto, A study on the effect of compaction on transport properties of soil gas and water. II: soil pore structure indices, Soil Till. Res. 143 (12) (2014) 180e187. J. Wang, Z. Pan, Y. Ma, Y. Lu, C. Shen, D. Cuiuri, H. Li, Characterization of wire arc additively manufactured titanium aluminide functionally graded material: microstructure, mechanical properties and oxidation behaviour, Mat. Sci. Eng. A-Struct. 734 (2018) 110e119. Ehab I. Salama, Sherry S. Morad, Amal M.K. Esawi, Fabrication and mechanical properties of aluminum-carbon nanotube functionally-graded cylinders, Materialia 7 (2019) 100351. A. Yusefi, N. Parvin, H. Mohammadi, WCu functionally graded material: low temperature fabrication and mechanical characterization, J. Phys. Chem. Solids 115 (2018) 26e35.