Influence of rice husk addition on the rheology and printability of alumina inks for direct ink writing

Abstract

Pluronic F-127 based alumina-rice husk inks with different weight % of rice husk (1, 3, 5, 7.5, 10 wt.%) were prepared and rheological characteristic of the inks was studied. The concentration of rice husk in the solid content of the ink was 1.5%, 4.6%, 7.7%, 11.5%, and 15.4%. The gelation temperature was observed to decrease with an increase in rice husk wt.% from 23 °C to 7 °C. The XRD of the sintered sample showed the presence of quartz and nepheline derived from rice husk and alumina. The microstructure of sintered alumina showed that porosity increased with increasing rice husk content. The relative density of the samples sintered at 1350 °C and 1600 °C was observed to be between 80.6% and 62.5%, and between 83.8% and 76.3%. This study shows that the microstructure of 3D-printed parts can be tailored by adding rice husk to alumina-PF127 inks by direct ink writing.

Keywords

alumina rice husk Pluronic F127 (PF127)direct ink writing rheology

Introduction

The addition of additives in ceramic plays a crucial role in the processing and properties of ceramic materials. Additives such as magnesium oxide (MgO) or yttrium oxide (Y₂O₃) are used to lower the sintering temperature and promote densification during the sintering process.^1,2 Certain additives, such as cerium oxide (CeO₂) or zirconium dioxide (ZrO₂), can inhibit excessive grain growth, leading to finer and more uniform microstructures.^3,4 Additives can also stabilize specific crystalline phases, improving the mechanical and thermal properties of the ceramic.⁵ Additives can be used to control the porosity of ceramics, either by acting as pore formers or by modifying the sintering behavior to promote porosity.⁶

Recently, additive manufacturing, specifically, direct ink writing (DIW), is being increasingly used to manufacture ceramic products due to its inherent advantages such as the use of inks consisting of ceramic particles in suitable binder.^7–9 Also, porous ceramics structures with intricate shapes are being obtained by suitably altering the process of DIW. Porous ceramics can be obtained through several methods, such as hierarchical porous structure by altering the infill density,^10,11 varying the solid loading in inks.^7,12 Different pore formers are used for obtaining porous structures by DIW. Hollow microspheres with polyvinyl alcohol (PVA) as binder were used as pore former in alumina for achieving porous alumina structure with 50%–90% porosity.¹³ Polymethyl methacrylate (PMMA) microspheres were used as sacrificial pore former^14,15 in alumina for achieving porous alumina structure where the binder was used PVA. The open porosity was found to be in the range of 58.2% ± 0.1%.¹⁶ Emulsion templating,^17,18 using foam-based inks,¹⁹ DIW combined with freeze-drying,^20,21 geopolymer mediated porous DIW,^22,23 multi-material and core shell printing^24,25 are also methods reported for obtaining porous structures. Post-processing techniques such as infiltration and partial densification were also used.²⁶ Low-density open porous alumina structures with relative density of 38% were obtained by PVA based alumina inks without using pore forming agents.²⁷

Porous alumina (Al₂O₃) has a wide range of applications due to its high temperature stability, chemical resistance, and mechanical strength. Some key applications include catalyst supports, filtration, biomedical application, insulation, and gas sensor.^28–33 In recent years, there has been a growing interest in incorporating sustainable and renewable materials into ceramic processing. Rice husk, an abundant agricultural byproduct, contains a high proportion of silica and has been explored as a pore-forming agent in ceramics.³⁴ Rice husk is also added in ceramics as a sintering aid and also for obtaining porous structures as natural pore former.^35,36 The addition of rice husk to alumina inks and subsequent printing and sintering can induce microstructural changes in sintered products. In addition, it will also result in alumina Al₂O₃–SiO₂ composite material after sintering as rice husk contains 10%–20 wt.% silica.³⁷

However, understanding the effect of rice husk addition on the rheological aspects of alumina inks is essential for optimizing the DIW process and achieving microstructural variation. The interplay between the rice husk particles, alumina, and binder system must be carefully studied to ensure consistent printability and desired material properties. This study aims to investigate the changes in viscosity, shear-thinning behavior, and viscoelastic properties of Pluronic F 127 (PF127) based alumina inks with varying amounts of rice husk for DIW applications.

Materials and methods

Alumina (99.9%, Rohini Industries) with a particle size of D₅₀ 0.46 µm was used. PF127 (Sigma Aldrich) was used as a thermos responsive binder in the ink formulation due to its excellent gelation behavior, as well as its biocompatible and biodegradable nature, which makes it suitable for environmentally friendly and potential biomedical applications. Hydrogel-based inks were prepared using PF127 and distilled water as the solvent. Ammonium poly acrylate (APA) was used as a dispersant in the inks. At first, 26 g of PF127 was dissolved in 74 ml of DI water to prepare the hydrogel solution. For the preparation of alumina inks 65 wt.% of alumina was mixed with 35 wt.% PF127 solution thoroughly in pot mill for obtaining homogeneous ink. To this ink, rice husk was added. Thus, the solid loading consisted of alumina and rice husk. The rice husk used was washed thoroughly, grinded and sieved through 45 µm sieve. The sieved rice husk was mixed with alumina in five variations using a pot milling machine, as shown in Table 1. A 0.32 gram of ammonium polyacrylate was added to this mixture and rigorously stirred to obtain a homogeneous ink. The relative ratios of alumina and rice husk were changed progressively by increasing the wt.% of rice husk and simultaneously decreasing wt.% alumina. The total solid loading was kept constant at 65 wt.%. The rheological analysis (viscosity, gelation transition, and thixotropy test) of the prepared ink was measured by MCR 72 rheometer (Anton Paar, Austria) using parallel plate of 25 mm diameter with 1 mm gap. TGA/DTA (Parkin Elmer, Japan) analysis was used for obtaining the debinding temperature. The DIW of the prepared ink was deposited by a customized 3D printer. The nozzle diameter used for printing was 2 mm. The extrusion was done using a pneumatic assembly, and the extrusion pressure was set to 5 bar. The prepared 3D models were converted by Ultimaker Cura 4.10.0 software to make them compatible for printing. The printing bed temperature was 50 °C. The sample designation used in this study is given in Table 1.

Table 1.

Ratio of alumina and rice husk.

Sample ID	Alumina (wt.%)	Raw rice husk powder (wt.%)	PF127 hydrogel (wt.%)	% of rice husk in solid loading of the ink
S0	65	-	35	-
S1	64	1	35	1.5
S2	62	3	35	4.6
S3	60	5	35	7.7
S4	57.5	7.5	35	11.5
S5	55	10	35	15.4

Results and discussion

Rheological characterization

The rheological properties of the inks must meet specific criteria to be suitable for the DIW process. The ink should be easily extrudable through a fine nozzle to produce continuous filaments without clogging. Once deposited, these filaments must retain their shape, accurately follow the printing path, and support the stacking of successive layers. The flow behavior of the ink (S1–S5) is shown in Figure 1(a). From the flow curve of viscosity vs shear rate, it is observed that with increasing shear rate, the viscosity decreases. This shows the shear-thinning behavior of the ink, which is essential for the smooth flow of the ink through the nozzle.

Figure 1.

For samples S1–S5 (a) viscosity versus shear rate of alumina rice husk inks (b) viscosity versus temperature of the inks (c) three interval thixotropy (3ITT) of alumina rice husk inks S1–S5 (d) initial and final elastic recovery from 3ITT test of inks S1–S5.

The viscosity versus temperature of samples S1 to S5 from 5 °C to 30 °C is shown in Figure 1(b). For comparison data of PF127 and PF127 based alumina ink (S0) is also shown to understand the effect of the rice husk addition. For PF127 hydrogel, the gelation temperature is observed to be at 16 °C. For PF127 based alumina ink (S0), the gelation temperature is observed to be at 12 °C. The addition of alumina to PF127 hydrogel has decreased the gelation temperature from 16 °C to 12 °C, with increase in viscosity.

It is observed that in the ink with 1 wt.% rice husk-loaded alumina (S1), the final viscosity was lower than that of PF127 hydrogel. The increase in the viscosity was observed at 23 °C which is the gelation transition temperature. For the ink S2, increase in viscosity was observed at 20 °C, which is the gelation transition. When the rice husk wt.% increases to 5 wt.% (S3) the gelation transition temperature of the ink increased to 25 °C. Further increase in the rice husk in the inks for S4 and S5, a change in the viscosity profile is observed wherein a sudden increase in viscosity is observed after gelation temperature. The viscosity of S4, is lower than that of S0, with a gelation transition temperature of 10 °C. For the sample S5, the viscosity was observed to be high among all the inks, with a gelation transition at 7 °C. From 7 °C onwards the viscosity increases sharply up to a temperature of 17 °C. However, from 17 °C to 30 °C, not much change in viscosity was observed. The sharp increase in viscosity of samples S1 to S5 with temperature shows the onset of gelation. With increasing concentration of rice husk in the inks the gelation temperature was observed to decrease from 23 °C to 7 °C, except for sample S3 which shows the initiation of gelation at 25 °C. Also, it is observed that for samples S1–S4, the viscosity was observed to be lower than S0, except for sample S5 for which the viscosity was observed to be higher. Thus, it is observed that the addition of rice husk and the relative ratio of alumina and rice husk in the solid loading significantly influence the viscosity and gelation transition temperature of the inks. Compared to S0, the gelation temperature was observed to increase with an increase in rice husk content for S1, S2, and S3. However, with further increase in rice husk content for S4 and S5, the gelation temperature decreased compared to S0. The variation in viscosity and gelation temperature with rice husk addition is attributed to the interaction between Pluronic F-127 micellar networks and the solid particles (alumina and rice husk). At lower rice husk concentrations, the particles interfere with the packing of PF127 micelles, leading to slight changes in gelation behavior and reduced viscosity compared to the pure hydrogel. With increasing rice husk content, particle–particle interactions and water absorption by the silica-rich rice husk promote the formation of a stronger particle network, resulting in earlier gelation and increased viscosity. These effects enable better control over ink rheology, which improves extrusion stability, filament shape retention, and overall printability in DIW of alumina structures.

In order to understand the behavior of the inks (S1–S5) by DIW which is subjected to rest-extrusion-rest conditions of ink during printing, a three interval thixotropy test (3ITT) was performed. Figure 1(c), shows the 3ITT test of S1–S5 at 23 °C using parallel plate geometry and an oscillation-oscillation-oscillation profile. Region-I shows the samples subjected to an oscillation with frequency of 1 Hz with shear strain of 0.002%, Region-II shows the sample subjected to higher shear strain 5% with frequency of 1 Hz. Region-III shows the sample with a frequency 1 Hz and shear strain 0.002%. From 3ITT test, it is observed that the storage modulus in Region-I is higher than that of Region-III for the entire sample S1–S5. In the Region-II the storage modulus for all the inks is constant. The initial percentage recovery of the inks is calculated by dividing storage modulus at starting of Region-III (SM_3I) to final storage modulus at the end point of Region-I (SM_I) shown in Figure 1(c) (initial recovery% = (SM_3I/SM_I) × 100).³⁸ The final recovery percentage of the ink is calculated by dividing storage modulus at the final point of Region-III (SM_3F) with storage modulus of end point of Region-I (SM_I), as shown in Figure 1(c) (final recovery % = (SM_3F/SM_I) × 100). At the end of the Region-I, for the sample S1, the storage modulus was 3.3 × 10⁵ Pa. With the increase of shear force the storage modulus drastically decreased to 1.8 × 10⁴ Pa in Region-II and when the shear force is released the ink starts to recover due to its elastic properties in Region-III. For sample S1 the initial recovery was observed to be 40% and by the end of the test in Region-III the final recovery was observed to be 71%. Similarly, the initial recovery for the sample S2, S3, S4, and S5 is observed to 38%, 20%, 32%, 46% and the final recovery of the ink was observed to be 71%, 75%, 36%, 62%, 58% respectively as shown in Figure 1(d). From the above observation, it is found that the initial elastic recovery of the inks decreased from 40% for S1 to 20% for S3. Thereafter, the elastic recovery is observed to increase up to 46% for S5. For final elastic recovery it is also observed that sample S3 shows minimum elastic recovery of 36% compared to all other samples which show higher elastic recovery of greater than 58%. The maximum recovery of 75% was observed for the sample S2.

Further, the amplitude sweep tests were conducted to assess the viscoelastic properties of the formulated inks and their suitability for DIW. The variation of storage modulus (G′), loss modulus (G″), and shear stress as a function of shear strain for the sample S1 and S5 only is shown in Figure 2(a) and (b). At low shear strain, all samples exhibited a linear viscoelastic region (LVR) up to Point-1, where G′ and G″ remain constant, indicating elastic behavior and structural integrity under minor deformations. The end of the LVR, marked as Point-1, represents the yield point, where G′ begins to decrease, indicating the onset of internal network breakdown. As the strain increases further, G′ intersects G″ at Point-2, denoting the flow point, where the material transitions from predominantly elastic to viscous behavior. The shear stresses corresponding to these points define the yield stress (τ_γ) and flow stress (τ_f), which are critical for understanding extrudability and post-print stability in DIW processes.

Figure 2.

(a) Amplitude sweep test sample S1 (1 wt.% rice husk + 64 wt.% alumina), point 1 yield point, point 2 flow point (b) amplitude sweep test sample S5 (10 wt.% rice husk + 55 wt.% alumina), point 1 yield point, point 2 flow point (c) yield stress (τ_γ) and flow stress (τ_f) values of sample S1 to S5 in bar graph.

Figure 2(c) shows a comparative chart of τ_γ, τ_f, for samples S1–S5. It is observed that the yield stress and flow stress of sample S1 are 376 and 1330 Pa. When the rice husk increases to 3 wt.% (sample S2) the yield stress and flow stress decrease to 352 and 1071 Pa. For sample S3 the yield stress and flow stress increased to 445 and 1348 Pa. For sample S4 the yield stress and flow stress further increased to 517 and 2845 Pa. As the amount of rice husk increases for sample S5, the yield stress and flow stress decreased to 495 and 1228 Pa. From the yield stress and flow stress value peak for sample S4 with increase to highest value. This suggests that the interplay between rice husk and alumina content enables tunability of viscoelastic properties, where rice husk reinforces the network while alumina controls flow resistance.

TGA of rice husk alumina Pluronic F127 inks

The thermogravimetric analysis (TGA) of the prepared inks was carried out for all samples S1–S5, as shown in Figure 3. The TGA, shows the % weight loss with temperature. Three distinct regions can be seen. In Region-I, there is a gradual weight loss up to 180 °C. In Region-II, from 180 °C to 400 °C, the weight loss is high. From temperature 400 °C onwards, the weight loss percentage is constant for samples S1 to S4, whereas for S5 weight loss percentage showed a gradual decrease. In the Region-II, S1 and S3 showed a similar decrease in terms of weight loss. For the samples S4 and S5, the weight loss begins at higher temperature and increases rapidly with temperature. For sample S2 however, the weight loss was observed to be initiated at lower temperatures. As all the samples contained rice husk and PF127, the weight loss is attributed to the decomposition of PF127 and rice husk. Also, both the materials show similar decomposition behavior due to the presence of carbonaceous material such as cellulose, hemicelluloses, lignin, and polyoxyethylene–polyoxypropylene–polyoxyethylene (PEO–PPO–PEO) triblock copolymers present in rice husk and PF127 respectively.^39,40

Figure 3.

TGA of all samples S1–S5.

DIW of alumina rice husk inks

The customized DIW system was used for DIW of the rice husk alumina ink. The system was a converted FDM printer with pneumatic system driven by compressed air for ink delivery. Ink discharge pressure was controlled using an air regulator. The ink was loaded into a syringe assembly connected to a 6 mm diameter polyurethane tube, with the other end of the tube attached to a 2 mm nozzle. In DIW, the rule-of-thumb for nozzle size and particle size is 10:1.⁴¹ As we have used 2 mm nozzle for printing, this would correspond to a minimum particle size of 200 microns. About 45 microns is significantly less than 200 μm. This will avoid clogging of the nozzle, ensure proper dispersion in alumina ink, and there is always sufficient alumina surrounding the rice husk. Moreover, this will not lead to any cross-sectional cracking of the samples due to rice husk in layered printing by DIW. A model of hexagon has been made, and the sliced STL file is shown in Figure 4(a). The dimension of the hexagon is a wall thickness of 2 mm and a height of 5 mm. The ink was deposited using a 2 mm nozzle, and the bed temperature was 50 °C. Figure 4(b), shows the printed sample using S1 ink by DIW in a hexagon shape. The printing structure largely retains the hexagonal shape. Some sagging and distortion were observed, especially at the top layer. The surface appears relatively smooth, and the line-by-line deposition layer is clearly observed. A total of five layers was observed after printing. Thus, it is observed that low rice husk content leads to relatively stable ink, but optimization of the ink with sufficient yield stress to maintain the perfect wall is still needed. The printed sample using S3 ink is shown in Figure 4(c). The sample S3 shows a more deformed structure and shrinkage of the green body from the actual model dimensions. Significant surface roughness and collapse around the edge are also observed on the sample surface. Figure 4(d), shows the ink S5 with 10 wt.% rice husk. From the printed sample, it is observed that the structure looks smoother compared to that of S3. Although some deformation exists, the hexagonal geometry is better preserved than in sample S3 (Figure 4(c)).

Figure 4.

DIW of hexagon with alumina rice husk ink (a) STL file of the printed hexagon (b) S1 (c) S3 (d) S5.

Sintering of printed rice husk alumina samples

The printed alumina rice husk inks deposited by customized 3D printer were sintered in atmospheric conditions. The TGA (Figure 3) shows that there is significant weight loss in the temperature region of 180–400 °C. For debinding of P127, a two stage debinding step was followed with the first debinding carried out at 180 °C for 30 min and then at 400 °C for 30 min. Thereafter the temperature was increased to 600 °C and hold 30 min. From 600 °C, the temperature was increased to 1600 °C. The sintering was carried out at 1600 °C for 90 min. The XRD analysis of sintered samples S1–S5 was carried out to analyze the effect of rice husk addition on phase formation of alumina. From the XRD, Figure 5(a), the higher intensity peaks identified with star (*) in the graph are for alumina,⁴² which matches with the JCPDS data card no: 96-900-8095. The low intensity peaks with black triangle (▴) in the graph Figure 5(a) is identified with quartz and is matching with JCPDS data card no: 96-901-2601. Interestingly, numerous low intensity peaks are identified with JCPDS data card no: 96-901-3313 which belongs to nepheline as shown by black square (▪). A shift in the most intense peak position of alumina was observed with increasing wt.% of rice husk as shown in the insert of Figure 5(a). Although the quartz and nepheline phases were observed in all the sintered samples, the intensity of the peaks is higher in the sintered sample S2. The nepheline peak at 10.25°, 17.73°, 21.24°, 27.20°, 30.93°, and 38.37° are only observed for sample S2 which is shown in the Figure 5(b). It is reported that the intense peaks of nepheline phase are obtained at a sintering temperature of 700 °C to 1000 °C and the peaks at 10.25°, 17.73°, 21.24° are observed in the temperature range of 800 °C to 900 °C.⁴³ Thus, from the XRD analysis it is found that the addition of rice husk into the alumina results in the formation of quartz and nepheline phases after sintering at 1600 °C. For S2, with 3 wt.% rice husk, resulted in the additional peaks of the phase nepheline compared to other samples.

Figure 5.

XRD graph of sintered sample S1, S2, S3, S4, S5 (a) identification of all peak (b) zoom in picture of smaller peak.

The SEM microstructure of the as printed samples (Figure 6(a), (d), and (g)), surface of sintered samples at 1350 °C for 5 h (Figure 6(b), (e), and (h)) and fracture surface of samples sintered at 1600 °C for 3 h (Figure 6(c), (f), and (i)) is shown for samples S1, S3, and S5. From Figure 6(a), (d), and (g), it is observed that alumina particles are uniformly packed and not much distinction has been observed from the as printed SEM microstructure. For samples sintered at 1350 °C, Figure 6(b), shows significant grain growth with rounded grains with clear grain boundaries, suggesting onset of grain growth for S1. Figure 6(e), for sample S3, shows needle shaped grains possibly due to the influence of rice husk derived silica on crystal growth direction indicating anisotropic grain growth. Figure 6(h), for sample S5, irregular grain growth is observed. Presence of small pores can also be observed showing that the rice husk in the inks can cause porosities after sintering. From Figure 6(c), for sample S1, the fracture surface shows porosity. The porosities are because of rice husk which burns to remove the cellulose materials retaining only SiO₂ at elevated temperatures as shown in XRD from Figure 5. From Figure 6(f), for sample S3, again, there is the presence of significant number of porosities from the fracture surface microstructure. This shows thermal degradation of organics and evolution of gases is leading to poor densification. From Figure 6(i), for sample S5, a highly porous microstructure is obtained. It is also observed that as the rice husk content increases in samples S1, S3, and S5, the porosity in the sintered samples also increases, showing that it is possible to prepare porous alumina structures by incorporating rice husk in alumina inks by DIW.

Figure 6.

SEM microstructure (a) surface of as printed sample S1 (b) surface of sample S1 sintered at 1350 °C (c) fracture surface sintered at 1600 °C for sample S1(d) surface of as printed sample (S3) with 5 wt.% rice husk (e) surface of sample S3 sintered at 1350 °C (f) fracture surface of sample S3 sintered at 1600 °C (g) surface of as printed sample S5 (h) surface of sample S5 sintered at 1350 °C (i) fracture surface of sample S5 sintered at 1600 °C.

For density measurements, two sets of samples were extruded from a syringe with 2 mm nozzle. Table 2 shows the density of the samples sintered at 1350 °C and 1600 °C measured by Archimedes principle. The relative density of the samples sintered at 1350 °C decreased from 80.6% for sample S1 to 62.5% for sample S5. The relative density of samples sintered at 1600 °C decreased from 83.8% to 76.3%. The density of the samples sintered at 1600 °C was higher than the samples sintered at 1350 °C. The difference in relative density of samples S1 and S5 for samples sintered at 1350 °C is 18% and for samples sintered at 1600 °C is 7.5%. This shows that the variation in density sintered at 1600 °C is much lower than that of samples sintered at 1350 °C. It is also observed that for the sample S1, the difference in density of samples sintered at 1350 °C and 1600 °C is 3.2% whereas the density difference increases to 13.8% for sample S5.

Table 2.

Density of the samples sintered at 1350 °C and 1600 °C.

Sample	% density of samples sintered at 1350 °C	% porosity of samples sintered at 1350 °C	% density of samples sintered at 1600 °C	% porosity of samples sintered at 1600 °C
S1	80.6	19.4	83.8	16.2
S2	72.3	27.7	79.3	20.7
S3	69.3	30.7	78.8	21.2
S4	65.5	34.5	77.8	22.2
S5	62.5	37.5	76.3	23.7

Rice husk is a biodegradable porogenic material with many advantages over other porogenic materials such as coconut shells, coconut fiber, groundnut shells and camphor. Among these materials rich husk has high mineral content rich in silica.⁴⁴ This offers many advantages for addition of rice husk to alumina as a porogenic material, as the burning of rice husk leaves porosities along with dispersion of silica. Also, grinding rice husk to different sizes can yield porous structures with varying pore sizes. The addition of silica to alumina will result in the formation of mullite at high temperatures.⁴⁵ For the samples S1–S5, Table 1 shows the concentration of the rice husk, wherein the concentration of the rice husk in S1 is 1.5 wt.% and the concentration of rice husk in S5 is 15.4 wt.%. With 19.6 wt.% of silica in rice husk (measured), this corresponds to a concentration of silica at 0.3 wt.% for S1 and 3.5 wt.% for S5 after sintering. The XRD shows the peaks of quartz and no mullite peaks are observed (Figure 5). This shows that silica is dispersed in alumina matrix. Silica has been added to alumina of 1–2 wt.% as a sintering aid to promote sintering of alumina.⁴⁶ Also, when dispersed in alumina and no mullite formation is observed, silica is diffused to grain boundary and leads to grain refinement.⁴⁷ This shows the advantages of addition of rice husk as pore former in alumina compared to other porogenic agents.

The variations in % porosity with rice husk additions to alumina as reported by researchers are shown in Table 3. It can be observed that the overall % porosity varies between 29.7% and 53%. For 5 wt.% rice husk addition, the % porosity varied between 29.7% and 46% whereas for 10 wt.% rice husk addition, the % porosity varied between 43% and 53%. For 15 wt.% rice husk the % porosity observed was 24% and for 20 wt.% rice husk addition, the % porosity was observed between 39% and 48%.

Table 3.

Rice husk additions to alumina and % porosity values.

Researcher	Materials	Process	Sintering temperature (°C)	Porosity (%)
Dele-Afolabi et al.⁴⁸	Alumina, Rice husk (<63 µm) and Sucrose solution as binder	Uniaxial pressing	1450 °C for 2 h	44% for 5 wt.% rice husks, 48% for 10 wt.% rice husks
Dele-Afolabi et al.⁴⁸	Alumina, Rice husk (63–125 µm) and Sucrose solution as binder	Uniaxial pressing	1450 °C for 2 h	46% for 5 wt.% rice husks, 53% for 10 wt.% rice husks
Ribeiro et al.⁴⁹	Alumina, Rice husk (<65 µm)	Uniaxial pressing	1450 °C for 1 h	29.7% for 5 wt.% rice husks, 24.1% for 15 wt.% rice husk
	Alumina, Rice husk (65–150 µm)	Uniaxial pressing	1450 °C for 1 h	31.7% for 5 wt.% rice husk
Jingjing et al.⁵⁰	Alumina, Carbonized rice husk, and PVA solution as binder	Uniaxial pressing	1600 °C for 2 h	42% for 5 wt.% carbonized rice husks, 48% for 10 wt.% carbonized rice husks
Ajay et al.⁵¹	Alumina, Rice husk (<75 µm), sucrose solution as binder	Uniaxial pressing	1700 °C for 2 h	39% porosity for 20 wt.% rice husks
Mohammed et al.⁵²	Alumina, Rice husk ash, sucrose solution as binder	Uniaxial pressing	1600 °C for 2 h	43% overall porosity for 10 wt.% rice husk ash, 48% for 20 wt.% rice husk ash content

A comparison of the porosity values from Table 3 with the present work is shown in Figure 7 for 5 wt.% and 10 wt.% rice husk. It can be observed that the porosity values with 5 wt.% rice husk addition sintered at 1350 °C is comparable to the values obtained by Ribeiro et al.⁴⁹ whereas for 10 wt.% rice husk the % porosity is very low. For 5 wt.% (S3) and 10 wt.% (S5) rice husk addition of samples sintered at 1600 °C has lower porosity values. In general, the % porosity values obtained in the present work are lower than those of the comparable conditions as shown in Table 3 and Figure 7.

Figure 7.

Comparison of porosity values with 5 and 10 wt.% rice husk.

Comparing the results for PF127 inks for samples S1–S5, the gelation temperature decreased with increasing rice husk and sample S5 showed marked gelation with increased viscosity compared to all other samples (Figure 1(a)). For printing, the S5 ink will be useful as the gelation temperature decreased compared to S0, with increased bed temperature allowing gelation faster, thereby enabling shape retention compared to other samples. This is particularly useful when bigger structures are to be printed. Also, from 3ITT test, the initial recovery of the S5 is much higher than all other samples (Figure 1(c)). This recovery aspect is also useful for shape retention. The yield stress and flow stress values are also within the limits for DIW applications.⁵³ Hence, sample S5 shows the established norms for DIW applications such as a marked gelation behavior, good recovery.

The % porosity of the sample S5 after sintering at 1350 °C is 37.5% and it decreased to 23.7% after sintering at 1600 °C. This shows a decrease of about 13.8%. For samples obtained by conventional compaction followed by sintering, the % porosity value varied in the range of 40%-55% as shown in Figure 7. This shows that by suitably changing the rice husk particle size, concentrations, and sintering temperature, the microstructure of alumina can be tailored to achieve suitable strength-weight ratio for desired applications. For obtaining more % porosity rice husk particle size greater than 45 microns will be useful along with sintering temperatures of 1350 °C-1450 °C. For obtaining denser structures sintering temperature of 1600 °C or more can be used with rice husk particle size of 45 microns.

Conclusion

PF127-based alumina ink with rice husk addition was successfully prepared, and its rheological characterization was carried out. The alumina-rice husk inks demonstrated a shear-thinning behavior, with viscosity decreasing as shear rate increased. A significant effect of temperature on viscosity was observed with a distinct behavior compared to PF127 hydrogel and pure alumina ink. The inks with rice husk exhibited lower viscosity than the pure alumina ink with the ink with 10 wt.% rice husk showing an exception with a higher viscosity. The addition of rice husk decreased the gelation temperature significantly from 27 °C to 7 °C as the rice husk content increased from 1 wt.% to 10 wt.%. 3ITT (three-interval thixotropy test) results showed initial recovery in the range of 32%–46% for inks with 1, 3, 7.5, and 10 wt.% rice husk, while the 5 wt.% sample showed a lower initial recovery of 20%. Final recovery values ranged from 75% to 58% with the 5 wt.% sample again showing reduced recovery at 37%. Amplitude sweep tests further revealed that both yield stress (τ_γ) and flow stress (τ_f) increased with rising rice husk content peaking in the sample with 7.5 wt.% rice husk, which displayed the highest yield stress (τ_γ) of 512 Pa and flow stress(τ_f) of 2845 Pa. TGA studies have shown that significant weight loss between 180 °C and 400 °C due to the decomposition of rice husk and PF127. XRD analysis of the sintered samples confirmed the presence of nepheline, quartz, and alumina. SEM microstructures showed a porous structure with porosities increasing with increased amount of rice husk.

The % porosity of the samples after sintering at 1350 °C was observed to be between 19.4% and 37.5%, and the % porosity of the samples sintered at 1600 °C was observed to be between 16.2% and 23.7% for samples with 1 wt.% to 10 wt.% rice husk. The % porosity was higher for samples sintered at 1350 °C. The % porosity was in a narrower range for sample sintered at 1600 °C. Higher porosity can be reached by sintering at 1350 °C, however if a narrower range with tighter control on porosity is desirable sintering has to be done at 1600 °C. The results show that the porosity in the sintered alumina can be varied by incorporating rice husk in DIW. Further studies can be undertaken for modifying the structures by changing the particle size of rice husk and varying the solid loading of alumina in the inks.

Footnotes

Acknowledgment

The authors would like to thank Anusandhan National Research Foundation (Grant no- CRG/2021/003423) in carrying out the research work.

ORCID iD

Debabrata Majumder

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Anusandhan National Research Foundation (ANRF), (grant number CRG/2021/003423).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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