Synthesis and Characterization of Nano-Aerogels

2.4. Summary

The direct sol-gel process in SCFs simplifies the conventional aerogel preparation by using a one-pot synthesis. The crystalline formation in the high temperature autoclave makes it possible to skip the downstream calcination process. Furthermore, the direct sol-gel process in SCFs promises a new route for synthesizing inorganic and hybrid materials with exciting morphologies, such as nanoparticles, films, and mesoporous and pillared materials.

For synthesis of inorganic materials, scCO₂ is a promising solvent due to its low critical temperature, and relatively low equipment cost. Supercritical water is also very attractive due to the direct formation of the crystalline phases. Its drawbacks include high operation temperature and corrosion of the autoclave materials. The supercritical alcohols suffer from safety issues, and very strict safety systems are required in the area in which the reactor is located.

Chapter 3. Materials and Methods

3.1. Outline

In this chapter, the experimental setup used for sol-gel synthesis and characterization methods used throughout this thesis are described.

The synthesis of the nanomaterials was carried out either in a high-pressure view cell reactor or an autoclave. The high-pressure reactors were connected with CO₂ and other liquid supplies, along with having temperature and pressure controllers, and in situ spectroscopy characterization equipment.

Characterization methods of the synthesized materials include: in situ attenuate total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), powder ATR-FTIR, differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), N₂ physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and scanning transmission electron microscopy (STEM).

3. 2. Synthesis Setup

For synthesizing a small amount of material, the experiments were carried out in 10 mL or 25 mL view cells made of stainless steel. For larger scale synthesis and in situ analysis, a 100 mL stainless steel autoclave equipped with ATR-FTIR spectrometry was used.

3.2.1. View Cell, Pump, Valves and Connections

View cell. Equipped with sapphire windows, the view cell is ideal for observation of phase equilibrium, particle formation and small-scale material synthesis in SCFs, especially for scCO₂.^146-148

The body of the view cell is made of stainless steel (Figure 3.1). There are 4 holes with threads on the wall. The big hole is for a pressure transducer, and the three small holes are for a thermocouple, inlet and outlet connections, respectively. The two openings are sealed using a Teflon™ O ring, sapphire window, copper washer and a stainless steel fastening part, respectively.

Figure 3.5. The view cell.

Pump. Syringe pumps (ISCO 100 or 260D) were used for pumping CO₂ or other liquids into the reactors. There are two modes for pumping operation using the syringe pumps: constant pressure and constant flow. Generally, when CO₂ was added into the reactor to initialize the reaction, the constant pressure mode was used; when using CO₂ to extract the liquid in the reactor after the reaction, the constant flow mode was used. The operation of the syringe pump was performed through LabView VI control or using the electronic controller provided by ISCO.

Valves, Tube and Connections. The valves and connections for the high-pressure reactors were provided by HIP. The specifications of the parts are:

Taper seal needle valves: 15-11 AF1 (1/16” tube), 15-11 AF2 (1/8” tube, straight), 15-12 AF2 (1/8” tube, angle).

Glands: 15-2 AM1 (1/16” tube), 15-2 AM2 (1/8” tube).

Sleeves: 15-2 A1 (1/16” tube), 15-2 A2 (1/16” tube).

Ball check valves: 15-41 AF1 (1/16” tube), 15-41 AF2 (1/8” tube)

3.2.2. View cell with LabView VI control

A 10 or 25 ml stainless steel view cell was connected with a syringe pump (ISCO 100 DM) for pumping CO₂ from a dip tube. The check valve next to the pumps was used to prevent possible back flow from the view cell. Temperature in the view cell was measured and controlled by means of a T-type thermocouple, a heating tape (Omega SRT051-040, 0.5×4 FT) and a temperature controller (Fuji PXZ-4). The pressure was controlled by means of a pressure transducer (Omega PX302-10KGV) and a control valve (Badger-78S), which were connected to a network interface (FieldPoint, National Instruments). The network interface communicated with a computer by means of LabView software (Figure 3.2).

Figure 3.6. Schematic of experimental setup: (A) computer with LabView Virtual Instrument, (B) FieldPoint by National Instruments, (C) temperature controller, (D) thermocouple, (E) pressure transducer (F) stainless steel view cell equipped with sapphire windows, (G) pneumatic control valve, (H) needle valve, (I) check valve, (J) syringe pump, (K) CO₂ cylinder.

3.2.3. Instrumentation

The syringe pump was controlled either by a local controller provided by ISCO or by a computer. A 9 pin 232 serial port and a driver were used for computer control. The driver for pump control was provided by ISCO.

The pneumatic control valve was controlled using the computer with “Pressure Control VI (Visual Instrument)” designed by the author. The front panel is shown in Figure 3.3, the block panel is provided in Appendix 6, and the software is provided in the attached CD. On the Front Panel, the pressure setpoint can be adjusted by clicking on the arrow buttons either before or during running the virtual instrument (VI). The real pressure is indicated by means of the blue level in the cylinder, the blue dots in the window, or numerically above the window. The real-time status of the pneumatic valve is indicated by means of the round indicator above the valve. The default PID Tuning Parameters were: Prop Band = 40 %, Reset Time = 0.08 min, Deriv Time = 0.02 min.

Figure 3.7. The Front Panel of Pressure Control. A: switch for automatic or manual control; B: pressure cylinder indicator; C: pneumatic valve open-close indicator; D: pneumatic valve; E: read error out; F: write error out; G: pressure setpoint; H: setpoint and real pressure indicator; I: PID tuning parameters; J: stop button.

The block diagram of pressure control was designed by using LabView 7.0 of National Instruments (Appendix 6). The analog input signals provided by FieldPoint FP-TP-10 channel 2 was translated from mV into pressure unit (psig) and displayed. The signal in engineering units was also transferred to percentage, and then sent to the PID subVI, together with the pressure setpoint. The output of the PID subVI was transferred from percentage into engineering units (mA), and then converted into dynamic data, which was sent to the pneumatic valve via FieldPoint FP-TP-10 channel 1. The instrumentation and configuration are summarized in Appendix 7.

3.2.4. Temperature Controller

In order to control the temperature of the view cells, a Fuji PXW-4 temperature controller and heating tape (Omega SRT051-040, 0.5×4 FT) were used.

3.2.5. Reactor Equipped with In situ FTIR

Figure 3.4 shows the schematic of the experimental reactor used in this research. Carbon dioxide and precursor were pumped using syringe pumps (ISCO 260D). All feed lines have check-valves to prevent backflow and rupture disks (HIP) for safety in case of over pressurization. The reactor is a 100-mL stainless steel stirred autoclave (Parr Instruments) with a turbine impeller to provide mixing of ingredients. The reactor is heated by a heating mantle, and has an installed pressure transducer (Ashcroft K25F) and a thermo well containing a thermocouple (Parr-A472E2). The reactor temperature was controlled using a digital controller (Parr 4842).



Figure 3.8. Schematic of experimental setup: autoclave with online FTIR and GC-MS. (A) computer; (B) FTIR; (C) temperature and RPM controller with pressure display; (D) 100 ml autoclave equipped with diamond IR probe; (E) needle valves; (F) check valves; (G) syringe pump; (H) container for carboxylic acid; (I) CO₂ cylinder.

Collection Vessel for RESS Process.

During the RESS process, particles were sprayed through a variable-volume nozzle (HIP) into a closed stainless steel chamber (home made, Figure 3.5) with a heating jacket (Glas-Col, 102A, 300W) to collect particles sprayed at a constant depressurizing rate. The inlet of the particle collection device was connected to the stirred reactor and the outlet went through a filter (HIP) and was vented to a high velocity fume hood.

Figure 3.9. Schematic of particle collection vessel for the RESS process.