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FABRICATION OF PIEZOELECTRIC CERAMlClPOLYMER COMPOSITES BY INJECTION MOLDING Leslie J Bowen and Kenneth W French Materials Systems Inc 53 Hillcrest Road Concord MA 01742 Abstract Research at the Materials Research Laboratory Pennsylvania State University has demonstrated the potential for improving hydrophone performance using piezoelectric ceramic polymer composites As part of an ONR funded initiative to develop cost effective manufacturing technology for these composites Materials Systems is pursuing an injection molding ceramic fabrication approach This paper briefly overviews key features of the ceramic injection molding process then describes the approach and methodology being used to fabricate PZT ceramic polymer composites Properties and applications of injection molded PZT ceramics are compared with conventionally processed material Introduction Piezoelectric ceramic polymer composites offer design versatility and performance advantages over both single phase ceramic and polymer piezoelectric materials in both sensing and actuating applications These composites have found use in high resolution medical ultrasound as well as developmental Navy applications Many composite configurations have been constructed and evaluated on a laboratory scale over the past thirteen years One of the most successful combinations designated 1 3 composite in Newnham s notation l 1 has a one dimensionally connected ceramic phase PZT fibers contained within a three dimensionally connected organic polymer phase Hydrophone figures of merit for this composite can be made over 10 000 times greater than those of solid PZT ceramic by appropriately selecting the phase characteristics and composite structure The Penn State composites were fabricated l by hand aligning extruded PZT ceramic rods in a jig and encapsulating in epoxy resin followed by slicing to the appropriate thickness and poling the ceramic Aside from demonstrating the performance advantages of this material the Penn State work highlighted the difficulties involved in fabricating 1 3 composites on a large scale or even for prototype purposes These are 11 The requirement to align and support large numbers of PZT fibers during encapsulation by the polymer 2 The high incidence of dielectric breakdown during poling arising from the significant probability of encountering one or more defective fibers in a typical large array Over the past five years several attempts have been made to simplify the assembly process for 1 3 transducers with the intention of improving manufacturing viability and lowering the material cost Early attempts involved dicing solid blocks of PZT ceramic into the desired configuration and back filling the spaces with a polymer phase This technique has found wide acceptance in the medical ultrasound industry for manufacturing high frequency transducers 2 More recently Fiber Materials Corp has demonstrated the applicability of its weaving technology for fiber reinforced composites to the assembly of piezoelectric composites 31 Another exploratory technique involves replicating porous fabrics having the appropriate connectivity 41 For extremely fine scale composites fibers having diameters in the order of 25 to 100 pn and aspect ratios in excess of five are required to meet device performance objectives As a result these difficulties are compounded by the additional challenge of forming and handling extremely fine fibers in large quantities without defects Recently researchers at Siemens Corp have shown that very fine scale composites can be produced by a fugitive mold technique However this method requires fabricating a new mold for every part 51 This paper describes a new approach to piezoelectric composite fabrication viz Ceramic injection molding Ceramic injection molding is a cost effective fabrication approach for both Navy piezoelectric ceramic polymer composites and for the fabrication of ultrafine scale piezoelectric composites such as those required for high frequency medical ultrasound and nondestructive evaluation The injection molding process overcomes the difficulty of assembling oriented ceramic fibers into composite transducers by net shape preforming ceramic fiber arrays Aside from this advantage the process makes possible the construction of composite transducers having more complex ceramic element geometries than those previously envisioned leading to greater design flexibility for improved acoustic impedance matching and lateral mode cancellation Process Descriotion Injection molding is widely used in the plastics industry as a means for rapid mass production of complex shapes at low cost Its application to ceramics has been most successful for small cross section shapes e g thread guides and large complex shapes which do not require sintering to high density such as turbine blade casting inserts More recently the process has been investigated as a production technology for heat engine turbine components 6 71 The injection molding process used for PZT molding is shown schematically in Figure 1 By injecting a hot thermoplastic mixture of ceramic powder and organic binder into a cooled mold complex shapes can be formed with the ease and rapidity normally associated with plastics molding Precautions such as hard facing the metal contact surfaces are important to minimize metallic contamination from the compounding and molding machinery For ceramics the binder must be removed nondestructively necessitating high solids loading careful control of the binder removal Powder Process i ng r 4 CERAMIC PREFORM Organic Binder I Granulate I PREFORM LAY UP TO FORM LARGER ARRAYS Lu i 1 1 and apply electrodes Figure 1 Injection Molding Process Route process and proper fixturing Once the binder is removed the subsequent firing poling and epoxy encapsulation processes are similar to those used for conventional PZTipolymer composites 11 I Thus the process offers the following advantages over alternative fabrication routes Complex near net shape capability for handling many fibers simultaneously rapid throughput typically seconds per part compatibility with statistical process control low material waste flexibility with respect to transducer design allows variation in PZT element spacing and shape and low cost in moderate to high volumes In general because of the high initial tooling cost the ceramics injection molding process is best applied to complex shaped components which require low cost in high volumes Comoosite Fabrication and Evaluation The approach taken to fabricate 1 3 piezoelectric composites is shown in Figure 2a which illustrates a PZT ceramic preform concept in which fiber positioning is achieved using a co molded integral ceramic base After polymer encapsulation the ceramic base is removed by grinding Aside from easlng the handling of many fibers this preform approach allows broad latitude in the selection of piezoelectric ceramic element geometry for composite performance optimization Tool design is important for successful injection molding of piezoelectric composites The approach shown in Figure 2b uses shaped tool inserts to allow changes in part design without incurring excessive retooling costs Figure 2c shows how individual preforms are configured to form larger arrays Figure 2a Preform Configuration Approx 400 ceramic elements REMOVABLE INSERT CAVITY TOOL BODY U SPRUE Figure 2b Injection Molding Tool Configuration Figure 2c Large Area Composite Arrays made from Preforms Figure 2 Preform Approach to Composite Fabrication In practice material and molding parameters must be optimized and integrated with injection molding tool design to realize intact preform ejection after molding Key parameters include PZT binder ratio PZT element diameter and taper PZT base thickness tool surface finish and the molded part ejection mechanism design In order to evaluate these process parameters without incurring excessive tool cost a tool design having only two rows of 19 PZT elements each has been adopted for experimental purposes Each row contains elements having three taper angles 0 1 and 2 degrees and two diameters 0 5 and lmm To accommodate molding shrinkage the size of the preform is maintained at 5Ox50mm to minimize the possibility of shearing off the outermost fibers during the cooling portion of the molding cycle Figure 3 Injection Molded 1 3 Composite Preforms 161 Figure 3 shows green ceramic preforms fabricated using this tool configuration Note that all of the PZT elements ejected intact after molding including those having no longitudinal tapering to facilitate ejection Slow heating in air has been found to be a suitable method for organic binder removal Finally the burned out preforms are sintered in a PbO rich atmosphere to 97 98 of the theoretical density No problems have been encountered with controlling the weight loss during sintering of these composite preforms even for those fine scale high surface area preforms which are intended for high frequency ultrasound L Figure 4 Scanning Electron Micrographs of As molded Upper and As sintered Lower Surfaces of PZT Fibers Figure 4 illustrates the surfaces of as molded and as sintered fibers showing the presence of shallow fold lines approximately 10pm wide which are characteristic of the injection molding process The fibers exhibit minor grooving along their length due to ejection from the tool Figure 5 shows the capability of near net shape molding for fabricating very fine scale preforms PZT element dimensions only 30pm wide have been demonstrated The as sintered surface of these elements indicates that the PZT ceramic microstructure is dense and uniform consisting of equiaxed grains 2 3pm in diameter Figure 5 Fine scale 2 2 Composite formed by Near Net shape Molding Upper Micrograph As sintered Surface Lower Micrograph In order to demonstrate the lay up approach for composite fabrication composites of approximately 10 volume percent PZT 5H fibers and Spurrs epoxy resin were fabricated by epoxy encapsulating laid up pairs of injection molded and sintered fiber rows followed by grinding away the PZT ceramic stock used to mold the composite preform Figure 6 shows composite samples made from freshly compounded PZT binder mixture and from reused material Recycling of the compounded and molded material appears to be entirely feasible and results in greatly enhanced material utilization Table 1 compares the piezoelectric and dielectric properties of injection molded PZT ceramic specimens with those reported for pressed PZT 5H samples prepared by the powder manufacturer When the sintering conditions are optimized for the PZT 5H formulation the piezoelectric and dielectric properties are comparable for both materials Since the donor doped PZT 5H formulation is expected to be particularly sensitive to iron contamination from the injection molding equipment the implication of these measurements is that such contamination is negligible in this injection molded PZT material Powder supplied by Morgan Matroc Inc Bedford Ohio Lot 105A 162 Table 1 Properties of Injection Molded Piezoelectric Ceramics Specimen Relative Dielectric d33 TY Pe Permittivity Loss 1 kHz1 pC N Die Pressed 3584 0 01 8 745 Inj Molded 3588 0 01 8 755 Aged 24 hours before measuremegt Poling conditions 2 4kV mm 60 C 2 minutes Figure 6 Injection Molded PZT Fiber Epoxy Resin Composites prepared by the Preform Lay up Method Summarv Ceramic injection molding has been shown to be a viable process for fabricating both PZT ceramics and piezoelectric ceramic polymer transducers The electrical properties of injection molded PZT ceramics are comparable with those prepared by conventional powder pressing with no evidence of deleterious effects from metallic contamination arising from contact with the compounding and molding equipment By using ceramic injection molding to fabricate composite preforms and then laying up the preforms to form larger composite arrays an approach has been demonstrated for net shape manufacturing of piezoelectric composite transducers in large quantities Ac knowledaements This work was funded by the Office of Naval Research under the direction of Mr Stephen E Newfield The authors wish to thank Ms Hong Pham for technical assistance and Dr Thomas Shrout of the Materials Research Laboratory Penn State University for electrical measurements References l R E Newnham et al Composite Piezoelectric Transducers Materials in Engineering Vol 2 pp 93 106 Dec 1980 21 C Nakaya et al IEEE Ultrasonics Symposium Proc Oct 16 18 1985 p 634 131 S D Darrah et al Large Area Piezoelectric Composites Proc of the ADPA Conference on Active Materials and Structures Alexandria Virginia Nov 4 8 1991 Ed G Knowles Institute of Physics Publishing pp 139 142 A Safari and D J Waller Fine Scale PZT Fiber Polymer Composites presented at the ADPA Conference on Active Materials and Structures Alexandria Virginia Nov 41 4 8 1991 5 U Bast D Cramer and A Wolff A New Technique for the Production of Piezoelectric Composites with 1 3 Connectivity Proc of the 7th CIMTEC Montecatini Italy June 24 30 1990 Ed P Vincenzini Elsevier pp 2005 201 5 G Bandyopadhyay and K W French Fabrication of Near net Shape Silicon Nitride Parts for Engine Application J Eng for Gas Turbines And Power 108 J Greim et al Injection Molded Sintered Turbocharger Rotors Proc 3rd Int Symp on Ceramic Materials and Components for Heat Engines Las Vegas Nev pp 1365 1375 Amer Cer Soc 1989 61 pp 536 539 1986 171 163 制作壓電陶瓷 萊斯利 J Bowen 和肯尼思 W 法國 材料 Systems 公司 53 Hillcrest 路 康科德 碩士 01742 摘要 在材料研究實(shí)驗(yàn)室 賓州州立大學(xué)研究已經(jīng)證明改進(jìn)水聽器表現(xiàn)的潛力使用壓電的制陶藝術(shù) 聚合物合成物 為這些合成物 材料系統(tǒng)有成本效益的制造業(yè)技術(shù)正追求一制陶藝術(shù)的制造接 近注射模塑 本文簡要概述主要特征的陶瓷注塑成型工藝 接著介紹的方式和方法可以被用來 制造壓電陶瓷 聚合物復(fù)合材料 性能和應(yīng)用注塑壓電陶瓷與常規(guī)加工材料 簡介 壓電的制陶藝術(shù) 聚合物合成物給予關(guān)于單相陶瓷和聚合物階段制陶藝術(shù)壓電的材料在遙感和實(shí)際應(yīng)用兩 方面設(shè)計(jì)多技能和表現(xiàn)優(yōu)勢 這些合成物已經(jīng)在以及發(fā)展的海軍應(yīng)用高分辨醫(yī)學(xué)超聲中找出使用 在過去 13 年對(duì)許多實(shí)驗(yàn)室進(jìn)行了已建成組合配置和評(píng)價(jià) 全球最成功的組合 指定 1 3 復(fù)合岡維爾的 五線譜 l 有一個(gè)連在尺寸上陶瓷相 壓電纖維 控制在一個(gè)三維連通有機(jī)高分子階段 水聽器 人物優(yōu)異這種復(fù)合材料可取得超過一萬倍以上的固體壓電陶瓷 由適當(dāng)選擇的階段性特征和復(fù)合 結(jié)構(gòu) 在賓夕法尼亞州立復(fù)合材料 l 進(jìn)行手工調(diào)擠壓壓電陶瓷棒在跳汰及封裝 環(huán)氧樹脂 然后切片 到適當(dāng)?shù)暮穸群蜆O化的陶瓷 除了展現(xiàn)優(yōu)越的技術(shù)性能 這種材料 在賓夕法尼亞州工作的突 出困難 編造 1 3 復(fù)合材料的大規(guī)模 甚至為原型的目的 這些措施包括 1 把許多的許多 PZT 纖維排成一排在包裝期間經(jīng)過聚合物和支撐要求 2 2 在滑行期間電介質(zhì)故障 起源于遭遇在一典型大陣列 中一根或更多有缺陷纖維的重要可 能性的高發(fā)生 過去五年已做了一些嘗試 以簡化裝配過程 1 3 傳感器與 有意提高制造業(yè)的可行性 并降低材 料成本 早期從事固體切丁塊壓電陶瓷成為理想的配置和回填土的空間內(nèi)的一種高分子 相 這項(xiàng)技術(shù)已廣泛接受了超聲醫(yī)學(xué)業(yè)生產(chǎn)高頻傳感器 2 最近 纖維材料股份有限公司已證明 適用其織造工藝?yán)w維復(fù)合材料向大會(huì)壓電陶瓷復(fù)合材料 3 另勘探技術(shù)涉及復(fù)制的多孔面料 有適當(dāng)?shù)倪B通 4 為極其好刻度合成物 纖維 大約 25 到 100 pn 和超過五一個(gè)尺寸與另一個(gè)尺寸之比有直徑需 要來遭遇裝置表現(xiàn)目標(biāo) 因此 這些困難被附加挑戰(zhàn)用形成沒有缺點(diǎn)和處理在大數(shù)量中極其好 纖維構(gòu)成了 最近 研究人員在西門子公司已表明很細(xì)尺度復(fù)合材料可以產(chǎn)生一個(gè)逃犯模具技 術(shù) 但是 這種方法需要制作一個(gè)新的模具 每部分 5 本文描述了一種新方法 壓電復(fù)合材料的制備 即 陶瓷注射成型 陶瓷注射成型技術(shù)是一種有 成本效益的制備方法雙方海軍的壓電陶瓷 聚合物復(fù)合材料的制備超細(xì) 規(guī)模壓電復(fù)合材料 例 如那些需要高頻醫(yī)用超聲和無損評(píng)價(jià) 注塑成型過程中 克服困難 裝配為主的陶瓷纖維復(fù)合成 換用網(wǎng)狀預(yù)成型品陶瓷纖維 陣列 除了這方面的優(yōu)勢 這一進(jìn)程使得有可能建造復(fù)合傳感器 具有更復(fù)雜的陶瓷元件幾何比原先設(shè)想 導(dǎo)致更大的靈活性 設(shè)計(jì)為改進(jìn)的聲阻抗匹配和橫向模 式取消 加工工藝 注塑廣泛應(yīng)用于塑料業(yè)作為一種手段 快速大量生產(chǎn) 形狀復(fù)雜 在 成本低 應(yīng)用陶瓷一直最成 功的小型截面形狀 例如 螺紋指南 及大型復(fù)雜形狀不需要燒結(jié)密度高 如渦輪葉片鑄造刀片 最近 這一進(jìn)程已展開調(diào)查 作為生產(chǎn)工藝熱發(fā)動(dòng)機(jī)渦輪部件 6 7 圖 1 注射模塑過程路線 注塑成型用于 PZT 成型見圖 1 注的熱點(diǎn)熱塑性混合陶瓷粉及有機(jī)結(jié)合成一個(gè)冷卻結(jié)晶 復(fù)雜 的形狀 可以形成與方便與快捷通常與塑料成型 防范措施 如硬面臨的金屬接觸面 這些都是 重要的 以盡量減少金屬污染的加劇和成型機(jī)器 陶瓷的粘結(jié)劑必須拆除非破壞性地 使成為必 要高固體量 嚴(yán)格控制粘結(jié)劑 拆除過程中 和正確裝夾 一旦粘結(jié)劑是拆掉 隨后射擊 極化和 環(huán)氧包封過程類似于常規(guī) pzti 聚合物復(fù)合 1 因此 過程具有以下優(yōu)點(diǎn)替代加工路線 復(fù)雜 近凈形能力處理許多纖維同時(shí)發(fā)生 快速吞吐量 通常每秒 兼容性與統(tǒng)計(jì)過程控制 低的材 料浪費(fèi) 靈活應(yīng)變傳感器設(shè)計(jì) 允許變化壓電元件間距和形狀 成本低 在中度到高度卷 總 的來說 由于高的初始成本 工裝 陶瓷注射成型是最好的應(yīng)用復(fù)雜形狀零件需要低成本高產(chǎn)量 制造和評(píng)價(jià) 采取這種辦法 編造 1 3 壓電復(fù)合載在圖 2a 它說明了壓電陶瓷預(yù)制棒的概念 光纖定位是實(shí) 現(xiàn)以共同塑造積分陶瓷基地 聚合物后封裝在陶瓷拆除磨 除了處理許多纖維 這預(yù)制棒做法 使廣大緯度在選擇壓電陶瓷元件幾何形狀的綜合性能優(yōu)化 工具的設(shè)計(jì)是成功的重要注塑壓電 陶瓷復(fù)合材料 辦法列于圖 2b 用途形工具刀片允許改變部分設(shè)計(jì) 又不過分更換工具成本 圖 2c 顯示了個(gè)體預(yù)制棒配置 能夠形成較大的陣列 圖 2 預(yù)制棒的方法綜合加工 在實(shí)踐中 材料成型參數(shù)必須優(yōu)化整合注塑模具設(shè)計(jì)實(shí)現(xiàn)完整的預(yù)制彈射后 成型 關(guān)鍵參數(shù) 包括 壓電 粘合劑比 壓電元件直徑和錐形 基地 PZT 的厚度 刀具表面光潔度 而塑造的一部 分彈射機(jī)制設(shè)計(jì) 為了評(píng)價(jià)這些工藝參數(shù) 又不過分工具成本 工具設(shè)計(jì) 僅有兩排 19PZT 的每 一分子已經(jīng)通過實(shí)驗(yàn)?zāi)康?每一行包含有三個(gè) 0 1 和 2 度 和兩個(gè)直徑 0 5lmm 容納成 型收縮 大小坯維持在 5ox50mm 以盡量剪過的最外層纖維在 冷卻部分的成型周期 圖 3 注射塑造 1 3 混合成的預(yù)成型品 圖 3 顯示綠色陶瓷預(yù)制裝配使用此工具配置 看到所有的 PZT 分子趕出完好無損后成型 包括 具有無縱一端漸漸變細(xì)變尖方便彈射 慢熱空氣已被認(rèn)為是一種合適的方法有機(jī)結(jié)合搬遷 最 后 燒出預(yù)制棒燒結(jié)在氣氛 97 98 的理論密度 沒有遇到任何問題與控制體重?zé)Y(jié)過程中 這 些組合坯 即使是這些優(yōu)秀的尺度 比表面積高預(yù)制棒是為高頻超聲波 圖 4 掃描電子顯微鏡對(duì)所塑造 上 以及燒結(jié) 下 面壓電纖維 圖 4 說明了表面為成型和燒結(jié)纖維 顯示駐留淺褶皺系大約寬十所特征的注塑成型過程 纖維 呈現(xiàn)輕微開槽沿其長度 由于噴出的工具 圖 5 顯示的能力近凈成型模具制作非常精細(xì)規(guī)模預(yù) 制棒 壓電元件尺寸只有 30pm 性已經(jīng)顯現(xiàn) 等離子燒結(jié)面這些元素顯示了壓電陶瓷微觀結(jié)構(gòu) 致密 均勻 為了展示裁員的辦法 為復(fù)合材料 復(fù)合材料約 10 體積 了 PZT 5H 纖維和 spurrs 環(huán)氧樹 脂材料環(huán)氧灌封奠定了對(duì) 注塑成型及燒結(jié)纖維整齊之后磨走了壓電陶瓷股票用來塑造復(fù)合預(yù)制 棒 f 5 26 顯示復(fù)合材料制成的鮮混合 pzt 粘合劑混合物和重用材料 循環(huán)中的復(fù)雜化 并 塑造材料似乎是完全可行的 結(jié)果大大提高材料的利用率 圖 5 細(xì) 2 2 級(jí)復(fù)合材料組成的近余量成形 上顯微照片 由于燒結(jié)面 下顯微照片 為了展示裁員的辦法 為復(fù)合材料 復(fù)合材料約 10 體積 了 PZT 5H 纖維和環(huán)氧樹脂材料環(huán)氧 灌封奠定了對(duì) 注塑成型及燒結(jié)纖維整齊之后磨走了壓電陶瓷股票用來塑造復(fù)合預(yù)制棒 f 5 26 顯示復(fù)合材料制成的混合 pzt 粘合劑混合物和重用材料 循環(huán)中的復(fù)雜化 并塑造材料似 乎是完全可行的 結(jié)果大大提高材料的利用率 表 1 比較了壓電及介電性能的注塑壓電陶瓷標(biāo)本所報(bào)道的壓了 PZT 5H 樣品制備的粉末的廠商 當(dāng)燒結(jié)條件 適合了 PZT 5H 制定 壓電和介電性能比較兩種材料 PZT 5H 制定預(yù)計(jì)將特別敏 感鐵污染的注塑設(shè)備 言下之意 這些測量是這種污染是微不足道 在這注塑壓電材料 概要 陶瓷注射成型技術(shù)已被證明是一種可行的工藝制備兩種壓電陶瓷和壓電陶瓷 聚合物換能器 電氣性能注塑壓電陶瓷媲美備傳統(tǒng)的粉末緊迫 但沒有證據(jù)的有害影響來自金屬污染引起的接 觸套匯和成型設(shè)備 用陶瓷注塑造復(fù)合坯 然后打下了預(yù)制棒 形成較大復(fù)合陣列 結(jié)束語 這項(xiàng)工作是由辦事處的海軍研究的指導(dǎo)下 斯蒂芬 e newfield 作者要感謝洪女士范用于技 術(shù)援助 而威博士 shrout 的材料研究實(shí)驗(yàn)室 西恩 州立大學(xué)電氣測量 參考文獻(xiàn) l R E Newnham et al 復(fù)合材料壓電換 材料工程卷 2 pp 93 106 1980年12 月 2 C Nakaya et al IEEE Ultrasonics Symposium Proc Oct 16 18 1985 p 634 3 S D Darrah 大面積壓電復(fù)合材料 proc 該adpa會(huì)議上活躍的材料和結(jié)構(gòu) 弗吉尼亞 州亞歷山 11月4日至8日 1991年版 g knowles物理研究所出版 pp139 142 4 A Safari and D J Waller 細(xì)尺度壓電纖維 高分子復(fù)合材料 會(huì)上adpa會(huì)議上活躍的材料 和結(jié)構(gòu) 弗吉尼亞州亞歷山 11月4日至8 日 1991年 5 U Bast D Cramer and A Wolff A New Technique for the Production of Piezoelectric Composites with 1 3 Connectivity Proc of the 7th CIMTEC Montecatini Italy June 24 30 1990 Ed P Vincenzini Elsevier pp 2005 201 5 6 G Bandyopadhyay and K W French Fabrication of Near net Shape Silicon Nitride Parts for Engine Application J Eng for Gas Turbines And Power 7 J Greim et al Injection Molded Sintered Turbocharger Rotors Proc 3rd Int Symp on Ceramic Materials and Components for Heat Engines Las Vegas Nev pp 1365 1375 Amer Cer Soc 1989