Low-Temperature Sintering of Porous Ceramics via Sodium Borate Addition

In the current study, sodium borate−bonded highly open porous ceramics successfully produced by starch consolidation technique. Open porous ceramic production was carried out by using an economical grade α−Si3N4, corn starch, CC31 commercial−grade kaolin, and borax decahydrate (Na2B4O7.10H2O). Borax decahydrate was used as a sintering aid in the system and total ceramic (α−Si3N4 + CC31):borax decahydrate ratio was kept constant at 5:1. Sintering studies of the shaped samples carried out in an air atmosphere at a relatively low sintering temperature, 1100oC, for one hour. Scanning electron microscopy investigations of the porous ceramic samples revealed that due to the high amount of borax based sintering additive a significant amount of liquid phase formed during the sintering process of the designed ceramics. Highly open porous (∼66 −74%) and lightweight (∼0.64 − 0.83 g/cm3) ceramics were produced via starch consolidation technique and low−temperature sintering at atmospheric conditions. Material Science Research India www.materialsciencejournal.org ISSN: 0973-3469, Vol.16, No.(1) 2019, Pg. 48-55 CONTACT Ayse Kalemtas ayse.kalemtas@btu.edu.tr Department of Metallurgical and Materials Engineering, Bursa Technical University, 16310 Bursa, Turkey. © 2019 The Author(s). Published by Oriental Scientific Publishing Company This is an Open Access article licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License Doi: http://dx.doi.org/10.13005/msri/160107 Article History Received: 19 December 2018 Accepted: 24 January 2018


Introduction
][3][4][5][6][7][8][9][10] Porous ceramic materials exhibit many distinct characteristics when compared with the metals and polymers, such as high hardness, specific surface area, chemical inertness, wear resistance, thermal shock resistance, corrosion resistance, excellent mechanical and chemical stability at elevated temperatures, low thermal expansion coefficient, and low density. 1,11,12ue to these characteristics properties porous ceramic materials are promising materials for    various functional and structural applications.The characteristics of the pores such as pore size and morphology are key factors that determines applications of these porous materials.The potential applications of porous ceramics are separation materials, 13 sensors, 14 catalyst supports, [15][16][17] filtration of fluids, 18 heat insulating 19 and implantable bioceramics, [20][21][22] etc.Some of the conventional processing methods used for the fabrication of porous ceramics are freeze casting, 20,[22][23][24] direct foaming method, [25][26][27][28] starch consolidation, [29][30][31][32][33] polymeric sponge method, [34][35][36][37] and adding pore former agents (such as starch, phosphoric acid, and carbon) [38][39][40][41] and gel−casting 42,43 method.All these processing methods have its advantages and limitations.Among these techniques, starch consolidation method is a novel promising near net−shaping and cost−effective method used for the production of various porous ceramic materials. 29,32Furthermore, starch consolidation does not require any specialized equipment, and it is relatively a simple processing method.This method provides a very systematic approach to control the microstructural development of the porous ceramics by designing the content of the ceramic slurry including the solid loading ratio, amount and type of the used starch, and by the rate of sintering achieved during the sintering step.[46][47][48][49][50][51] The objective of this study was to produce highly porous ceramics via starch consolidation technique.
In this study, starch was used for both as pore former and gelling agent.XRD pattern of the used CC31 commercial -grade kaolin powder is given in Fig. 2. XRD analysis result of the CC31 showed mainly a typical diffraction pattern of kaolinite phase with some illite and free quartz as an impurity.
Designed compositions (Table 1) and deionized water mixed via wet milling process by using a planetary ball mill to achieve a homogenous slurry.
Prepared ceramic slurries poured into plastic molds and then placed into an oven that was 80 0 C. One hour dwell time was applied to achieve gelatinization of the starch content of the samples.After the gelatinization process, obtained green samples dried at room temperature for at least 24 hours.
A simultaneous thermogravimetry and differential thermal analyzer (TG/DTA, Netzsch STA 449F3) was used to determine binder burn out process conditions for the starch removal process.Investigations were done by TG/DTA in an air atmosphere at a constant heating rate of 10 0 C/minute.It was determined that the corn starch was removed entirely from the structure just below 560 0 C (Fig. 3).
Binder burn out studies of the ceramic green bodies were done at 650 0 C for one hour under an air atmosphere.Applied binder burn out process heating rate was 1 0 C/minute.Sintering studies of the prepared samples carried out with a heating rate of 5 0 C/minute up to 1100 0 C under air atmosphere.Archimedes displacement method used to determine bulk density and open porosity amount of the fabricated porous ceramic samples.Phase content of the samples was determined via X-ray diffraction (XRD) analysis.XRD investigations were performed by using monochromatic Cu-K α radiation.Before XRD analysis, sintered porous ceramic samples crushed and sieved down to<63 μm.Starting materials and fracture surface of the sintered porous ceramic samples investigated by using scanning electron microscopy (SEM).SEM micrographs of the used corn starch are given in Fig. 4. It was determined that the average particle size of the corn starch particles was approximately 15 -20 μm (Fig. 4).

Results and Discussion
Bulk density and open porosity measurement results of the sintered ceramics are given in Table 2. Depending on the increased starch content of the samples, open porosity amount of the samples slightly increased, whereas the bulk density of the samples decreased.It was determined that highly open porous (∼66−74%) and light-weight (∼0.64−0.83g/cm 3 ) ceramic samples were produced.
Scanning electron microscopy investigations of the sintered porous ceramic samples revealed that a three-dimensional interconnected porous network structure obtained for all samples (Fig. 5).It was observed that there were two different sizes of pores, big pores (10 -50 μm) developed by the burn out of the starch particles and fine pores (smaller than a few μm) formed between the ceramic particles (Fig. 5).Microstructural analysis revealed that a high amount of liquid phase formed in all samples during the heat treatment process.Due to the sintering process performed under atmospheric conditions, starting α−Si 3 N 4 powder can readily give a reaction with the oxygen in air and silica can form in the samples.Besides, some reactions may take place between silica and borax decahydrate sintering additive in the system, which may result in a liquid phase formation that has a low melting temperature.Phase analysis of the fabricated porous ceramic samples is given in Fig. 6.XRD results showed that cristobalite phase formed in all the samples due to the applied heat treatments under the atmospheric conditions.It was determined that starting α−Si 3 N 4 powder significantly consumed during the sintering process via reacting with the oxygen in the air atmosphere.It is believed that a liquid phase formation that has a low melting temperature took place due to borax containing sintering additive and this liquid phase formation has a considerable effect on the development of the cristobalite formation.

Table 2 : Open porosity and bulk density measurement results
Samples were pressureless sintered under the atmospheric conditions at 1100 0 C for one hour.Microstructure development, open porosity, bulk density, and phase content of the fabricated porous ceramics were investigated.Open porous ceramic production was carried out by using an economical grade α−Si 3 N 4 , corn starch, CC31 commercial -grade kaolin, and borax decahydrate (Na 2 B 4 O 7 .10H 2 O).Borax decahydrate was used as a sintering aid in the system and total ceramic (α−Si 3 N 4 + CC31):borax decahydrate ratio was kept constant at 5:1.Scanning electron Starch is a well−known readily available, environmentally friendly, lowcost polysaccharide synthesized by plants and it has relatively low burn out temperature without generation of any toxic materials.
Si 3 N 4 ceramic content of the samples considerably consumed during the sintering process via reacting with the oxygen in the air atmosphere.XRD analysis of the samples revealed that a considerable amount of cristobalite phase formed in all samples due to the sintering step under atmospheric conditions and used borax decahydrate (Na 2 B 4 O 7 .10H 2 O) sintering additive.• It was determined that highly open porous structures achieved for all the sintered ceramic samples.There were two different sizes of pores in the microstructure, big pores (10 -50 μm) developed by the burn out of the starch particles and fine pores (smaller than a few μm) formed just between the ceramic particles.