We report on the design and fabrication of a simple and versatile elastic canister for the compaction and hot-pressing of air-sensitive materials. This device consists of a heated double-ended floating die assembly, enclosed in a compressible stainless steel bellows that allows the action of an external hydraulic press in a uniaxial motion. The enclosure is fitted with vacuum, gas, and electrical feedthroughs to allow for atmosphere control, heating, and in situ process monitoring. The overall chamber is compact enough to be portable and transferrable into and out of a standard laboratory glovebox, thus eliminating the problem of exposing samples to ambient atmosphere during loading and unloading. Our design has been tested up to 600 °C and 7500 kg-force applied load, conditions within which transparent ceramics of anhydrous halides can be produced.

Humidity, oxygen, and carbon dioxide are constant environmental factors that affect many technologies and industries. Whether in the bulk, finely divided, or molten state, air-sensitive materials require particular attention during their preparation, use, and characterization so as to prevent contamination, decomposition, and handling hazards. Pyrophoric metals, toxic materials, metal hydrides for hydrogen storage, chalcogenide-based infrared optics, halide scintillators (NaI, SrI2, LaBr3), organometallics for solar cells ((CH3NH3)PbI3), and OLEDs, are just a few examples of a long list of materials for which extensive efforts are being deployed to limit oxygen and water contamination levels.1–8 In glass systems, for example, a ten-fold increase in moisture, between 40 and 700 ppm, causes the glass transition temperature to drop by 25 °C9 and the optical absorption at 2.5 μm to increase from 0.3 to 1.8 dB/m.10 Similarly, low oxygen contamination levels have been shown to adversely affect optical absorption,5,11–13 electron trapping,14 and radiation hardness15 in various optical materials. Because of their increased specific area, such contamination issues are all the more concern when materials are prepared from the powder state. In addition to altering physical properties of fully dense materials, moisture absorbed on particle surfaces can alter densificataion and grain growth rates, as well as lead to residual phases at grain boundaries.16 While dry-compaction can be carried out in a glovebox to control these effects, the loading of a hot-press by transporting powders through the air can be more problematic. The use of slip-fit die sets and evacuation/purge cycles under inert gas helps bring the partial pressure of O2 and H2O down to acceptable levels before compaction. However, depending on the application and the reactivity of the material, such conditions may not suffice to avoid trace levels of oxide and hydroxide contamination. At the other end of the spectrum, more radical approaches resort to miniaturizing hot-presses for use inside a standard glove box17 and building large air-tight enclosures around industrial-sized spark plasma sintering systems.18 

Here, we describe a low-cost, compact, and elastic pressing chamber, which can be loaded and sealed inside a glovebox and transferred to a floor-standing external hydraulic press. The modular design of the chamber can accommodate a heater and function as an airtight hot-press for the consolidation of a wide range of materials including metals, alloys, glasses, ceramics, and organics under vacuum or inert atmosphere. This concept solves the issue of brief exposure of air-sensitive materials during the loading and unloading stages of a powder consolidation process.

The setup consists of a double-ended floating die assembly placed inside a flexible airtight container (Fig. 1). The container is made of a standard iso-160 stainless steel vacuum bellows (Kurt J. Lesker Company), measuring 150 mm internal diameter by 150 mm tall. The bellows is hermetically clamped to a stainless steel base and an iso-160 lid using two sets of silicone O-rings and twelve aluminum claw clamps. Inside the chamber, the compaction die is surrounded by a 1.4 kW 100 mm-diameter cylindrical resistive heater (Hyndman Industrial Products) and by a 25 mm-thick layer of quartz wool to provide thermal insulation. The base is drilled out to allow the installation of an electrical feedthrough for powering the heater. Additional holes and feedthroughs allow for connections to a vacuum/purge port and to a type K thermocouple for monitoring the die temperature. The entire assembly weighs about 25 kg and can be easily positioned on a 20-ton shop press (Fig. 2(a)). This compact assembly fits the antechamber of most commercial gloveboxes (Fig. 2(b)). This is a key feature of the present design, which enables keeping the raw powders, loaded in the die, and the final pellet obtained after pressing away from any atmospheric contamination during the entirety of the compaction process.

FIG. 1.

Schematic view of the flexible chamber showing (1) thermocouple feedthrough, (2) quartz wool insulation, (3) heater, (4) die assembly, (5) claw clamp, (6) silicon O-ring, (7) vacuum/purge port, and (8) power feedthrough (hidden).

FIG. 1.

Schematic view of the flexible chamber showing (1) thermocouple feedthrough, (2) quartz wool insulation, (3) heater, (4) die assembly, (5) claw clamp, (6) silicon O-ring, (7) vacuum/purge port, and (8) power feedthrough (hidden).

Close modal
FIG. 2.

(a) The pressing chamber mounted to a hydraulic pressing frame. (b) Inside view of the pressing chamber as it sits inside the glovebox during loading/unloading with the top lid removed. The top plunger of the die assembly and the quartz wool insulation are visible in the center of the canister.

FIG. 2.

(a) The pressing chamber mounted to a hydraulic pressing frame. (b) Inside view of the pressing chamber as it sits inside the glovebox during loading/unloading with the top lid removed. The top plunger of the die assembly and the quartz wool insulation are visible in the center of the canister.

Close modal

When used as a hot-press, the heater is connected to an external power unit equipped with a temperature controller (Eurotherm). For powder compaction and sintering studies, a linear variable differential transformer (LVDT) is mounted on the outside of the chamber, between the base plate and the lid, to monitor sample displacement, while an electronic pressure gauge is installed on the hydraulic line actuating the press. Additional sensors such as oxygen, moisture, or residual gas analyzers can easily be mounted on the chamber. The output of these sensors can be then simultaneously recorded using a data acquisition device (Fig. 3).

FIG. 3.

Example of a sintering curve obtained with the present hot-pressing system.

FIG. 3.

Example of a sintering curve obtained with the present hot-pressing system.

Close modal

The limited machining of the base and the use of off-the-shelf components make the present design low-cost and easy to assemble. Furthermore, the limited use of fibrous insulation and dust-generating graphite dies provides less opportunity for particulate contamination than many commercial systems. A comparison of the key properties of our device with those of widely used commercial hot-press and spark-plasma sintering (SPS) systems is highlighted in Table I. The temperature, pressure, and atmospheric capabilities are largely dictated by material selection and, to a lesser extent, by the design of the press. These aspects are discussed in Sec. II B.

TABLE I.

Selected properties of major pressure-assisted sintering technologies.

This pressHot pressSPS
Cost (k$) 250 500 
System height (m) 0.3 
Temperature (°C) 1000 2000 2500 
Pressure (MPa) 400 400 100a 
Heating rate (°C/min) 10 10 1000 
Cleanliness + + + + + + 
This pressHot pressSPS
Cost (k$) 250 500 
System height (m) 0.3 
Temperature (°C) 1000 2000 2500 
Pressure (MPa) 400 400 100a 
Heating rate (°C/min) 10 10 1000 
Cleanliness + + + + + + 
a

SPS is limited to graphite tooling, while this press and conventional hot-presses can employ high-strength steels, refractory metals, or carbides.

1. Pressure

The maximum pressure rating of the chamber is determined by the strength of the die-set material and by the footprint of the plungers at the base plate and top lid, so as to prevent permanent deformation by yielding. Die-sets are made of stiff and strong materials including graphite, high-strength steels, molybdenum, or tungsten carbide, whose properties as they relate to pressing dies are discussed at length elsewhere.19–21 With compressive strength on the order of 100 MPa, high-density fine grain graphite is most commonly used in high-temperature hot-presses. It is, however, brittle and its purity and reducing behavior at high temperature can lead to sample contamination issues.22,23 With average room temperature compressive strengths of 400 MPa, high-strength steels and molybdenum are well suited to drive performance to higher pressures, but the significant temperature dependency of their behavior must be taken into account in the design. Cemented tungsten carbide die assemblies, on the other hand, allow extending applicable pressures towards the gigapascal range.

2. Temperature

Throughout the design, thermal modeling (Energy2D software24) was used to test possible implementations of the press and their effect on (i) the maximum achievable temperature at the sample location, (ii) the temperature profile across the sample, and (iii) the maximum achievable heating and cooling rates, as these characteristics affect the sintering behavior of materials. To this end, a 2-dimensional cross-section of the setup was drafted and the appropriate thermal properties of each component were defined. The heating element was treated as a source of constant heat flux and conductive and convective heat transfer were monitored over time. In this compact system, the maximum temperature achievable at the sample location is largely determined by the power of the heating element and the maximum temperature that the O-rings can withstand (200 °C). Simulations also found that adding an alumina ceramic spacer between the plungers and the base or top flange limits heat flux loss by up to 32% for a 14 mm spacer thickness. Consequently, the maximum average heating rate between 25 and 600 °C is expected to increase by 7% and 10% for a 7-mm and 14-mm spacer, respectively.

In testing, the die temperature was raised to 600 °C. After a 1-h dwell at this temperature, the top flange, just above the O-ring, reached 85 °C and 100 °C after 2 h. This is in reasonable agreement with our simulation, which indicates that the O-ring reaches 84 °C and 147 °C after 1 and 2 h, respectively. Safe and prolonged use of the press at temperatures well beyond 600 °C should pose no issue, however, active air or water-cooling of the bellows and top flange would be required. In addition, our simulation indicates that, after 1 h equilibration, radial temperature gradients at the sample location are minimal and range from 0.1 to 0.3 °C/mm in the case of graphite and high-strength steel dies, respectively. During pressing runs, typical heating rates are 8 °C/min with the heater operated at 55% of its capacity (Fig. 3). Simulations indicate that the maximum heating and cooling rates can reach 15 °C/min.

The pressing chamber is loaded in a glovebox as shown in Fig. 1. A typical procedure consists of loading the press with a 13 mm-diameter, 6 mm-thick powder-compact made with a separate arbor press located inside the glovebox. These consolidated pellets are either loaded directly between the plungers of the pressing chamber or embedded in a soft pressure-transmitting medium, such as anhydrous sodium chloride. Anti-adhesive gaskets (e.g., graphite) or coatings (e.g., boron nitride spray) may be used on the plungers depending on the material to be pressed. Once loaded, the top plunger is placed into the die and the top flange is clamped down to seal the chamber. A 1-mm deep recess machined in the base and lid prevents the die from shifting during transport. The hermetically sealed chamber is then transferred through the antechamber of the glovebox and mounted on the hydraulic press frame. The electrical and ground wiring are connected as well as the sensors, vacuum, and inert-gas lines. After compaction, the chamber is transferred back into the glovebox for unloading.

As emphasized in the Introduction, the reduction of atmospheric impurity levels is paramount in a variety of applications. Optical grade alkaline-earth halides, used as UV and infrared windows, scintillators, or laser materials, for example, have well-documented sensitivity to the ambient atmosphere and require special handling during their fabrication25 and characterization.26 We have used the present hot-press design, fitted with a tungsten carbide die-set, to produce transparent ceramics of halide materials. Figure 4 shows the world-first transparent ceramic of anhydrous barium chloride, a recently identified gamma-ray detector.27,28 The optical transparency of this sample is clearly superior to the best results obtained with a commercial hot-press utilizing a graphite heating element and die-set. A backlight is used in Figure 4(b) to show particulate contamination and the moderate translucency of the conventionally processed sample. The difference between the two pictured samples is attributed to a combination of the 20 ppm moisture level maintained throughout processing, a carbon-free environment, and the ability to access a higher pressure with a high-strength tungsten carbide die set.

FIG. 4.

(a) A 25-mm diameter BaCl2 sample fabricated in a conventional hot-press and (b) the same sample illuminated with a backlight to show carbon inclusions. (c) 13-mm diameter BaCl2 transparent ceramic embedded in a NaCl pressure transmitting medium (white rim) made in the press described in this paper.

FIG. 4.

(a) A 25-mm diameter BaCl2 sample fabricated in a conventional hot-press and (b) the same sample illuminated with a backlight to show carbon inclusions. (c) 13-mm diameter BaCl2 transparent ceramic embedded in a NaCl pressure transmitting medium (white rim) made in the press described in this paper.

Close modal

This work was supported by the U.S. Department of Energy/NNSA/DNN R&D under Contract No. AC02-05CH11231. This support does not constitute an express or implied endorsement on the part of the Government.

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